Balancing care and conflict: towards a better understanding of maternal aggression in canaries

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Yet, seemingly maladaptive parental behaviours such as directed aggression towards the offspring have been reported in a variety of species. While maternal aggression - defined as aggressive interactions from mothers to the offspring within the family context - may be a seemingly maladaptive behaviour, it could also be an adaptive strategy allowing optimal resource allocation for current and future reproduction in the face of evolutionary conflicts of interest. This study investigated associations between maternal aggression and altered offspring development in domestic canaries ( Serinus canaria ). Offspring exposed to maternal aggression showed reduced growth, while no differences in survival were observed. In addition, juvenile males, but not females, exposed to maternal aggression displayed increased threatening behaviours, highlighting the importance of considering long-term effects when interpreting the significance of aggressive parenting styles. Females that exhibited maternal aggression did not lay larger second clutches, as would be expected if aggression during the first reproductive event was directed at prioritising future reproduction. However, they laid larger and less variable clutches overall, suggesting that females that engaged in maternal aggression may be less flexible and more prone to high investment at egg laying. Biological sciences/Developmental biology Biological sciences/Ecology/Evolutionary ecology Biological sciences/Evolution/Evolutionary developmental biology Biological sciences/Genetics/Animal breeding Biological sciences/Genetics/Development early-life experience maternal aggression maternal behaviour offspring development Figures Figure 1 Figure 2 Figure 3 Figure 4 INTRODUCTION Parental care is a widespread phenomenon in the animal kingdom and is defined as any parental behaviour likely to enhance offspring quality and survival [ 1 ]. The extent and nature of parental care varies considerably between and within species. It can range from nest site selection and egg guarding to more complex forms such as incubation, brooding and provisioning of dependent offspring [ 2 , 3 ]. Once offspring are born, the level of parental care is expected to be aligned with the needs of the offspring, which are signalled by a variety of traits. However, instead of responding to these signals of need, parents sometimes show neglect and aggression towards their offspring (e.g. [ 4 ] in rhesus macaques; [ 5 ] in humans; [ 6 ] in seals; [ 7 ] in American coots; [ 8 ] in moorhens). Previous studies proposed that such aggression may arise as a side-effect of other parental traits [ 4 ], be linked to genetic abnormalities [ 9 ] or be due to environmental stressors [ 10 ]. Intriguingly, however, parenting styles, including aggressive and “less caring” ones, are often transmitted across generations [ 5 , 11 ], raising the question of whether they represent an adaptive parental strategy in specific contexts, rather than a seemingly maladaptive behaviour that reduces reproductive success and population viability [ 4 ] [ 12 ]. When the expected fitness gain from the investment in the current offspring is low, maternal aggression towards the offspring could represent an adaptive parental strategy to reduce or terminate the investment in the current reproductive event [ 13 ]. Indeed, maternal aggression (i.e., aggressive behaviour of the mothers towards their offspring) and, in extreme cases, infanticide and cannibalism has been found to facilitate the selective survival of other offspring, while aspects, such as neglect or abandonment are thought to promote offspring independence (i.e., in the context of so-called weaning age conflicts; [ 6 , 7 ]. Such weaning age conflicts are rooted in evolutionary conflicts of interest, as the offspring are selected to demand more immediate investment for their own growth and survival than the parents are willing to provide, while the parents are often inclined to preserve resources for other current or future offspring [ 3 , 14 ]. The resulting parent-offspring conflicts may lead to a tug-of-war over resource allocation and caregiving, often involving parental aggression and, in particular, neglect, especially when the costs of caregiving outweigh the benefits [ 13 ]. Traditionally, though, parental aggression has mainly been interpreted in terms of short-term cost-benefit balance for the parents. However, it is known that parental behaviour can have long-lasting consequences on offspring phenotype [ 15 ]. For example, juvenile rats deprived of maternal care show inappropriate social play behaviour [ 16 ], and adult male rats exposed to maternal separation show higher levels of aggression [ 17 ]. Thus, these early environmental and parental effects can modify the developmental trajectory of the offspring, allowing for phenotypic adjustments i.e. developmental plasticity [ 18 ]. An adaptation to current conditions may come at a cost in the future, if developmental plasticity leads to negative lasting changes in their phenotype [ 19 ]. For example, adverse early-life environments have been associated with accelerated cellular ageing and increased inflammation in adulthood e.g., [ 20 ]. However, such adjustments may facilitate survival under potentially suboptimal conditions. For example, an altered phenotype as a consequence of early-life social adversity or maternal aggression (see e.g., [ 16 , 17 ] could eventually allow the offspring to benefit from higher levels of aggression in harsh environments if this allows them, for example, to access higher dominance ranks [ 17 , 21 ]. Early environmental and parental effects are therefore also interpreted as a form of adaptive developmental plasticity that is beneficial in some specific contexts [ 18 , 22 – 26 ]. Specifically, he match/mismatch hypothesis suggests that individuals are developmentally programmed to cope with the environment encountered during sensitive developmental periods [ 27 , 28 ]. However, experimental evidence on this topic is still scarce. Here, we investigated the functional consequences of maternal aggression (hereafter MA) using the domestic canary ( Serinus canaria ) as a model system, to test whether MA could be part of a context-dependent adaptive parental strategy or a maladaptive behaviour. In this species, mothers often behave aggressively towards their offspring (i.e., by pecking and plucking their feathers before fledging) from around day 14 post-hatching onwards, and eventually throw them out of the nest (unpublished observations from our lab). First, we expected MA to be associated with reduced offspring growth (prediction 1) and ultimately higher early-life mortality (prediction 2), reflecting short-term effects of MA. Then, we investigated potential lasting consequences on the behavioural phenotype. As MA has been linked to higher levels of aggression in mammals e.g., [ 17 ], we expected that experiencing MA during early life would be associated with a higher tendency to use direct aggression to resolve stressful conflicts (prediction 3), here tested in a social defeat test (i.e., long-term associations). Finally, if MA is driven by parental preferences for future over current offspring as a form of reducing or terminating current investment, it should be associated with a greater investment in future reproduction (prediction 4). Thus, we expect a shorter time interval between the first and the second clutch, as well as larger second clutches. On the other hand, if MA is a sign of conflict, it could lead to more variability in parental investment, while non-aggressive mothers might show more consistent clutch sizes [ 29 ]. Otherwise, if MA is maladaptive it should always deteriorate offspring development and reduce the overall reproductive success of parents. MATERIALS AND METHODS Study Species and Breeding In this study, we bred 197 first-year canaries in two different years. Birds originated from a lab-bred population (LA-number: LA 1100 161) of captive Fife fancy canaries kept at the University of Antwerp [ 33 ], supplemented each year in late autumn with new birds from local breeders to ensure genetic diversity. In both years, experiments were conducted under the same conditions and following the same procedures. In February, all individuals that were selected for breeding (approx. 50% local: 50% new birds) were kept five weeks prior to pair formation in single-sex aviaries at a room temperature of 20–24°C with long artificial daylight (14h light: 10h dark) [ 30 ]. We then formed 80 couples in 2022 and 117 in 2023, by placing first the male and then a female into a breeding cage (50 x 64 x 40 cm 3 , GEHU cages, The Netherlands). Mating pairs were created randomly, but pair composition was subsequently checked to make sure that the partners were unrelated. Individuals were replaced by new males or females if one of the pair showed signs of sickness or died. The breeding cages were equipped with two perches, a nest cup, nesting material, cuttlefish bones, shell sand, canary seed mixture (Van Camp, Belgium), egg food (provided twice a week; Van Camp, Belgium) and water ad libitum . After the breeding pairs completed nest-building, we monitored egg laying each morning and weighed the freshly laid eggs. Ethical Note The experiments described in this study were carried with the approval of the Ethical Committee for Animal Experimentation (ECD) at the University of Antwerp under license ECD-2022-87 and were performed in accordance with Belgian and Flemish legislation regarding animal welfare, adhered to the ASAB/ABS guidelines for the use of animals in behavioural research and teaching, and complied with the ARRIVE guidelines. During the experiments, the birds were kept in standard breeding cages for canaries, otherwise, they were kept in mixed-sex aviaries (2 x 2 x 2 m 3 ). A constant effort was made to keep the birds in semi-natural conditions and in good health. To this end, we frequently sought advice from breeders and veterinarians to further optimize the housing conditions. The invasiveness of all observational procedures described below and the handling time were minimized. For a more detailed description on the measures taken during the social defeat test to minimise stress, please see below the corresponding methods section. Body condition and behaviour were monitored throughout the whole experiment as it reflects general well-being based on pre-defined criteria described in [ 31 ]. At the end of the experiment, all the canaries were kept in the lab-bred population at the University of Antwerp as they are part of a transgenerational transmission experiment and will be allowed to breed in the following years. Nestling Growth and Survival From 14 days after the first egg of the first clutch was laid (i.e., minimum incubation period), we checked daily for newly hatched chicks. When hatched, nestlings were marked with a non-toxic coloured marker for individual identification. From then on, the parents were given daily egg food, which was regularly supplemented with freshly germinated seeds. Nestlings were weighed every other day until fledging (± 25 days old) with an electronic balance, and survival was checked daily until birds were 100 days old. When the nestling mass reached 7 g, they received a numbered plastic ring for further individual identification. For all nests, we noted the hatching order as “A” for the first hatched chick, “B” for the intermediate chicks and “C” for the last hatched chick. All nestlings were molecularly sexed at the end of the breeding season using blood samples (50 µl of blood collected from the alar wing vein), which were collected at fledging (± 25 days old). Chicks that did not survive to blood sampling were excluded from the final growth analyses as their sex was not known (n = 54 nestlings). At fledgling, birds were separated from their parents and subsequently kept in mixed-sex aviaries (2 x 2 x 2 m 3 ). Temperature conditions were kept constant (22–24°C), and light was stepwise adjusted in function of the natural light-dark cycle (in one hour steps starting from a 15h light-9 h dark cycle and ending with a 10 h light: 14 h dark cycle). Aviaries were equipped with wooden perches, shell sand, and ad libitum access to canary seed mixture (Van Camp, Belgium) and water. Egg food (Van Camp, Belgium) was provided twice a week. Identification and Categorization of Exposure to Maternal Aggression The criterion for early-life MA was the evidence of feather plucking in the nestlings. To confirm that nestling feather plucking was an indicator of MA, we conducted direct behavioural observations on a subset of 20 nests. The nests were monitored by video for a total of 19 hours and 40 minutes during the period when feather plucking is usually observed (between days 14 and 19 after hatching). Our observations confirmed that in 17 out of 20 nests, feather plucking was exclusively performed by the female. In two nests both parents engaged in pecking although the female remained the predominant aggressor (ratios of 19:5 and 4:1 female-to-male pecks), in both cases, the female initiated the aggression. Only in one nest did the male appear to be the sole aggressor. Based on these observations, we will keep the label MA. Overall, feather plucking (i.e., MA) occurred in 40% of the nests. Importantly, not all nestlings from a nest with a female that exerted MA were necessarily affected. For the statistical analyses that involve individual data of nestlings, we categorized MA at the individual level, marking each chick as having experienced MA “yes” or “no” based on whether feather plucking occurred. Long-lasting Associations between Maternal Aggression and Offspring Behavioural Phenotype (Social Defeat Test) In 2022, a behavioural test, i.e., a social defeat test, was conducted on a subset of randomly selected individuals when they were 100 days old. Two weeks prior to this test, focal individuals were transferred to breeding cages, where they were kept individually. The focal birds were then subjected to a “social defeat” by being confronted with heavier, territorial adult individuals in the opponents’ cage (hereafter, resident birds). The resident birds were kept under the same housing conditions as the focal birds for two weeks to become territorial. The resident birds were adult females and males of unknown genetic background. The focal birds were at least 2g lighter than the selected resident bird, and opponents were always same-sex individuals. The resident birds were used in a maximum of two social interactions, once with an opponent who experienced MA and once with an opponent who did not experience MA. These individuals were selected at random. On the day of testing, the focal individual was placed in a cage that was adjacent to the resident cage, separated only by an opaque removable partition. Both resident and focal birds were food-deprived for 90 minutes. A dish with palatable food was then placed in the resident’s cage and the opaque partition was removed allowing the focal individual to enter the resident’s cage (this always occurred within 10–30 s). The partition was placed back when the focal bird entered the resident’s cage. The test was stopped after 15 minutes or when one of the two individuals was clearly defeated, i.e., when one of the following criteria, established in [ 32 ], was met: one of the birds sits immobile for 5 min; is chased for 3 minutes; or is physically attacked > 10 times. However, none of these thresholds was reached. At the end of the test, the opaque partition was again removed allowing the focal individual to go back to the empty half of the cage, which it usually entered on its own (otherwise, it was gently pushed to do so). The test was video-recorded, and aggressive behaviour was annotated by scoring two types of responses from the videos: the duration of threat postures (referred to as “time threatening”, including chest-raising, beak-opening with or without vocalizations, and wing-flapping) and the frequency of direct aggression (pecks and displacements). However, due to the low frequency of direct aggression, we used its presence or absence for statistical analyses instead. Parental Investment: Current versus Future Reproduction Female canaries typically lay a second clutch at about the time that the nestlings of their first brood reach independence [ 33 ]. Eggs from the second clutch were weighed on the day they were laid. In both study years, we measured clutch size (i.e., the number of eggs) for both the first and the second clutch, and the interval between clutches. These parameters of the second clutch were taken as measures of investment in future reproduction. Statistical Analysis All statistical analyses were performed in R 4.1.3 [ 34 ]. Backward elimination for non-significant interactions (α = .05) based on the Akaike Information Criterium (AIC) value was used to build the minimal models; we used lmerTest (v3.1.3, [ 35 ]) to calculate p-values (by default this package provides a type-III Anova). Depending on the dependent variable, we ran a linear model (LMs; for time interval between clutches), a linear mixed effects model (LMMs; lme4 package, v1.1.33, [ 36 ]; for clutch size, nestling weight, and time spent threatening another individual during the social defeat test), a Cox proportional hazards model ( survival package; v3.5.5, [ 37 ], for survival), and a Fisher exact test for direct aggression during the social defeat test. For all statistical tests, the significance level was set at α = .05. To inspect violations of the model assumptions, lmm plots were generated using the packages performance (v0.10.3, [ 38 ]) and DHARMa (v0.4.6, [ 39 ]). A. Nestling Growth and Survival To test prediction 1 (i.e., the association between MA and growth; N = 212 individuals) we fitted a linear mixed effects model. We included the time period between day 0 and 25 to identify whether potential differences on growth trajectories were evident from the time of MA onset or whether they were already present from hatching. We used body mass as the response variable and age (included as a fourth order polynomial) as the explanatory variable as this was the simplest model (i.e., minimum degree needed) to explain the relationship between weight and age . Higher-order polynomials are a useful approach to model growth trajectories as they can improve the fit of the model to the data by accounting for nonlinear patterns and potential variability in growth rates or inflection points over time [ 40 , 41 ]. Additional fixed effects were maternal aggression (i.e., experienced early-life MA, yes or no), hatching order (hatching position A, B, or C), year (2022 or 2023), and nestling sex (female or male), as well as the interaction between maternal aggression and age , and the interaction between nestling sex and age . To account for dependency between observations, random intercepts were added for nest identity and chick identity nested within nest identity [ 42 ], as well as a random slope term for age (again, included as a fourth order polynomial). Here we report results of the model excluding the individuals that were not sexed, but a model including all individuals measured can be found in the Supplementary information (Supplementary Table S1 ). To test prediction 2 (i.e., the relationship between MA and offspring survival; N = 309 individuals) we fitted a Cox proportional hazards model. The model included maternal aggression (yes or no), hatching order (hatching position A, B, or C), body mass at hatch , and year (2022 or 2023) as predictor variables. Z-values were used to establish the significance of the fixed effects. We created Kaplan-Meyer plots to visualize the survival curves using the survminer package (v0.4.9, [ 43 ]). B. Long-lasting Associations between MA and Offspring Behavioural Phenotype (Social Defeat Test) To test prediction 3 (i.e., the relationship between MA and the offspring behavioural phenotype; N = 23), we fitted a linear mixed effects model for the time spent threatening. The model included maternal aggression (yes or no), nestling sex (female or male), and the interaction between both terms as predictor variables. The response variable, time threatening , was square-root transformed to meet the model assumptions. Nest identity was added as a random-effect term to account for dependency between observations of nestlings originating from the same nest. Direct aggression was analysed with a Fisher exact test given that the sample sizes were small (i.e., of the 23 individuals tested, only 12 individuals showed any type of direct aggression) and that the observations were independent. We used two categorical variables each one with two levels: maternal aggression (yes or no), and direct aggression (i.e., presence or absence). C. Parental Investment: Current versus Future Reproduction To test prediction 4 (i.e., the relationship between exerting MA and reproductive investment; N = 91) clutch size was analysed in a linear mixed effects model as the count data followed a normal distribution. We included as fixed effects maternal aggression (yes or no, based on the first clutch data), year (2022 or 2023), clutch (first or second), and the interactions between maternal aggression and year and maternal aggression and clutch . Nest identity was included as a random effect as there were two observations per individual (i.e., one for the first clutch and one for the second clutch). A Levene’s test ( car package, v3.1.2, [ 44 ]) was used to compare the overall variation in clutch size between females that exerted MA and those who didn’t. The time interval between laying the first and the second clutches was analysed using a linear model with maternal aggression (yes or no), brood size of the first brood at day 10, year (2022 or 2023) and the pairwise interactions as fixed effects on the full model. Here we report results of the reduced models, but full models can be found in the Supplementary Information (Supplementary Table S2). RESULTS Nestling Growth and Survival There was no main effect of MA on nestling growth (Table 1), but we did find a significant two-way interaction between age and maternal aggression ( maternal aggression x age , p=0.001), suggesting that the association between MA and slower growth became stronger as nestlings aged (Figure 1). As expected, there was also a significant interaction effect between age and nestling sex ( nestling sex x age , p<0.001), with male nestlings becoming heavier than female nestlings with increasing age. Overall, body mass increased with age in a non-linear way reflecting growth ( polynomials of age (k=4), p<0.001; Table 1). Table 1. Effects of MA on nestling growth. Outcome of a linear mixed effects model (polynomial, 4 th order). Bold values indicate p <0.05. Growth trajectory Estimate (SE) F df p Intercept 331.38 (192.92) - - - poly (Age , k=4) - 538 (4, 159.56) <0.001 Maternal aggression -0.02 (0.09) 0.08 (1, 217.19) 0.76 Nestling sex 0.002 (0.08) 0.01 (1, 440.92) 0.97 Year -0.15 (0.09) 2.78 (1, 87.2) 0.09 Hatching order -0.06 (0.04) 1.90 (1, 260.87) 0.16 Maternal aggression: Age 0.03 (0.01) 11.13 (1, 158.91) 0.001 Nestling sex: Age 0.04 (0.01) 19.53 (1, 215.74) <0.001 There was no significant effect of MA on the survival probability of the nestlings ( maternal aggression , Z=0.94, p=0.34, Figure 2; Table 2). However, there was a significant negative effect of weight at hatch , with lighter hatchlings having lower survival probabilities (Z=-2.12, p=0.03; Table 2) and a negative effect of hatching order , with later hatched nestlings having lower survival probabilities (Z=5.43, p<0.001; Table 2). Table 2. Full model of the effects of MA on nestling survival. Outcome of a Cox proportional hazards model. Bold values indicate p< 0.05. Survival coef exp(coef) se(coef) Z p Maternal aggression 0.18 1.20 0.19 0.94 0.34 Weight at hatch -0.74 0.47 0.35 -2.12 0.03 Hatching order 0.68 1.98 0.12 5.43 <0.001 Year 0.11 1.12 0.19 0.59 0.55 Long-lasting Associations between MA and Offspring Behavioural Phenotype (Social Defeat Test) Table 3 Effects of MA on time spent threatening during the social defeat test. Model outcomes of a linear mixed effects models. Bold values indicate p<0.05. Time in Threat Position Estimate (SE) F df p Intercept 0.92 (0.48) - - Maternal aggression -0.19 (0.66) 3.34 (1, 19) 0.08 Sex Maternal aggression: Sex -0.55 (0.72) 2.23 (1.00) 1.24 4.93 (1, 19) (1, 19) 0.27 0.03 There was no significant association between MA and time threatening ( maternal aggression, p=0.08). However, there was a significant interaction effect between MA and sex on the time threatening ( maternal aggression x sex, p= 0.03; Table 3; Figure 3), with males that experienced MA spending more time threatening the resident male, whereas this pattern was not found for females. There was no significant association between experiencing MA and the number of direct aggressions exerted during the test (Fisher exact test, p=0.31, Figure 3). Parental Investment: Current versus Future Reproduction There was a significant positive association between MA and clutch size ( maternal aggression , p=0.03, Table 4). We also found a negative effect of clutch order ( clutch ID , p=0.02, Table 4; Figure 4) with second clutches having fewer eggs than first clutches. Variation in clutch size was significantly lower in clutches laid by females that exerted MA compared to clutches laid by females that did not exert MA (females that exerted MA: SD=0.88; females that did not exert MA: SD=1.27; Levene’s test: F 1-180 =7.35, p=0.007). There was no main effect of MA on the time interval between laying the first and the second clutch ( maternal aggression , p=0.51, Table 4; Figure 4). Table 4 The relationship between MA and parental investment (i.e., clutch size of both the first and the second clutch), second clutch mass and time interval between the first and the second clutch. Outcome of a linear mixed model for the clutch size, and outcome of linear models for the total mass of the second clutch (with robust standard errors, RSE) and the interval between clutches. Bold values indicate p<0.05. Clutch size Estimate (SE) df t p Intercept 4.59 (0.15) 130.97 28.92 <0.001 Maternal aggression -0.37 (0.17) 88 -2.19 0.03 Clutch ID -0.32 (0.14) 90 -2.28 0.02 Year -0.15 (0.16) 88 -0.93 0.35 Time interval between clutches Estimate (SE) t p Intercept 36.01 (1.19) 30.01 <0.001 Maternal aggression -0.47 (0.72) -0.65 0.51 Brood size day 10 0.64 (0.34) 1.89 0.06 Year 0.17 (0.72) 0.24 0.80 DISCUSSION This study investigated the association between maternal aggression (MA), a seemingly maladaptive parental behaviour, and variation in offspring development and behavioural phenotype. In addition, we explored whether MA is related to a trade-off between current and future reproduction. We found that offspring exposed to MA showed reduced growth, while no significant differences in survival were observed. Intriguingly, we observed a sex-specific effect on aggression, with male offspring exposed to MA early in life displaying more threatening behaviours towards other males, which may be beneficial in certain contexts. Finally, we found no evidence that MA was associated with a trade-off between current and future reproduction. However, females expressing MA laid, on average, larger and less variable clutches. These results are discussed in detail below. We hypothesized that MA would be associated with retarded growth (prediction 1), which should be evident from the time of MA onset and potentially reduce offspring survival probabilities (prediction 2). Consistent with the first part of the hypothesis, nestlings exposed to MA during early life showed a lower increase in body mass, with differences becoming evident from the onset of MA (around day 14 after hatching). There are two non-mutually exclusive explanations for this result. First, MA could be associated with neglect or a reduced responsiveness to needs of the offspring, both of which could result in poorer food provisioning and a consequent reduction in body mass gain [45]. Second, early-life exposure to MA could increase stress in nestlings, leading to elevated glucocorticoid levels [46], which can constrain energy allocation to growth [47,48]. Elevated corticosterone may also explain why exposure to stressors early in life can lead to an altered energy metabolism, as has been shown in humans and other animals, [49,50], as well as to impaired brain development [51], and predisposition to disease [52]. Thus, MA is associated with negative effects early in life, such as reduced body mass gain in the offspring, which could potentially have long-lasting consequences for the offspring’s adult phenotype and fitness (see below). Contrary to our expectations, there was no significant association between MA and the offspring survival (prediction 2), at least up to 100 days of age. This contrasts with findings in other bird species, such as American coots [7,53] and moorhens [8], where similar forms of aggression (e.g., tousling) have been linked to increased mortality. In those species, MA has been hypothesized to promote independent feeding in nestlings. However, in our study, canary nestlings were still unable to feed independently at the age at which MA started (canaries reach independence at 25-30 days of age; [33]), making this explanation unlikely. An alternative explanation is that MA may be a response to specific nestling behaviours potentially functioning as a form of punishment or social regulation [4,54-55]. For example, in macaques, MA has been observed in response to pestering behaviour by the young [54], which is generally thought to increase maternal irritability [55]. Further research is needed to determine whether certain offspring behaviours trigger MA in this species, and whether such aggression is effective in reducing those behaviours or in increasing maternal control. Regarding the lasting effects on the offspring behavioural phenotype (prediction 3), we predicted that exposure to early-life MA would be associated with altered behavioural phenotypes, specifically in the form of heightened aggressive behaviour in response to stressful conflicts. Consistent with this hypothesis, juvenile males reared under MA displayed significantly more threatening behaviour than juveniles reared by non-aggressive mothers. This pattern was not found in females. However, contrary to our expectations and to previous studies (in birds: [56]; in mammals: [57]), MA-exposed juveniles did not engage in more direct aggressions. This is also inconsistent with the “cycle of violence” hypothesis described in humans, where early exposure to aggression is linked to increased violent behaviour in adulthood [58]. One possible explanation for the lack of association between MA and the frequency of direct aggression, as well as with the absence of effects in female offspring, is the limited sample size in our test, which may have hindered the detection of subtle or sex-specific effects. Males may be more prone to engage in intra-sexual aggression and dominance displays throughout the year [59,60], whereas in canaries, females typically display higher levels of aggression closed to the laying period [61]. Additionally, previous work suggests that early-life experiences may have sex-specific impacts on the behavioural phenotype of the offspring [62]. This way, while the mechanisms underlying these patterns remain speculative, previous studies in rodents and humans suggest that early-life adversity, such as MA, can alter stress reactivity and increase impulsive, reactive aggression in response to ‘provocative’ stimuli or social challenges [16, 63-65]. Moreover, MA-exposed individuals may be more likely to perceive social stimuli as threatening [66,67], or to adopt aggressive behaviour to achieve desirable outcomes in social interactions [68]. Future studies should investigate whether enhanced threat displays contribute to increased competitiveness or fitness in challenging environments [24,69], so that the effects of MA on the offspring’s phenotype could be considered adaptive at least in certain contexts. However, demonstrating adaptive plasticity requires direct measures of fitness outcomes in the offspring [18]. It is important to note that the observed patterns could be influenced by genetic inheritance. Offspring from aggressive mothers may have inherited genes that predispose them to have lower quality, reduced growth, or higher aggression levels [70,71]. Future studies are needed to disentangle potential genetic contributions from developmental effects by incorporating, for instance, cross-fostering designs. Regarding our results on the parental investment in the current and future reproduction, we predicted that females showing aggression towards their first clutch offspring, would intend to reduce or terminate their investment in current offspring in order to save resources that can be invested in future reproduction, that is, in their second clutch (prediction 4). However, instead of a trade-off between current and future reproduction, we found a statistically significant main difference in clutch size of both the first and second clutch between females that exhibited MA compared to those that did not, while the variance in clutch size was lower among MA females. This result suggests phenotypic differences or different reproductive strategies associated with MA. It could be speculated that MA females overestimated their rearing capacity when laying larger “optimistic” clutches, and subsequently adjusted their investment in the current brood afterwards to match their own and their partner’s rearing capacity [72]. However, if this was the case, it did not result in fewer offspring in our study case, but possibly lower quality offspring. The increased variance in clutch size in females that did not exert MA towards the offspring could reflect greater plasticity in their parental investment and possibly reproductive strategies, by adapting clutch size to their current condition, their partner’s willingness and capacity to contribute to parental care and/or the resources available [29]. The latter, however, is unlikely in our study given the ad libitum access to food resources in captivity. However, we acknowledge that, although the differences in clutch size are statistically significant, these results should be interpreted with caution due to the small effect size, leaving some uncertainty about the biological significance and its potential impact on reproductive success. Declarations Author Contribution C.G. and W.M. were in charge of the conceptualization. C.G developed the methodology and investigation with the support of W.M. C.G was in charge of data curation, the formal analysis and visualization and the writing (original draft and editing). W.M provided the resources. J.M, F.V and W.M were in charge of the funding acquisition, supervision, validation and reviewing and editing the writing. Acknowledgement We thank Peter Scheys for his assistance in taking care of the birds. This work was supported by the FWO Flanders (CG: 1173825N). FV was supported by an ERC Consolidator grant (European Union’s Horizon 2020 research and innovation programme, grant agreement No 769595) and Methusalem Project 01M00221 (Ghent University). A research stay waspossible thanks to project PID2022-139166NB-I00 (to JM) funded by MCIN/AEI/https://doi.org/10.13039/501100011033 and “ERDF A way of making Europe”. Data Availability The data used in this study is archived in Mendeley Data in the following link: https://data.mendeley.com/datasets/svbwhszz3r/1. The videos analysed are available from the corresponding author (Clara Garcia-Co) upon request. The authors declare no competing or financial interests. References Clutton-Brock, T. H. The evolution of parental care (Vol. 10). (Princeton University Press, 1991). Kokko, H., & Jennions, M. D. Parental investment, sexual selection, and sex ratios. J. Evol. Biol. 21 (4), 919–948 (2008). Royle, N. J., Smiseth, P. T., & Kölliker, M. (Eds.). The evolution of parental care . 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Song as an honest signal of developmental stress in the zebra finch ( Taeniopygia guttata ). Hormones and Behavior, 44 (2), 132–139 (2003). Sadoul, B., & Vijayan, M. M. Stress and growth in Fish physiology (ed. Schreck, C., B., Tort, L., Farrell, A. P. & Brauner, C. J.), 35 , 167–205 (Academic Press, 2016). O'Regan, D., Kenyon, C. J., Seckl, J. R., & Holmes, M. C. Glucocorticoid exposure in late gestation in the rat permanently programs gender-specific differences in adult cardiovascular and metabolic physiology. Am. J. Physiol. Endocrinol. Metab . 287 (5), E863-E870 (2004). Harris, A., & Seckl, J. Glucocorticoids, prenatal stress, and the programming of disease. Hormones and Behavior 59 , 279–289 (2011). Buchanan, K. L., Leitner, S., Spencer, K. A., Goldsmith, A. R., & Catchpole, C. K. Developmental stress selectively affects the song control nucleus HVC in the zebra finch. Proc. R. Soc. B. 271 , 2381–2386 (2004). Shanks, N. Early life environment: Does it have implications for predisposition to disease? Acta Neuropsychiatr. 14 (6), 292–302 (2002). Horsfall, J. A. Brood reduction and brood division in coots. Anim. Behav. 32 (1), 216–225 (1984). Jensen, G. D., Bobbitt, R. A., & Gordon, B. N. Patterns and sequences of hitting behavior in mother and infant monkeys ( Macaca nemestrina ). J. Psychiatr. Res. (1969). Negayama, K. Maternal aggression to its offspring in Japanese monkeys. JHE 10.7, 523-527 (1981). Müller, M. S. et al. Maltreated nestlings exhibit correlated maltreatment as adults: Evidence of a “cycle of violence” in Nazca boobies ( Sula granti ). The Auk, 128 (4), 615–619 (2011). Maestripieri, D. Early experience affects the intergenerational transmission of infant abuse in rhesus monkeys. PNAS, 102 (27), 9726–9729 (2005). Heyman, R. E., & Slep, A. M. S. Do child abuse and interparental violence lead to adulthood family violence? JMF, 64 (4), 864–870 (2002). Dingemanse, N. J., & de Goede, P. The relation between dominance and exploratory behavior is context-dependent in wild great tits. Behav. Ecol. 15 (6), 1023-1030 (2004). O’Shea, W., Serrano-Davies, E., & Quinn, J. L. Do personality and innovativeness influence competitive ability? An experimental test in the great tit. Behav. Ecol. 28 (6), 1435-1444 (2017). Shoemaker, H. H. Social hierarchy in flocks of the canary. The Auk , 381-406 (1939). Kundakovic, M. et al. Sex-specific epigenetic disruption and behavioral changes following low-dose in utero bisphenol A exposure. PNAS , 110 (24), 9956-9961 (2013). Lovallo, W. R. Early life adversity reduces stress reactivity and enhances impulsive behavior: Implications for health behaviors. Int. J. Psychophysiol. 90 (1), 8–16 (2013). Gagnon, J. et al. An ERP study on hostile attribution bias in aggressive and nonaggressive individuals. Aggress. Behav. 43 , 217–229 (2016). Zhu, W., Chen, Y., & Xia, L. X. Childhood maltreatment and aggression: The mediating roles of hostile attribution bias and anger rumination. Pers. Individ. Differ. 162 , 110007 (2020). Price, J. M., & Glad, K. Hostile attributional tendencies in maltreated children. J. Abnorm. Child Psychol. 31 , 329–343 (2003). Card, N. A., & Little, T. D. Proactive and reactive aggression in childhood and adolescence: A meta-analysis of differential relations with psychosocial adjustment. Int. J. Behav. Dev. 30 (5), 466–480 (2006). Li, X., Wang, Y., Li, J., Tang, J., Zhang, J., Wang, M., & Jiang, S. Violence exposure across multiple contexts as predictors of reactive and proactive aggression in Chinese preadolescents. Aggress. Behav. 48 (3), 319–330 (2022). Ellis, B. J., & Boyce, W. T. Biological sensitivity to context. Curr. Dir. Psychol. Sci. 17 (3), 183–187 (2008). Veroude, K. et al. Genetics of aggressive behavior: an overview. AJMGB, 171 (1), 3-43 (2016). Dragovich, A. Y., & Borinskaya, S. A. Genetic and genomic basis of aggressive behavior. Russ. J. Genet. 55, 1445-1459 (2019). Forbes, L. S., & Mock, D. W. Food, information and avian bread reduction. Ecoscience , 3 (1), 45-53 (1996). Additional Declarations No competing interests reported. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-6479850","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":456467483,"identity":"4b9c85df-ad32-40c2-8e3f-46b1efd90bf6","order_by":0,"name":"Clara Garcia-Co","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAABPUlEQVRIie2RPWrDMBSAZQz2IupVJaW5goWHdkjToReRMCRL4sVLBg9qDPaSAzg0hzD0ADUI7MUlq7uUlEAnDy5dPJhSxekQqOjPVoo/EE9/H+89CYCOjj+IEbZhKIbCAPHA6eGpJlMQb4P9oaTA2s++UMyrNiT7lTIHlH2rqOpzVYOkfxby6+qJDcZxdp9t4ezRMXpM21QyRbOWC5DgVU59RNloGucO9WHuuserRMeRTBHVQ5ApEVKYUPg0TibYnwaExgXRelCm6K9KA/ilUPxaKGNzXQrljdC7ndLI2oeWCgGnQgkQnXNiFrssTGRBQpG0jzh01RPTtiNIg3OSjvCyKPFNkxIXFTTEi8+KEYa3SjkbXkR6tn2ovUH/aD0xXyKPOEZkp5ta9sxteZI9cvA/P4T87npHR0fHP+YdTNVsZV0LrFkAAAAASUVORK5CYII=","orcid":"","institution":"University of Antwerp","correspondingAuthor":true,"prefix":"","firstName":"Clara","middleName":"","lastName":"Garcia-Co","suffix":""},{"id":456467484,"identity":"022180ee-c01b-402b-b422-ac1550957ef5","order_by":1,"name":"Frederick Verbruggen","email":"","orcid":"","institution":"Ghent University","correspondingAuthor":false,"prefix":"","firstName":"Frederick","middleName":"","lastName":"Verbruggen","suffix":""},{"id":456467485,"identity":"536b3bcd-6440-453c-9f54-525fce643a26","order_by":2,"name":"Judith Morales","email":"","orcid":"","institution":"National Museum of Natural Sciences, Spanish National Research Council (MNCN-CSIC","correspondingAuthor":false,"prefix":"","firstName":"Judith","middleName":"","lastName":"Morales","suffix":""},{"id":456467486,"identity":"47114861-2110-4f75-a5e8-43e0a84c785b","order_by":3,"name":"Wendt Müller","email":"","orcid":"","institution":"University of Antwerp","correspondingAuthor":false,"prefix":"","firstName":"Wendt","middleName":"","lastName":"Müller","suffix":""}],"badges":[],"createdAt":"2025-04-18 14:53:04","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-6479850/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-6479850/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1038/s41598-025-10698-4","type":"published","date":"2025-07-19T16:05:39+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":82792523,"identity":"2565a41f-3149-4c14-bbc5-a3ae168287c1","added_by":"auto","created_at":"2025-05-15 10:23:14","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":146026,"visible":true,"origin":"","legend":"\u003cp\u003eEffect of MA (yes in red; no in grey) on nestling growth. Plotted lines are regression lines of the two-way interaction term “maternal aggression x age”. Shaded regions represent the model prediction (±95% confidence interval).\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-6479850/v1/20f5da77d1adfcaa7a395dd5.png"},{"id":82792015,"identity":"36e26c13-2aee-4304-9b4e-431bd6f842e3","added_by":"auto","created_at":"2025-05-15 10:15:14","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":38324,"visible":true,"origin":"","legend":"\u003cp\u003eEffect of MA (experienced MA in red, or did not experience it, in grey) on the survivorship curve with 95% confidence intervals.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-6479850/v1/363ddfda5fe8acd307ce4731.png"},{"id":82792531,"identity":"4381bf10-4cc9-4840-94d9-ce1318005ebb","added_by":"auto","created_at":"2025-05-15 10:23:14","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":68740,"visible":true,"origin":"","legend":"\u003cp\u003eEffects of MA on aggressiveness of the offspring during a social defeat test. A: Time threatening (in seconds) separated by sex (F=Female; M=Male) and MA (yes or no) and \u003cstrong\u003eB\u003c/strong\u003e the frequency of pecks and displacements, i.e., directed aggressions.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-6479850/v1/5eae5061bb067ebee4a72065.png"},{"id":82792023,"identity":"044adb52-6088-43dd-bea5-fc4bc93cfe3d","added_by":"auto","created_at":"2025-05-15 10:15:14","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":84961,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eEffect of MA on parental investment\u003c/strong\u003e.\u003cstrong\u003e \u003c/strong\u003eViolin plots of (A) clutch size of the first and second clutch; and (B) the time interval (days) between laying the first clutch and initiating the second clutch. In both graphs, the data of the two consecutive breeding seasons (2022 and 2023) are combined. The black diamonds show the mean, grey dots are the individual data points.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-6479850/v1/5c56a7dc68158b0e90b72a6f.png"},{"id":88506119,"identity":"abedafdd-9527-490e-8f12-b489c86145d0","added_by":"auto","created_at":"2025-08-07 07:31:20","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1255404,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-6479850/v1/1619b7ab-4f0b-4b75-b72e-469d774d9a9f.pdf"},{"id":82792019,"identity":"02ac94c0-4507-4b9d-85b0-91d9f77e5bb3","added_by":"auto","created_at":"2025-05-15 10:15:14","extension":"docx","order_by":0,"title":"","display":"","copyAsset":false,"role":"supplement","size":17348,"visible":true,"origin":"","legend":"","description":"","filename":"supplementaryinformation.docx","url":"https://assets-eu.researchsquare.com/files/rs-6479850/v1/656bc4cbf31fe4dbf8139db5.docx"}],"financialInterests":"No competing interests reported.","formattedTitle":"Balancing care and conflict: towards a better understanding of maternal aggression in canaries","fulltext":[{"header":"INTRODUCTION","content":"\u003cp\u003eParental care is a widespread phenomenon in the animal kingdom and is defined as any parental behaviour likely to enhance offspring quality and survival [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]. The extent and nature of parental care varies considerably between and within species. It can range from nest site selection and egg guarding to more complex forms such as incubation, brooding and provisioning of dependent offspring [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e, \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]. Once offspring are born, the level of parental care is expected to be aligned with the needs of the offspring, which are signalled by a variety of traits. However, instead of responding to these signals of need, parents sometimes show neglect and aggression towards their offspring (e.g. [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e] in rhesus macaques; [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e] in humans; [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e] in seals; [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e] in American coots; [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e] in moorhens). Previous studies proposed that such aggression may arise as a side-effect of other parental traits [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e], be linked to genetic abnormalities [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e] or be due to environmental stressors [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]. Intriguingly, however, parenting styles, including aggressive and \u0026ldquo;less caring\u0026rdquo; ones, are often transmitted across generations [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e, \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e], raising the question of whether they represent an adaptive parental strategy in specific contexts, rather than a seemingly maladaptive behaviour that reduces reproductive success and population viability [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e] [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eWhen the expected fitness gain from the investment in the current offspring is low, maternal aggression towards the offspring could represent an adaptive parental strategy to reduce or terminate the investment in the current reproductive event [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]. Indeed, maternal aggression (i.e., aggressive behaviour of the mothers towards their offspring) and, in extreme cases, infanticide and cannibalism has been found to facilitate the selective survival of other offspring, while aspects, such as neglect or abandonment are thought to promote offspring independence (i.e., in the context of so-called weaning age conflicts; [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e, \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]. Such weaning age conflicts are rooted in evolutionary conflicts of interest, as the offspring are selected to demand more immediate investment for their own growth and survival than the parents are willing to provide, while the parents are often inclined to preserve resources for other current or future offspring [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e, \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]. The resulting parent-offspring conflicts may lead to a tug-of-war over resource allocation and caregiving, often involving parental aggression and, in particular, neglect, especially when the costs of caregiving outweigh the benefits [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]. Traditionally, though, parental aggression has mainly been interpreted in terms of short-term cost-benefit balance for the parents. However, it is known that parental behaviour can have long-lasting consequences on offspring phenotype [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]. For example, juvenile rats deprived of maternal care show inappropriate social play behaviour [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e], and adult male rats exposed to maternal separation show higher levels of aggression [\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eThus, these early environmental and parental effects can modify the developmental trajectory of the offspring, allowing for phenotypic adjustments i.e. developmental plasticity [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]. An adaptation to current conditions may come at a cost in the future, if developmental plasticity leads to negative lasting changes in their phenotype [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e]. For example, adverse early-life environments have been associated with accelerated cellular ageing and increased inflammation in adulthood e.g., [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]. However, such adjustments may facilitate survival under potentially suboptimal conditions. For example, an altered phenotype as a consequence of early-life social adversity or maternal aggression (see e.g., [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e, \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e] could eventually allow the offspring to benefit from higher levels of aggression in harsh environments if this allows them, for example, to access higher dominance ranks [\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e, \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]. Early environmental and parental effects are therefore also interpreted as a form of adaptive developmental plasticity that is beneficial in some specific contexts [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e, \u003cspan additionalcitationids=\"CR23 CR24 CR25\" citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e]. Specifically, he match/mismatch hypothesis suggests that individuals are developmentally programmed to cope with the environment encountered during sensitive developmental periods [\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e, \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e]. However, experimental evidence on this topic is still scarce.\u003c/p\u003e \u003cp\u003eHere, we investigated the functional consequences of maternal aggression (hereafter MA) using the domestic canary (\u003cem\u003eSerinus canaria\u003c/em\u003e) as a model system, to test whether MA could be part of a context-dependent adaptive parental strategy or a maladaptive behaviour. In this species, mothers often behave aggressively towards their offspring (i.e., by pecking and plucking their feathers before fledging) from around day 14 post-hatching onwards, and eventually throw them out of the nest (unpublished observations from our lab). First, we expected MA to be associated with reduced offspring growth (prediction 1) and ultimately higher early-life mortality (prediction 2), reflecting short-term effects of MA. Then, we investigated potential lasting consequences on the behavioural phenotype. As MA has been linked to higher levels of aggression in mammals e.g., [\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e], we expected that experiencing MA during early life would be associated with a higher tendency to use direct aggression to resolve stressful conflicts (prediction 3), here tested in a social defeat test (i.e., long-term associations). Finally, if MA is driven by parental preferences for future over current offspring as a form of reducing or terminating current investment, it should be associated with a greater investment in future reproduction (prediction 4). Thus, we expect a shorter time interval between the first and the second clutch, as well as larger second clutches. On the other hand, if MA is a sign of conflict, it could lead to more variability in parental investment, while non-aggressive mothers might show more consistent clutch sizes [\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e]. Otherwise, if MA is maladaptive it should always deteriorate offspring development and reduce the overall reproductive success of parents.\u003c/p\u003e"},{"header":"MATERIALS AND METHODS","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\n \u003ch2\u003eStudy Species and Breeding\u003c/h2\u003e\n \u003cp\u003eIn this study, we bred 197 first-year canaries in two different years. Birds originated from a lab-bred population (LA-number: LA 1100 161) of captive Fife fancy canaries kept at the University of Antwerp [\u003cspan class=\"CitationRef\"\u003e33\u003c/span\u003e], supplemented each year in late autumn with new birds from local breeders to ensure genetic diversity. In both years, experiments were conducted under the same conditions and following the same procedures. In February, all individuals that were selected for breeding (approx. 50% local: 50% new birds) were kept five weeks prior to pair formation in single-sex aviaries at a room temperature of 20\u0026ndash;24\u0026deg;C with long artificial daylight (14h light: 10h dark) [\u003cspan class=\"CitationRef\"\u003e30\u003c/span\u003e]. We then formed 80 couples in 2022 and 117 in 2023, by placing first the male and then a female into a breeding cage (50 x 64 x 40 cm\u003csup\u003e3\u003c/sup\u003e, GEHU cages, The Netherlands). Mating pairs were created randomly, but pair composition was subsequently checked to make sure that the partners were unrelated. Individuals were replaced by new males or females if one of the pair showed signs of sickness or died. The breeding cages were equipped with two perches, a nest cup, nesting material, cuttlefish bones, shell sand, canary seed mixture (Van Camp, Belgium), egg food (provided twice a week; Van Camp, Belgium) and water \u003cem\u003ead libitum\u003c/em\u003e. After the breeding pairs completed nest-building, we monitored egg laying each morning and weighed the freshly laid eggs.\u003c/p\u003e\n\u003c/div\u003e\n\u003ch3\u003eEthical Note\u003c/h3\u003e\n\u003cp\u003eThe experiments described in this study were carried with the approval of the Ethical Committee for Animal Experimentation (ECD) at the University of Antwerp under license ECD-2022-87 and were performed in accordance with Belgian and Flemish legislation regarding animal welfare, adhered to the ASAB/ABS guidelines for the use of animals in behavioural research and teaching, and complied with the ARRIVE guidelines. During the experiments, the birds were kept in standard breeding cages for canaries, otherwise, they were kept in mixed-sex aviaries (2 x 2 x 2 m\u003csup\u003e3\u003c/sup\u003e). A constant effort was made to keep the birds in semi-natural conditions and in good health. To this end, we frequently sought advice from breeders and veterinarians to further optimize the housing conditions. The invasiveness of all observational procedures described below and the handling time were minimized. For a more detailed description on the measures taken during the social defeat test to minimise stress, please see below the corresponding methods section. Body condition and behaviour were monitored throughout the whole experiment as it reflects general well-being based on pre-defined criteria described in [\u003cspan class=\"CitationRef\"\u003e31\u003c/span\u003e]. At the end of the experiment, all the canaries were kept in the lab-bred population at the University of Antwerp as they are part of a transgenerational transmission experiment and will be allowed to breed in the following years.\u003c/p\u003e\n\u003ch3\u003eNestling Growth and Survival\u003c/h3\u003e\n\u003cp\u003eFrom 14 days after the first egg of the first clutch was laid (i.e., minimum incubation period), we checked daily for newly hatched chicks. When hatched, nestlings were marked with a non-toxic coloured marker for individual identification. From then on, the parents were given daily egg food, which was regularly supplemented with freshly germinated seeds. Nestlings were weighed every other day until fledging (\u0026plusmn;\u0026thinsp;25 days old) with an electronic balance, and survival was checked daily until birds were 100 days old. When the nestling mass reached 7 g, they received a numbered plastic ring for further individual identification. For all nests, we noted the hatching order as \u0026ldquo;A\u0026rdquo; for the first hatched chick, \u0026ldquo;B\u0026rdquo; for the intermediate chicks and \u0026ldquo;C\u0026rdquo; for the last hatched chick. All nestlings were molecularly sexed at the end of the breeding season using blood samples (50 \u0026micro;l of blood collected from the alar wing vein), which were collected at fledging (\u0026plusmn;\u0026thinsp;25 days old). Chicks that did not survive to blood sampling were excluded from the final growth analyses as their sex was not known (n\u0026thinsp;=\u0026thinsp;54 nestlings). At fledgling, birds were separated from their parents and subsequently kept in mixed-sex aviaries (2 x 2 x 2 m\u003csup\u003e3\u003c/sup\u003e). Temperature conditions were kept constant (22\u0026ndash;24\u0026deg;C), and light was stepwise adjusted in function of the natural light-dark cycle (in one hour steps starting from a 15h light-9 h dark cycle and ending with a 10 h light: 14 h dark cycle). Aviaries were equipped with wooden perches, shell sand, and \u003cem\u003ead libitum\u003c/em\u003e access to canary seed mixture (Van Camp, Belgium) and water. Egg food (Van Camp, Belgium) was provided twice a week.\u003c/p\u003e\n\u003ch3\u003eIdentification and Categorization of Exposure to Maternal Aggression\u003c/h3\u003e\n\u003cp\u003eThe criterion for early-life MA was the evidence of feather plucking in the nestlings. To confirm that nestling feather plucking was an indicator of MA, we conducted direct behavioural observations on a subset of 20 nests. The nests were monitored by video for a total of 19 hours and 40 minutes during the period when feather plucking is usually observed (between days 14 and 19 after hatching). Our observations confirmed that in 17 out of 20 nests, feather plucking was exclusively performed by the female. In two nests both parents engaged in pecking although the female remained the predominant aggressor (ratios of 19:5 and 4:1 female-to-male pecks), in both cases, the female initiated the aggression. Only in one nest did the male appear to be the sole aggressor. Based on these observations, we will keep the label MA. Overall, feather plucking (i.e., MA) occurred in 40% of the nests. Importantly, not all nestlings from a nest with a female that exerted MA were necessarily affected. For the statistical analyses that involve individual data of nestlings, we categorized MA at the individual level, marking each chick as having experienced MA \u0026ldquo;yes\u0026rdquo; or \u0026ldquo;no\u0026rdquo; based on whether feather plucking occurred.\u003c/p\u003e\n\u003ch3\u003eLong-lasting Associations between Maternal Aggression and Offspring Behavioural Phenotype (Social Defeat Test)\u003c/h3\u003e\n\u003cp\u003eIn 2022, a behavioural test, i.e., a social defeat test, was conducted on a subset of randomly selected individuals when they were 100 days old. Two weeks prior to this test, focal individuals were transferred to breeding cages, where they were kept individually. The focal birds were then subjected to a \u0026ldquo;social defeat\u0026rdquo; by being confronted with heavier, territorial adult individuals in the opponents\u0026rsquo; cage (hereafter, resident birds). The resident birds were kept under the same housing conditions as the focal birds for two weeks to become territorial. The resident birds were adult females and males of unknown genetic background. The focal birds were at least 2g lighter than the selected resident bird, and opponents were always same-sex individuals. The resident birds were used in a maximum of two social interactions, once with an opponent who experienced MA and once with an opponent who did not experience MA. These individuals were selected at random.\u003c/p\u003e\n\u003cp\u003eOn the day of testing, the focal individual was placed in a cage that was adjacent to the resident cage, separated only by an opaque removable partition. Both resident and focal birds were food-deprived for 90 minutes. A dish with palatable food was then placed in the resident\u0026rsquo;s cage and the opaque partition was removed allowing the focal individual to enter the resident\u0026rsquo;s cage (this always occurred within 10\u0026ndash;30 s). The partition was placed back when the focal bird entered the resident\u0026rsquo;s cage. The test was stopped after 15 minutes or when one of the two individuals was clearly defeated, i.e., when one of the following criteria, established in [\u003cspan class=\"CitationRef\"\u003e32\u003c/span\u003e], was met: one of the birds sits immobile for 5 min; is chased for 3 minutes; or is physically attacked\u0026thinsp;\u0026gt;\u0026thinsp;10 times. However, none of these thresholds was reached. At the end of the test, the opaque partition was again removed allowing the focal individual to go back to the empty half of the cage, which it usually entered on its own (otherwise, it was gently pushed to do so).\u003c/p\u003e\n\u003cp\u003eThe test was video-recorded, and aggressive behaviour was annotated by scoring two types of responses from the videos: the duration of threat postures (referred to as \u0026ldquo;time threatening\u0026rdquo;, including chest-raising, beak-opening with or without vocalizations, and wing-flapping) and the frequency of direct aggression (pecks and displacements). However, due to the low frequency of direct aggression, we used its presence or absence for statistical analyses instead.\u003c/p\u003e\n\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e\n \u003ch2\u003eParental Investment: Current versus Future Reproduction\u003c/h2\u003e\n \u003cp\u003eFemale canaries typically lay a second clutch at about the time that the nestlings of their first brood reach independence [\u003cspan class=\"CitationRef\"\u003e33\u003c/span\u003e]. Eggs from the second clutch were weighed on the day they were laid. In both study years, we measured clutch size (i.e., the number of eggs) for both the first and the second clutch, and the interval between clutches. These parameters of the second clutch were taken as measures of investment in future reproduction.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec9\" class=\"Section2\"\u003e\n \u003ch2\u003eStatistical Analysis\u003c/h2\u003e\n \u003cp\u003eAll statistical analyses were performed in R 4.1.3 [\u003cspan class=\"CitationRef\"\u003e34\u003c/span\u003e]. Backward elimination for non-significant interactions (\u0026alpha;\u0026thinsp;=\u0026thinsp;.05) based on the Akaike Information Criterium (AIC) value was used to build the minimal models; we used \u003cem\u003elmerTest\u003c/em\u003e (v3.1.3, [\u003cspan class=\"CitationRef\"\u003e35\u003c/span\u003e]) to calculate p-values (by default this package provides a type-III Anova). Depending on the dependent variable, we ran a linear model (LMs; for time interval between clutches), a linear mixed effects model (LMMs; \u003cem\u003elme4\u003c/em\u003e package, v1.1.33, [\u003cspan class=\"CitationRef\"\u003e36\u003c/span\u003e]; for clutch size, nestling weight, and time spent threatening another individual during the social defeat test), a Cox proportional hazards model (\u003cem\u003esurvival\u003c/em\u003e package; v3.5.5, [\u003cspan class=\"CitationRef\"\u003e37\u003c/span\u003e], for survival), and a Fisher exact test for direct aggression during the social defeat test. For all statistical tests, the significance level was set at \u003cem\u003e\u0026alpha;\u003c/em\u003e\u0026thinsp;=\u0026thinsp;.05. To inspect violations of the model assumptions, lmm plots were generated using the packages \u003cem\u003eperformance\u003c/em\u003e (v0.10.3, [\u003cspan class=\"CitationRef\"\u003e38\u003c/span\u003e]) and \u003cem\u003eDHARMa\u003c/em\u003e (v0.4.6, [\u003cspan class=\"CitationRef\"\u003e39\u003c/span\u003e]).\u003c/p\u003e\u003cspan\u003e\n \u003cp\u003eA. Nestling Growth and Survival\u003c/p\u003e\n \u003c/span\u003e\n \u003cp\u003eTo test prediction 1 (i.e., the association between MA and growth; N\u0026thinsp;=\u0026thinsp;212 individuals) we fitted a linear mixed effects model. We included the time period between day 0 and 25 to identify whether potential differences on growth trajectories were evident from the time of MA onset or whether they were already present from hatching. We used body mass as the response variable and \u003cem\u003eage\u003c/em\u003e (included as a fourth order polynomial) as the explanatory variable as this was the simplest model (i.e., minimum degree needed) to explain the relationship between \u003cem\u003eweight\u003c/em\u003e and \u003cem\u003eage\u003c/em\u003e. Higher-order polynomials are a useful approach to model growth trajectories as they can improve the fit of the model to the data by accounting for nonlinear patterns and potential variability in growth rates or inflection points over time [\u003cspan class=\"CitationRef\"\u003e40\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e41\u003c/span\u003e]. Additional fixed effects were \u003cem\u003ematernal aggression\u003c/em\u003e (i.e., experienced early-life MA, yes or no), \u003cem\u003ehatching order\u003c/em\u003e (hatching position A, B, or C), \u003cem\u003eyear\u003c/em\u003e (2022 or 2023), and \u003cem\u003enestling sex\u003c/em\u003e (female or male), as well as the interaction between \u003cem\u003ematernal aggression\u003c/em\u003e and \u003cem\u003eage\u003c/em\u003e, and the interaction between \u003cem\u003enestling sex\u003c/em\u003e and \u003cem\u003eage\u003c/em\u003e. To account for dependency between observations, random intercepts were added for \u003cem\u003enest identity\u003c/em\u003e and \u003cem\u003echick identity\u003c/em\u003e nested within \u003cem\u003enest identity\u003c/em\u003e [\u003cspan class=\"CitationRef\"\u003e42\u003c/span\u003e], as well as a random slope term for \u003cem\u003eage\u003c/em\u003e (again, included as a fourth order polynomial). Here we report results of the model excluding the individuals that were not sexed, but a model including all individuals measured can be found in the Supplementary information (Supplementary Table \u003cspan class=\"InternalRef\"\u003eS1\u003c/span\u003e).\u003c/p\u003e\n \u003cp\u003eTo test prediction 2 (i.e., the relationship between MA and offspring survival; N\u0026thinsp;=\u0026thinsp;309 individuals) we fitted a Cox proportional hazards model. The model included \u003cem\u003ematernal aggression\u003c/em\u003e (yes or no), \u003cem\u003ehatching order\u003c/em\u003e (hatching position A, B, or C), \u003cem\u003ebody mass at hatch\u003c/em\u003e, and \u003cem\u003eyear\u003c/em\u003e (2022 or 2023) as predictor variables. Z-values were used to establish the significance of the fixed effects. We created Kaplan-Meyer plots to visualize the survival curves using the \u003cem\u003esurvminer\u003c/em\u003e package (v0.4.9, [\u003cspan class=\"CitationRef\"\u003e43\u003c/span\u003e]).\u003c/p\u003e\u003cspan\u003e\n \u003cp\u003eB. Long-lasting Associations between MA and Offspring Behavioural Phenotype (Social Defeat Test)\u003c/p\u003e\n \u003c/span\u003e\n \u003cp\u003eTo test prediction 3 (i.e., the relationship between MA and the offspring behavioural phenotype; N\u0026thinsp;=\u0026thinsp;23), we fitted a linear mixed effects model for the time spent threatening. The model included \u003cem\u003ematernal aggression\u003c/em\u003e (yes or no), \u003cem\u003enestling sex\u003c/em\u003e (female or male), and the interaction between both terms as predictor variables. The response variable, \u003cem\u003etime threatening\u003c/em\u003e, was square-root transformed to meet the model assumptions. \u003cem\u003eNest identity\u003c/em\u003e was added as a random-effect term to account for dependency between observations of nestlings originating from the same nest.\u003c/p\u003e\n \u003cp\u003eDirect aggression was analysed with a Fisher exact test given that the sample sizes were small (i.e., of the 23 individuals tested, only 12 individuals showed any type of direct aggression) and that the observations were independent. We used two categorical variables each one with two levels: \u003cem\u003ematernal aggression\u003c/em\u003e (yes or no), and \u003cem\u003edirect aggression\u003c/em\u003e (i.e., presence or absence).\u003c/p\u003e\u003cspan\u003e\n \u003cp\u003eC. Parental Investment: Current versus Future Reproduction\u003c/p\u003e\n \u003c/span\u003e\n \u003cp\u003eTo test prediction 4 (i.e., the relationship between exerting MA and reproductive investment; N\u0026thinsp;=\u0026thinsp;91) clutch size was analysed in a linear mixed effects model as the count data followed a normal distribution. We included as fixed effects \u003cem\u003ematernal aggression\u003c/em\u003e (yes or no, based on the first clutch data), \u003cem\u003eyear\u003c/em\u003e (2022 or 2023), \u003cem\u003eclutch\u003c/em\u003e (first or second), and the interactions between \u003cem\u003ematernal aggression\u003c/em\u003e and \u003cem\u003eyear\u003c/em\u003e and \u003cem\u003ematernal aggression\u003c/em\u003e and \u003cem\u003eclutch\u003c/em\u003e. \u003cem\u003eNest identity\u003c/em\u003e was included as a random effect as there were two observations per individual (i.e., one for the first clutch and one for the second clutch). A Levene\u0026rsquo;s test (\u003cem\u003ecar\u003c/em\u003e package, v3.1.2, [\u003cspan class=\"CitationRef\"\u003e44\u003c/span\u003e]) was used to compare the overall variation in clutch size between females that exerted MA and those who didn\u0026rsquo;t.\u003c/p\u003e\n \u003cp\u003eThe time interval between laying the first and the second clutches was analysed using a linear model with \u003cem\u003ematernal aggression\u003c/em\u003e (yes or no), \u003cem\u003ebrood size\u003c/em\u003e of the first brood at day 10, \u003cem\u003eyear\u003c/em\u003e (2022 or 2023) and the pairwise interactions as fixed effects on the full model. Here we report results of the reduced models, but full models can be found in the Supplementary Information (Supplementary Table S2).\u003c/p\u003e\n\u003c/div\u003e"},{"header":"RESULTS","content":"\u003cp\u003e\u003cem\u003e\u0026nbsp;Nestling Growth and Survival\u003c/em\u003e\u003c/p\u003e\n\u003cp\u003eThere was no main effect of MA on nestling growth (Table 1), but we did find a significant two-way interaction between \u003cem\u003eage\u003c/em\u003e and \u003cem\u003ematernal aggression\u003c/em\u003e (\u003cem\u003ematernal aggression x age\u003c/em\u003e, p=0.001), suggesting that the association between MA and slower growth became stronger as nestlings aged (Figure 1). As expected, there was also a significant interaction effect between \u003cem\u003eage\u003c/em\u003e and \u003cem\u003enestling sex\u003c/em\u003e (\u003cem\u003enestling sex x age\u003c/em\u003e, p\u0026lt;0.001), with male nestlings becoming heavier than female nestlings with increasing age. Overall, body mass increased with \u003cem\u003eage\u003c/em\u003e in a non-linear way reflecting growth (\u003cem\u003epolynomials of age (k=4),\u003c/em\u003e p\u0026lt;0.001; Table 1).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eTable 1.\u0026nbsp;\u003c/strong\u003eEffects of MA on nestling growth.\u0026nbsp;Outcome of a linear mixed effects model (polynomial, 4\u003csup\u003eth\u003c/sup\u003e order). Bold values indicate p \u0026lt;0.05.\u003c/p\u003e\n\u003ctable border=\"0\" cellspacing=\"0\" cellpadding=\"0\" width=\"564\"\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 160px;\"\u003e\n \u003cp\u003e\u003cem\u003e\u0026nbsp;\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 98px;\"\u003e\n \u003cp\u003e\u003cem\u003e\u0026nbsp;\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd colspan=\"3\" valign=\"top\" style=\"width: 306px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e\u003cem\u003eGrowth trajectory\u003c/em\u003e\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 160px;\"\u003e\n \u003cp\u003e\u003cem\u003e\u0026nbsp;\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 98px;\"\u003e\n \u003cp\u003e\u003cem\u003eEstimate (SE)\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 92px;\"\u003e\n \u003cp\u003e\u003cem\u003eF\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 103px;\"\u003e\n \u003cp\u003e\u003cem\u003edf\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 111px;\"\u003e\n \u003cp\u003e\u003cem\u003ep\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 160px;\"\u003e\n \u003cp\u003e\u003cem\u003eIntercept\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 98px;\"\u003e\n \u003cp\u003e331.38 (192.92)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 92px;\"\u003e\n \u003cp\u003e-\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 103px;\"\u003e\n \u003cp\u003e-\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 111px;\"\u003e\n \u003cp\u003e-\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 160px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e\u003cem\u003epoly (Age , k=4)\u003c/em\u003e\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 98px;\"\u003e\n \u003cp\u003e-\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 92px;\"\u003e\n \u003cp\u003e538\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 103px;\"\u003e\n \u003cp\u003e(4, 159.56)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 111px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e\u0026lt;0.001\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 160px;\"\u003e\n \u003cp\u003e\u003cem\u003eMaternal aggression\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 98px;\"\u003e\n \u003cp\u003e-0.02 (0.09)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 92px;\"\u003e\n \u003cp\u003e0.08\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 103px;\"\u003e\n \u003cp\u003e(1, 217.19)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 111px;\"\u003e\n \u003cp\u003e0.76\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 160px;\"\u003e\n \u003cp\u003e\u003cem\u003eNestling sex\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 98px;\"\u003e\n \u003cp\u003e0.002 (0.08)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 92px;\"\u003e\n \u003cp\u003e0.01\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 103px;\"\u003e\n \u003cp\u003e(1, 440.92)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 111px;\"\u003e\n \u003cp\u003e0.97\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 160px;\"\u003e\n \u003cp\u003e\u003cem\u003eYear\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 98px;\"\u003e\n \u003cp\u003e-0.15 (0.09)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 92px;\"\u003e\n \u003cp\u003e2.78\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 103px;\"\u003e\n \u003cp\u003e(1, 87.2)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 111px;\"\u003e\n \u003cp\u003e0.09\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 160px;\"\u003e\n \u003cp\u003e\u003cem\u003eHatching order\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 98px;\"\u003e\n \u003cp\u003e-0.06 (0.04)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 92px;\"\u003e\n \u003cp\u003e1.90\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 103px;\"\u003e\n \u003cp\u003e(1, 260.87)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 111px;\"\u003e\n \u003cp\u003e0.16\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 160px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e\u003cem\u003eMaternal aggression: Age\u003c/em\u003e\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 98px;\"\u003e\n \u003cp\u003e0.03 (0.01)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 92px;\"\u003e\n \u003cp\u003e11.13\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 103px;\"\u003e\n \u003cp\u003e(1, 158.91)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 111px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e0.001\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 160px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e\u003cem\u003eNestling sex: Age\u003c/em\u003e\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 98px;\"\u003e\n \u003cp\u003e0.04 (0.01)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 92px;\"\u003e\n \u003cp\u003e19.53\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 103px;\"\u003e\n \u003cp\u003e(1, 215.74)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 111px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e\u0026lt;0.001\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n\u003c/table\u003e\n\u003cp\u003eThere was no significant effect of MA on the survival probability of the nestlings (\u003cem\u003ematernal aggression\u003c/em\u003e, Z=0.94, p=0.34, Figure 2; Table 2). However, there was a significant negative effect of \u003cem\u003eweight at hatch\u003c/em\u003e, with lighter hatchlings having lower survival probabilities (Z=-2.12, p=0.03; Table 2) and a negative effect of \u003cem\u003ehatching order\u003c/em\u003e, with later hatched nestlings having lower survival probabilities (Z=5.43, p\u0026lt;0.001; Table 2).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eTable 2.\u0026nbsp;\u003c/strong\u003eFull model of the effects of MA on nestling survival. Outcome of a Cox proportional hazards model. Bold values indicate p\u0026lt; 0.05.\u003c/p\u003e\n\u003ctable border=\"0\" cellspacing=\"0\" cellpadding=\"0\" width=\"566\"\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 151px;\"\u003e\n \u003cp\u003e\u003cem\u003e\u0026nbsp;\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 57px;\"\u003e\n \u003cp\u003e\u003cem\u003e\u0026nbsp;\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd colspan=\"3\" valign=\"top\" style=\"width: 0px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e\u003cem\u003eSurvival\u003c/em\u003e\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 0px;\"\u003e\n \u003cp\u003e\u003cem\u003e\u0026nbsp;\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 151px;\"\u003e\n \u003cp\u003e\u003cem\u003e\u0026nbsp;\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 57px;\"\u003e\n \u003cp\u003e\u003cem\u003ecoef\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 66px;\"\u003e\n \u003cp\u003e\u003cem\u003eexp(coef)\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 104px;\"\u003e\n \u003cp\u003e\u003cem\u003ese(coef)\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 95px;\"\u003e\n \u003cp\u003e\u003cem\u003eZ\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 94px;\"\u003e\n \u003cp\u003e\u003cem\u003ep\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 151px;\"\u003e\n \u003cp\u003e\u003cem\u003eMaternal aggression\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 57px;\"\u003e\n \u003cp\u003e0.18\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 66px;\"\u003e\n \u003cp\u003e1.20\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 104px;\"\u003e\n \u003cp\u003e0.19\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 95px;\"\u003e\n \u003cp\u003e0.94\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 94px;\"\u003e\n \u003cp\u003e0.34\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 151px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e\u003cem\u003eWeight at hatch\u003c/em\u003e\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 57px;\"\u003e\n \u003cp\u003e-0.74\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 66px;\"\u003e\n \u003cp\u003e0.47\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 104px;\"\u003e\n \u003cp\u003e0.35\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 95px;\"\u003e\n \u003cp\u003e-2.12\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 94px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e0.03\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 151px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e\u003cem\u003eHatching order\u003c/em\u003e\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 57px;\"\u003e\n \u003cp\u003e0.68\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 66px;\"\u003e\n \u003cp\u003e1.98\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 104px;\"\u003e\n \u003cp\u003e0.12\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 95px;\"\u003e\n \u003cp\u003e5.43\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 94px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e\u0026lt;0.001\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 151px;\"\u003e\n \u003cp\u003e\u003cem\u003eYear\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 57px;\"\u003e\n \u003cp\u003e0.11\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 66px;\"\u003e\n \u003cp\u003e1.12\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 104px;\"\u003e\n \u003cp\u003e0.19\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 95px;\"\u003e\n \u003cp\u003e0.59\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 94px;\"\u003e\n \u003cp\u003e0.55\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n\u003c/table\u003e\n\u003cp\u003e\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003e\u003cem\u003eLong-lasting Associations between MA and Offspring Behavioural Phenotype (Social Defeat Test)\u003c/em\u003e\u003c/p\u003e\n\u003cp\u003e\u003cem\u003e\u003cstrong\u003eTable 3\u003c/strong\u003e\u003cem\u003e\u0026nbsp;\u003c/em\u003eEffects of MA on time spent threatening during the social defeat test. Model outcomes of a linear mixed effects models. Bold values indicate p\u0026lt;0.05.\u003c/em\u003e\u003c/p\u003e\n\u003ctable border=\"0\" cellspacing=\"0\" cellpadding=\"0\" width=\"567\"\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 187px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd colspan=\"2\" valign=\"top\" style=\"width: 102px;\"\u003e\n \u003cp\u003e\u003cem\u003e\u0026nbsp;\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd colspan=\"3\" valign=\"top\" style=\"width: 278px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e\u003cem\u003eTime in Threat Position\u003c/em\u003e\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 187px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 97px;\"\u003e\n \u003cp\u003e\u003cem\u003eEstimate (SE)\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd colspan=\"2\" valign=\"top\" style=\"width: 108px;\"\u003e\n \u003cp\u003e\u003cem\u003eF\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 97px;\"\u003e\n \u003cp\u003e\u003cem\u003edf\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 79px;\"\u003e\n \u003cp\u003e\u003cem\u003ep\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 187px;\"\u003e\n \u003cp\u003e\u003cem\u003eIntercept\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 97px;\"\u003e\n \u003cp\u003e0.92 (0.48)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd colspan=\"2\" valign=\"top\" style=\"width: 108px;\"\u003e\n \u003cp\u003e-\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 97px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 79px;\"\u003e\n \u003cp\u003e-\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 187px;\"\u003e\n \u003cp\u003e\u003cem\u003eMaternal aggression\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 97px;\"\u003e\n \u003cp\u003e-0.19 (0.66)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd colspan=\"2\" valign=\"top\" style=\"width: 108px;\"\u003e\n \u003cp\u003e3.34\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 97px;\"\u003e\n \u003cp\u003e(1, 19)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 79px;\"\u003e\n \u003cp\u003e0.08\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 187px;\"\u003e\n \u003cp\u003e\u003cem\u003eSex\u003c/em\u003e\u003c/p\u003e\n \u003cp\u003e\u003cstrong\u003e\u003cem\u003eMaternal aggression: Sex\u003c/em\u003e\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 97px;\"\u003e\n \u003cp\u003e-0.55 (0.72)\u003c/p\u003e\n \u003cp\u003e2.23 (1.00)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd colspan=\"2\" valign=\"top\" style=\"width: 108px;\"\u003e\n \u003cp\u003e1.24\u0026nbsp;\u003c/p\u003e\n \u003cp\u003e4.93\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 97px;\"\u003e\n \u003cp\u003e(1, 19)\u003c/p\u003e\n \u003cp\u003e(1, 19)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 79px;\"\u003e\n \u003cp\u003e0.27\u003c/p\u003e\n \u003cp\u003e\u003cstrong\u003e0.03\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n\u003c/table\u003e\n\u003cp\u003eThere was no significant association between MA and time threatening (\u003cem\u003ematernal aggression,\u0026nbsp;\u003c/em\u003ep=0.08). However, there was a significant interaction effect between MA and sex on the time threatening (\u003cem\u003ematernal aggression x sex,\u003c/em\u003e p= 0.03; Table 3; Figure 3), with males that experienced MA spending more time threatening the resident male, whereas this pattern was not found for females. There was no significant association between experiencing MA and the number of direct aggressions exerted during the test (Fisher exact test, p=0.31, Figure 3).\u003c/p\u003e\n\u003cp\u003e\u003cem\u003eParental Investment: Current versus Future Reproduction\u0026nbsp;\u003c/em\u003e\u003c/p\u003e\n\u003cp\u003eThere was a significant positive association between MA and clutch size (\u003cem\u003ematernal aggression\u003c/em\u003e, p=0.03, Table 4). We also found a negative effect of \u003cem\u003eclutch order\u003c/em\u003e (\u003cem\u003eclutch ID\u003c/em\u003e, p=0.02, Table 4; Figure 4) with second clutches having fewer eggs than first clutches. Variation in clutch size was significantly lower in clutches laid by females that exerted MA compared to clutches laid by females that did not exert MA (females that exerted MA: SD=0.88; females that did not exert MA: SD=1.27; Levene\u0026rsquo;s test: F\u003csub\u003e1-180\u003c/sub\u003e=7.35, p=0.007).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThere was no main effect of MA on the time interval between laying the first and the second clutch (\u003cem\u003ematernal aggression\u003c/em\u003e, p=0.51, Table 4; Figure 4).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eTable 4\u0026nbsp;\u003c/strong\u003eThe relationship between\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003eMA and parental investment\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003e(i.e., clutch size of both the first and the second clutch), second clutch mass\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003eand time interval between the first and the second clutch. Outcome of a linear mixed model for the clutch size, and outcome of linear models for the total mass of the second clutch (with robust standard errors, RSE) and the interval between clutches. Bold values indicate p\u0026lt;0.05.\u003c/p\u003e\n\u003ctable border=\"0\" cellspacing=\"0\" cellpadding=\"0\" width=\"557\"\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd colspan=\"5\" valign=\"top\" style=\"width: 557px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e\u003cem\u003eClutch size\u003c/em\u003e\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 226px;\"\u003e\n \u003cp\u003e\u003cem\u003e\u0026nbsp;\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 127px;\"\u003e\n \u003cp\u003e\u003cem\u003eEstimate (SE)\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 91px;\"\u003e\n \u003cp\u003e\u003cem\u003edf\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 54px;\"\u003e\n \u003cp\u003e\u003cem\u003et\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 59px;\"\u003e\n \u003cp\u003e\u003cem\u003ep\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 226px;\"\u003e\n \u003cp\u003e\u003cem\u003eIntercept\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 127px;\"\u003e\n \u003cp\u003e4.59 (0.15)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 91px;\"\u003e\n \u003cp\u003e130.97\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 54px;\"\u003e\n \u003cp\u003e28.92\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 59px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e\u0026lt;0.001\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 226px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e\u003cem\u003eMaternal aggression\u003c/em\u003e\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 127px;\"\u003e\n \u003cp\u003e-0.37 (0.17)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 91px;\"\u003e\n \u003cp\u003e88\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 54px;\"\u003e\n \u003cp\u003e-2.19\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 59px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e0.03\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 226px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e\u003cem\u003eClutch ID\u003c/em\u003e\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 127px;\"\u003e\n \u003cp\u003e-0.32 (0.14)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 91px;\"\u003e\n \u003cp\u003e90\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 54px;\"\u003e\n \u003cp\u003e-2.28\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 59px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e0.02\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 226px;\"\u003e\n \u003cp\u003e\u003cem\u003eYear\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 127px;\"\u003e\n \u003cp\u003e-0.15 (0.16)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 91px;\"\u003e\n \u003cp\u003e88\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 54px;\"\u003e\n \u003cp\u003e-0.93\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 59px;\"\u003e\n \u003cp\u003e0.35\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd colspan=\"5\" valign=\"top\" style=\"width: 557px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e\u003cem\u003eTime interval between clutches\u003c/em\u003e\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 226px;\"\u003e\n \u003cp\u003e\u003cem\u003e\u0026nbsp;\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 127px;\"\u003e\n \u003cp\u003e\u003cem\u003eEstimate (SE)\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 91px;\"\u003e\n \u003cp\u003e\u003cem\u003e\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;t\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 54px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 59px;\"\u003e\n \u003cp\u003e\u003cem\u003ep\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 226px;\"\u003e\n \u003cp\u003e\u003cem\u003eIntercept\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 127px;\"\u003e\n \u003cp\u003e36.01 (1.19)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd colspan=\"2\" valign=\"top\" style=\"width: 145px;\"\u003e\n \u003cp\u003e30.01\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 59px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e\u0026lt;0.001\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 226px;\"\u003e\n \u003cp\u003e\u003cem\u003eMaternal aggression\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 127px;\"\u003e\n \u003cp\u003e-0.47 (0.72)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd colspan=\"2\" valign=\"top\" style=\"width: 145px;\"\u003e\n \u003cp\u003e-0.65\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 59px;\"\u003e\n \u003cp\u003e0.51\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 226px;\"\u003e\n \u003cp\u003e\u003cem\u003eBrood size day 10\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 127px;\"\u003e\n \u003cp\u003e0.64 (0.34)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd colspan=\"2\" valign=\"top\" style=\"width: 145px;\"\u003e\n \u003cp\u003e1.89\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 59px;\"\u003e\n \u003cp\u003e0.06\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 226px;\"\u003e\n \u003cp\u003e\u003cem\u003eYear\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 127px;\"\u003e\n \u003cp\u003e0.17 (0.72)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd colspan=\"2\" valign=\"top\" style=\"width: 145px;\"\u003e\n \u003cp\u003e0.24\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 59px;\"\u003e\n \u003cp\u003e0.80\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n\u003c/table\u003e"},{"header":"DISCUSSION","content":"\u003cp\u003eThis study investigated the association between maternal aggression (MA), a seemingly maladaptive parental behaviour, and variation in offspring development and behavioural phenotype. In addition, we explored whether MA is related to a trade-off between current and future reproduction. We found that offspring exposed to MA showed reduced growth, while no significant differences in survival were observed. Intriguingly, we observed a sex-specific effect on aggression, with male offspring exposed to MA early in life displaying more threatening behaviours towards other males, which may be beneficial in certain contexts. Finally, we found no evidence that MA was associated with a trade-off between current and future reproduction. However, females expressing MA laid, on average, larger and less variable clutches. These results are discussed in detail below.\u003c/p\u003e\n\u003cp\u003eWe hypothesized that MA would be associated with retarded growth (prediction 1), which should be evident from the time of MA onset and potentially reduce offspring survival probabilities (prediction 2). Consistent with the first part of the hypothesis, nestlings exposed to MA during early life showed a lower increase in body mass, with differences becoming evident from the onset of MA (around day 14 after hatching). There are two non-mutually exclusive explanations for this result. First, MA could be associated with neglect or a reduced responsiveness to needs of the offspring, both of which could result in poorer food provisioning and a consequent reduction in body mass gain [45]. Second, early-life exposure to MA could increase stress in nestlings, leading to elevated glucocorticoid levels [46], which can constrain energy allocation to growth [47,48]. Elevated corticosterone may also explain why exposure to stressors early in life can lead to an altered energy metabolism, as has been shown in humans and other animals, [49,50], as well as to impaired brain development [51], and predisposition to disease [52]. Thus, MA is associated with negative effects early in life, such as reduced body mass gain in the offspring, which could potentially have long-lasting consequences for the offspring\u0026rsquo;s adult phenotype and fitness (see below).\u003c/p\u003e\n\u003cp\u003eContrary to our expectations, there was no significant association between MA and the offspring survival (prediction 2), at least up to 100 days of age. This contrasts with findings in other bird species, such as American coots [7,53] and moorhens [8], where similar forms of aggression (e.g., tousling) have been linked to increased mortality. In those species, MA has been hypothesized to promote independent feeding in nestlings. However, in our study, canary nestlings were still unable to feed independently at the age at which MA started (canaries reach independence at 25-30 days of age; [33]), making this explanation unlikely. An alternative explanation is that MA may be a response to specific nestling behaviours potentially functioning as a form of punishment or social regulation [4,54-55]. For example, in macaques, MA has been observed in response to pestering behaviour by the young [54], which is generally thought to increase maternal irritability [55]. Further research is needed to determine whether certain offspring behaviours trigger MA in this species, and whether such aggression is effective in reducing those behaviours or in increasing maternal control.\u003c/p\u003e\n\u003cp\u003eRegarding the lasting effects on the offspring behavioural phenotype (prediction 3), we predicted that exposure to early-life MA would be associated with altered behavioural phenotypes, specifically in the form of heightened aggressive behaviour in response to stressful conflicts. Consistent with this hypothesis, juvenile males reared under MA displayed significantly more threatening behaviour than juveniles reared by non-aggressive mothers. This pattern was not found in females. However, contrary to our expectations and to previous studies (in birds: [56]; in mammals: [57]), MA-exposed juveniles did not engage in more direct aggressions. This is also inconsistent with the \u0026ldquo;cycle of violence\u0026rdquo; hypothesis described in humans, where early exposure to aggression is linked to increased violent behaviour in adulthood [58]. One possible explanation for the lack of association between MA and the frequency of direct aggression, as well as with the absence of effects in female offspring, is the limited sample size in our test, which may have hindered the detection of subtle or sex-specific effects. Males may be more prone to engage in intra-sexual aggression and dominance displays throughout the year [59,60], whereas in canaries, females typically display higher levels of aggression closed to the laying period [61]. Additionally, previous work suggests that early-life experiences may have sex-specific impacts on the behavioural phenotype of the offspring [62].\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThis way, while the mechanisms underlying these patterns remain speculative, previous studies in rodents and humans suggest that early-life adversity, such as MA, can alter stress reactivity and increase impulsive, reactive aggression in response to \u0026lsquo;provocative\u0026rsquo; stimuli or social challenges [16, 63-65]. Moreover, MA-exposed individuals may be more likely to perceive social stimuli as threatening [66,67], or to adopt aggressive behaviour to achieve desirable outcomes in social interactions [68]. Future studies should investigate whether enhanced threat displays contribute to increased competitiveness or fitness in challenging environments [24,69], so that the effects of MA on the offspring\u0026rsquo;s phenotype could be considered adaptive at least in certain contexts. However, demonstrating adaptive plasticity requires direct measures of fitness outcomes in the offspring [18].\u003c/p\u003e\n\u003cp\u003eIt is important to note that the observed patterns could be influenced by genetic inheritance. Offspring from aggressive mothers may have inherited genes that predispose them to have lower quality, reduced growth, or higher aggression levels [70,71]. Future studies are needed to disentangle potential genetic contributions from developmental effects by incorporating, for instance, cross-fostering designs.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eRegarding our results on the parental investment in the current and future reproduction, we predicted that females showing aggression towards their first clutch offspring, would intend to reduce or terminate their investment in current offspring in order to save resources that can be invested in future reproduction, that is, in their second clutch (prediction 4). However, instead of a trade-off between current and future reproduction, we found a statistically significant main difference in clutch size of both the first and second clutch between females that exhibited MA compared to those that did not, while the variance in clutch size was lower among MA females. This result suggests phenotypic differences or different reproductive strategies associated with MA. It could be speculated that MA females overestimated their rearing capacity when laying larger \u0026ldquo;optimistic\u0026rdquo; clutches, and subsequently adjusted their investment in the current brood afterwards to match their own and their partner\u0026rsquo;s rearing capacity [72]. However, if this was the case, it did not result in fewer offspring in our study case, but possibly lower quality offspring. The increased variance in clutch size in females that did not exert MA towards the offspring could reflect greater plasticity in their parental investment and possibly reproductive strategies, by adapting clutch size to their current condition, their partner\u0026rsquo;s willingness and capacity to contribute to parental care and/or the resources available [29]. The latter, however, is unlikely in our study given the \u003cem\u003ead libitum\u003c/em\u003e access to food resources in captivity. However, we acknowledge that, although the differences in clutch size are statistically significant, these results should be interpreted with caution due to the small effect size, leaving some uncertainty about the biological significance and its potential impact on reproductive success.\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eC.G. and W.M. were in charge of the conceptualization. C.G developed the methodology and investigation with the support of W.M. C.G was in charge of data curation, the formal analysis and visualization and the writing (original draft and editing). W.M provided the resources. J.M, F.V and W.M were in charge of the funding acquisition, supervision, validation and reviewing and editing the writing.\u003c/p\u003e\u003ch2\u003eAcknowledgement\u003c/h2\u003e\u003cp\u003eWe thank Peter Scheys for his assistance in taking care of the birds. This work was supported by the FWO Flanders (CG: 1173825N). FV was supported by an ERC Consolidator grant (European Union\u0026rsquo;s Horizon 2020 research and innovation programme, grant agreement No 769595) and Methusalem Project 01M00221 (Ghent University). A research stay waspossible thanks to project PID2022-139166NB-I00 (to JM) funded by MCIN/AEI/https://doi.org/10.13039/501100011033 and \u0026ldquo;ERDF A way of making Europe\u0026rdquo;.\u003c/p\u003e\u003ch2\u003eData Availability\u003c/h2\u003e\u003cp\u003eThe data used in this study is archived in Mendeley Data in the following link: https://data.mendeley.com/datasets/svbwhszz3r/1. The videos analysed are available from the corresponding author (Clara Garcia-Co) upon request.\u003c/p\u003e\n\u003cp\u003eThe authors declare no competing or financial interests.\u003c/p\u003e\n"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eClutton-Brock, T. 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Food, information and avian bread reduction. \u003cem\u003eEcoscience\u003c/em\u003e , \u003cstrong\u003e3\u003c/strong\u003e (1), 45-53 (1996). \u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"scientific-reports","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"scirep","sideBox":"Learn more about [Scientific Reports](http://www.nature.com/srep/)","snPcode":"","submissionUrl":"","title":"Scientific Reports","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Scientific Reports","inReviewEnabled":true,"inReviewRevisionsEnabled":true},"keywords":"early-life experience, maternal aggression, maternal behaviour, offspring development ","lastPublishedDoi":"10.21203/rs.3.rs-6479850/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-6479850/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eParental care improves offspring quality and survival, thereby enhancing parental fitness. Yet, seemingly maladaptive parental behaviours such as directed aggression towards the offspring have been reported in a variety of species. While maternal aggression - defined as aggressive interactions from mothers to the offspring within the family context - may be a seemingly maladaptive behaviour, it could also be an adaptive strategy allowing optimal resource allocation for current and future reproduction in the face of evolutionary conflicts of interest. This study investigated associations between maternal aggression and altered offspring development in domestic canaries (\u003cem\u003eSerinus canaria\u003c/em\u003e). Offspring exposed to maternal aggression showed reduced growth, while no differences in survival were observed. In addition, juvenile males, but not females, exposed to maternal aggression displayed increased threatening behaviours, highlighting the importance of considering long-term effects when interpreting the significance of aggressive parenting styles. Females that exhibited maternal aggression did not lay larger second clutches, as would be expected if aggression during the first reproductive event was directed at prioritising future reproduction. However, they laid larger and less variable clutches overall, suggesting that females that engaged in maternal aggression may be less flexible and more prone to high investment at egg laying.\u003c/p\u003e","manuscriptTitle":"Balancing care and conflict: towards a better understanding of maternal aggression in canaries","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-05-15 10:15:09","doi":"10.21203/rs.3.rs-6479850/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2025-05-26T18:04:44+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-05-24T10:12:05+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-05-19T15:44:09+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"155145404644951621119486115283105127356","date":"2025-05-14T10:20:15+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"74517841599192626246476942797911211391","date":"2025-05-13T14:30:51+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-05-12T23:34:47+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-05-12T09:34:51+00:00","index":"","fulltext":""},{"type":"editorInvited","content":"","date":"2025-04-28T09:18:45+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2025-04-25T10:42:32+00:00","index":"","fulltext":""},{"type":"submitted","content":"Scientific Reports","date":"2025-04-18T14:38:06+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"scientific-reports","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"scirep","sideBox":"Learn more about [Scientific Reports](http://www.nature.com/srep/)","snPcode":"","submissionUrl":"","title":"Scientific Reports","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Scientific Reports","inReviewEnabled":true,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"b1728253-2b8e-40b0-90b8-ea1a12c32114","owner":[],"postedDate":"May 15th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[{"id":48517526,"name":"Biological sciences/Developmental biology"},{"id":48517527,"name":"Biological sciences/Ecology/Evolutionary ecology"},{"id":48517528,"name":"Biological sciences/Evolution/Evolutionary developmental biology"},{"id":48517529,"name":"Biological sciences/Genetics/Animal breeding"},{"id":48517530,"name":"Biological sciences/Genetics/Development"}],"tags":[],"updatedAt":"2025-08-07T07:15:05+00:00","versionOfRecord":{"articleIdentity":"rs-6479850","link":"https://doi.org/10.1038/s41598-025-10698-4","journal":{"identity":"scientific-reports","isVorOnly":false,"title":"Scientific Reports"},"publishedOn":"2025-07-19 16:05:39","publishedOnDateReadable":"July 19th, 2025"},"versionCreatedAt":"2025-05-15 10:15:09","video":"","vorDoi":"10.1038/s41598-025-10698-4","vorDoiUrl":"https://doi.org/10.1038/s41598-025-10698-4","workflowStages":[]},"version":"v1","identity":"rs-6479850","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-6479850","identity":"rs-6479850","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}