Is Bergmann's rule valid for terrestrial vertebrates?

Is Bergmann's rule valid for terrestrial vertebrates? | Authorea try { document.documentElement.classList.add('js'); } catch (e) { } var _gaq = _gaq || []; _gaq.push(['_setAccount', 'G-8VDV14Y67G']); _gaq.push(['_trackPageview']); (function() { var ga = document.createElement('script'); ga.type = 'text/javascript'; ga.async = true; ga.src = ('https:' == document.location.protocol ? 'https://ssl' : 'http://www') + '.google-analytics.com/ga.js'; var s = document.getElementsByTagName('script')[0]; s.parentNode.insertBefore(ga, s); })(); Skip to main content Preprints Collections Wiley Open Research IET Open Research Ecological Society of Japan All Collections About About Authorea FAQs Contact Us Quick Search anywhere Search for preprint articles, keywords, etc. Search Search ADVANCED SEARCH SCROLL This is a preprint and has not been peer reviewed. Data may be preliminary. 18 April 2025 V1 Latest version Share on Is Bergmann's rule valid for terrestrial vertebrates? Authors : Oleksandra Oskyrko 0000-0003-0092-4193 , Jiajia Liu 0000-0002-1923-5964 , and Weiguo Du 0000-0002-1868-5664 [email protected] Authors Info & Affiliations https://doi.org/10.22541/au.174498491.11294167/v1 Published Ecology Version of record Peer review timeline 522 views 224 downloads Contents Abstract Information & Authors Metrics & Citations View Options References Figures Tables Media Share Abstract Bergmann’s rule suggests that animals in colder climates tend to be larger, but does this pattern hold across all vertebrates? While traditionally thought to apply more strongly to endotherms, evidence remains inconsistent across species and ecosystems. We analysed body size trends across latitude in major terrestrial vertebrate—amphibians, reptiles, birds, and mammals—to assess the rule’s validity. Birds and mammals followed Bergmann’s rule, while amphibians and reptiles showed no consistent pattern. In addition, we observed a stronger support for Bergmann’s rule in endotherms compared to ectotherms at the order/family level, but a slightly higher support at the species level. Contrary to expectation, however, Bergmann’s rule is not stronger at the population level than at the species level. These results highlight that Bergmann’s rule is context-dependent, shaped by factors like body temperature regulation and evolutionary history, and underscore the need to consider these differences when predicting species’ vulnerability to climate change. Is Bergmann’s rule valid for terrestrial vertebrates? Oleksandra Oskyrko 1,2,3 , Jiajia Liu 1 , Weiguo Du 1* 1 State Key Laboratory of Wetland Conservation and Restoration, National Observations and Research Station for Wetland Ecosystems of the Yangtze Estuary, Ministry of Education Key Laboratory for Biodiversity Science and Ecological Engineering, and Institute of Eco-Chongming, School of Life Sciences, Fudan University, Songhu Road 2005, Shanghai, 200438, China 2 Key Laboratory of Animal Ecology and Conservation Biology, Institute of Zoology, Chinese Academy of Sciences, 1 Beichen West Road, Chaoyang District, Beijing, 100101, China 3 University of Chinese Academy of Sciences, No.1 Yanqihu East Rd, Huairou District, Beijing, 101408, China [email protected] , [email protected] , [email protected] *Corresponding author: Weiguo Du, [email protected] Short running title : Bergmann’s rule for vertebrates Keywords : Bergmann’s rule, latitude, temperature, interspecific, intraspecific Type of article : Letter The number of words in the abstract: 150 The number of words in the main text: 3840 The number of references: 81 The number of figures: 3 The number of tables: 1 Author Contributions OO, and WD contributed to the study of conception and design. WD supervised this study. OO performed material preparation, data collection and analysis. OO, JL and WD wrote the manuscript. All authors read and approved of the final manuscript. Data accessibility statement The Supplement figures and tables supporting the results have been in Supplement materials. Bergmann’s rule suggests that animals in colder climates tend to be larger, but does this pattern hold across all vertebrates? While traditionally thought to apply more strongly to endotherms, evidence remains inconsistent across species and ecosystems. We analysed body size trends across latitude in major terrestrial vertebrate—amphibians, reptiles, birds, and mammals—to assess the rule’s validity. Birds and mammals followed Bergmann’s rule, while amphibians and reptiles showed no consistent pattern. In addition, we observed a stronger support for Bergmann’s rule in endotherms compared to ectotherms at the order/family level, but a slightly higher support at the species level. Contrary to expectation, however, Bergmann’s rule is not stronger at the population level than at the species level. These results highlight that Bergmann’s rule is context-dependent, shaped by factors like body temperature regulation and evolutionary history, and underscore the need to consider these differences when predicting species’ vulnerability to climate change. Introduction Predictable latitudinal variations in climatic variables, particularly temperature and seasonality, impose strong selection pressures on different physiological and life-history traits such as body size ( Bergmann 1847) , metabolism (Tieleman et al. 2006), and longevity ( Scharf et al. 2015; Wikelski et al. 2003). These pressures create consistent patterns of life-history strategies across latitudes ( Bergmann 1847; Gould 1997; Mayr 1956 ). Consequently, general ecological and evolutionary principles, such as Bergmann’s rule, Allen’s and Gloger’s rules, have emerged. However, their universality remains a topic of debate ( Gould 1997; Mayr 1956; Meiri 2011; Riemer et al. 2018 ). Among these principles, Bergmann’s rule is particularly prominent, proposing that body size increases with latitude due to decreasing environmental temperatures. Many studies on birds and mammals have explored geographic variation in body size, generally supporting the Bergmann’s rule, though some have found no clear evidence for its applicability in these groups ( Blackburn & Hawkins 2004; Gaston & Chown 1999; He et al. 2023; Ramirez et al. 2008 ). For example, one review of 243 bird and mammal species found that over 65% species followed Bergmann’s rule ( Meiri & Dayan 2003 ), while other studies concluded that the rule is not universally applicable (Meiri 2011; Riemer et al. 2018) . In contrast, research on terrestrial ectotherms is more limited and has shown inconsistent patterns ( Angilletta et al. 2002; Ashton & Feldman 2003; Bansal & Thaker 2021 ). Some taxa (e.g., turtles) adhere to Bergmann’s rule, while others (e.g., lizards and snakes) exhibit the opposite trend ( Meiri 2011) . These discrepancies may stem from differences in thermoregulatory mechanisms between endotherms and ectotherms. While endotherms rely on metabolic heat production, making them more likely to follow Bergmann’s rule due to heat retention advantages ( Bergmann 1847; James 1970 ), ectotherms depend on external heat sources, potentially leading to different responses to latitudinal gradients ( Angilletta et al. 2002; Meiri 2011 ). Bergmann’s rule is also predicted to operate more strongly at the population level than among closely related species ( Bergmann 1847; Mayr 1956; Meiri 2011) . This is because population-level size differences are more likely to reflect direct adaptations to local environmental conditions, whereas species-level differences may arise from factors such as phylogeny ( Mayr 1956) . Despite this, previous studies have focused on interspecific patterns ( Calder 1974; He et al. 2023; James 1970; Romano et al. 2020 ), overlooking population-level trends that are critical for understanding variation within lineages. Additionally, while some studies have documented large-scale assemblage patterns and identified ecological and biological drivers ( Ashton et al. 2000; Blackburn & Hawkins 2004; Dunbar 1990; Rosenzweig 1968 ), the mechanisms underlying these patterns remain poorly understood, revealing complex and often context-dependent effects of different predictors. Despite decades of research, a global assessment of Bergmann’s rule across diverse vertebrate lineages remains lacking, leaving these fundamental questions regarding its applicability unanswered. To fill this gap, we investigated body size variation across latitudes at both species and population levels for all terrestrial vertebrate groups—amphibians, reptiles, birds, and mammals. This approach aimed to clarify how Bergmann’s rule operates across different taxonomical groups and biological scales. Specifically, we sought to answer the following questions: (1) Is Bergmann’s Rule stronger in endotherms than in ectotherms? (2) Is Bergmann’s Rule stronger at the population level than at the species level? Material and methods Data collection at the species level This study included all major terrestrial vertebrate groups — amphibians, reptiles, birds and mammals. For species-level analyses, maximum recorded body size (in millimeters, mm) was used. In amphibians and reptiles, snout-vent length (SVL) was used, except in turtles, where maximum carapace length (CL) was applied. For birds and mammals, body size was represented by individual-level body length (head-to-tail length) data. Body length was chosen as a proxy for body size because of its strong correlation with body mass ( Sheard et al. 2020) and its importance in determining individual fitness ( Niklas 1994 ). Although body mass as a direct measure of size has been used in previous studies to test Bergmann’s rule in birds and mammals ( He et al. 2023; Meiri 2011; Riemer et al. 2018 ), it is often influenced by factors such as dehydration, life stage, and seasonality during measurement ( Doughty & Shine 1998; Murie & Boag 1984; Speakman & Racey 1986; van Gils et al. 2016 ), making body length a more reliable metric. When multiple sources provided body size data for a species, the maximum value was selected. Data for reptiles were obtained from the ReptTraits dataset ( Oskyrko et al. 2024 ). Data for other vertebrate groups was compiled through an extensive literature review of 10 scientific sources (5 for amphibians, 2 for birds, and 3 for mammals; see Table S10). We used absolute latitude for the analysis, which was calculated as the centroid of each species’ distribution range. Reptile centroids were sourced from the Global Assessment of Reptile Distributions (GARD; Roll & Meiri 2022) . For amphibians and mammals, latitudinal centroids were extracted from IUCN Red List distribution data ( IUCN 2024 ). Bird latitudinal ranges were derived from BirdLife International data ( BirdLife 2024 ). Species without reliable centroid estimates or had too large distribution ranges (for example distribution range in two continents) were excluded from analyses. We considered only the breeding ranges of all vertebrates for central latitude and excluded insular (island endemic) species from our analysis. Amphibian taxonomy was based on Amphibian Species of the World ( Frost 2024 ), while reptile taxonomy followed the Reptile Database ( Uetz et al. 2024 ). Bird taxonomy adhered to BirdLife (2024), and mammal taxonomy followed the IUCN (2024). We constructed using the Hackett constraint generating posterior distributions of 100 trees, and mammal phylogenies for amphibians, reptiles, birds and mammals ( Jetz et al. 2012; Jetz & Pyron 2018; Thomson et al. 2021; Tonini et al. 2016; Upham et al. 2019 ). We collected body size data for 33,284 terrestrial vertebrates, including 7,467 amphibians, 9,519 reptiles, 9,732 birds and 6,566 mammals. After, to address missing data, phylogenetic trees were pruned, and trait values were interpolated using the phyEstimate function ( Kembel et al. 2010 ) in R. Imputation was based on the phylogenetic signal of species with known traits, ensuring accuracy. Imputed estimates were filtered conservatively, retaining only those with standard errors smaller than the estimated trait value ( Goberna & Verdú 2016 ). Finally, our dataset includes body size and latitude data for 19,877 vertebrate species, comprising 4,217 amphibians, 4,840 reptiles, 6,534 birds, and 4,286 mammals. To ensure the feasibility of the body size–latitude correlations, we also included only orders or families with more than 10 species in this analysis. Orders were used for birds and mammals, while families were used for amphibians and reptiles, as these groups typically contain only 2–3 orders. Data collection for population level For the population-level analysis, body size data were collected from 803 published references (see Table S10). These records included body size measurements (in mm) and the geographic coordinates of collection sites. When coordinates were unavailable, location names were used. Records were excluded if they lacked species identification, geographic coordinates, or location information. To ensure consistency, only data from wild populations were included; records from laboratory-reared or captive animals were excluded. The analysis focused on adult individuals, omitting records where the life stage was unknown or identified as non-adult. Additionally, 1791 field records were incorporated: 1434 for 18 amphibian species and 357 for 30 reptile species. Anomalous measurements, defined as those deviating by more than 20% from the median body size for each species, were excluded to maintain data quality. To ensure robust estimates of intraspecific body size variation, only species with data from more than five populations and at least 10 individuals per population were retained. This filtering minimized bias and ensured adequate trait representation. The final dataset comprised 786 species, including 149 amphibians, 126 reptiles, 282 birds, and 229 mammals. Climate variables For this study, we utilized two key climate variables from the WorldClim dataset (version 2.0; Fick & Hijmans 2017) at a 30-second spatial resolution: Bio1 (Annual Mean Temperature) and Bio12 (Annual Precipitation). Other Predictors To identify life history traits as potential drivers of Bergmann’s rule, species were classified into ecological groups based on various predictors, including zone, ecosystem, trophic level, nesting site, reproductive mode, sex determination, foraging mode, activity time, habitat openness, migration, and hibernation. Data limitations led to some variation in life-history traits across animal lineages. To test the retention mechanism hypothesis ( Steudel et al. 1994 ), species were categorized into tropical and non-tropical zones based on the absolute midpoint latitude of their geographic range, using 23.5° latitude as the cutoff (He et al. 2023). Habitat types, such as ecosystems and nesting sites, or the ability to utilize diverse habitats, were used as proxies for species niche breadth, which is expected to be positively correlated with body size variation across latitude as predicted by the niche variation hypothesis ( Scheele et al. 2017; Van Valen 1962 ). Diet, a key ecological trait often overlooked in large-scale analyses, was also considered, as herbivores generally have higher energetic demands than carnivores and are more likely to follow Bergmann’s rule ( Burness et al. 2001; Clauss et al. 2013; Espinoza et al. 2004 ). Reproductive output can be influenced by biomass production per unit of body size across latitudes. Therefore, reproductive mode and sex determination were separately analyzed to assess their potential role in explaining body size patterns in the context of Bergmann’s rule ( Charnov et al. 2007; Williams 1966 ). Species-inherent traits, such as habitat openness, activity time, migration, and hibernation, were also considered as potential factors contributing to the understanding of body size patterns in the context of Bergmann’s rule (He et al. 2023). Data was sourced from the most updated references, or, if unavailable, from the next most recent datasets (see Table S10), to investigate ecological and evolutionary mechanisms driving body size variation and to test the role of ecological traits in shaping Bergmanian trends. Statistical analysis We conducted statistical analyses on a dataset encompassing body size information for all major vertebrate groups (amphibians, reptiles, birds and mammals) to maximize taxonomic coverage. Species lacking available DNA data were excluded, and analyses were repeated using phylogenies reconstructed from molecular data. To account for evolutionary and ecological variation, models were also constructed separately for each order or family. Additionally, we constructed models for amphibian orders (Anura, Caudata, and Gymnophiona) and reptiles (Squamata and Testudines). To evaluate the hypothesis that latitudinal gradients influence body size at the species level, we employed linear mixed models (LMMs) with log-transformed body size as the response variable. This transformation ensured the assumptions of normality and homogeneity of variance were met ( Changyong et al. 2014 ). Separate models were constructed for each trait, with latitude as a fixed predictor, analysed using ordinary least squares regression within the LMM framework ( Clobert et al. 1998; Dunham & Miles 1985 ). To account for the nested structure of the data, taxonomic family was included as a random effect (R package lmerTest; Kuznetsova et al. 2017) . Also, we quantified the relationship between species’ body size (log-transformed) and the midpoint of their absolute latitudinal range using Bayesian phylogenetic generalized linear mixed models (MCMCglmm; Hadfield 2017) . Phylogenetic covariance was incorporated by including a phylogenetic variance-covariance matrix as a random effect ( Freckleton et al. 2002 ). To address phylogenetic uncertainty, we ran models across a sample of 100 phylogenetic trees and summarized slope estimates for the latitudinal effect by calculating mean values across these models. Priors for the phylogenetic and residual variances were set at V=1 and 𝜈=0.02 ( Hadfield 2017 ). Each model was run for 11,000 iterations with a burn-in of 1,000 and a thinning interval of 100, yielding 100 effective samples. We report posterior means, 95% credible intervals, and pMCMC values for slope estimates, with effects considered significant if the 95% credible interval did not overlap zero. Results from linear mixed models (LMMs) and MCMCglmm models were qualitatively similar across all traits. Thus, we present MCMCglmm results in the main text, while LMM results are provided in Table S1. Phylogenetic signal was assessed by estimating heritability (h 2 ), representing the proportion of variance attributable to phylogeny ( Freckleton et al. 2002) , analogous to Blomberg’s K ( Blomberg et al. 2003 ) or Pagel’s λ ( Pagel 1999 ). To assess body size variability at the population level, we log-transformed body size data and included only species with data from at least five distinct populations. A meta-analysis framework was applied using the rma function from the metafor package ( Viechtbauer 2010 ) to evaluate general trends across populations. We categorized species based on their conformity to Bergmann’s rule, identifying those with positive trends, negative trends, or non-significant relationships (Table S5). To examine whether temperature correlates with latitude, we developed models incorporating temperature as a fixed predictor. Both LMMs and MCMCglmm models were fitted with body size as the response variable and temperature as the predictor. Additionally, given the known influence of precipitation on body size in some vertebrates ( Adolph & Porter 1996; Angilletta et al. 2004 ), we constructed further models to test body size responses to precipitation, using the same methodology as for temperature. To test the robustness of our results, we conducted a sensitivity analysis 9 for species-level data using alternative datasets and life-history traits. Recognizing that body size itself is a life-history trait, we focused on comparing differences in other life-history traits among groups of animals at the species level that either adhere to or deviate from Bergmann’s rule. For population-level data, which had non-normal distribution, we used Chi-Squared tests ( Okoye & Hosseini 2024 ) to analyse differences in body size categories across life-history traits and the independent effects of life-history traits among the three Bergmann’s rule categories ( p > 0.05): species exhibiting larger body sizes with increasing latitude (”follow”), species showing smaller body sizes with increasing latitude (”reverse”), and species with no significant latitudinal trends (”not significant”). Results Testing Bergmann’s rule at the species level We used Bayesian phylogenetic generalized linear mixed models (MCMCglmm) to test Bergmann’s rule across major terrestrial vertebrate groups. The posterior mean slope estimates for body size–latitude correlations revealed significant variation among groups (Table 1). In ectothermic vertebrates, amphibians (β = 0.0008, pMCMC = 0.06) and reptiles (β = -0.0002, pMCMC = 0.92) showed a weak, non-significant relationship between body size and latitude. In contrast, endothermic vertebrates displayed a clear positive body size–latitude relationship (birds: β = 0.0021, pMCMC < 0.001; mammals: β = 0.0018, pMCMC < 0.001), consistent with Bergmann’s rule. These findings indicated that Bergmann’s rule was strongly supported in endotherms but not in ectotherms at the species level (Table 1 and Fig. 1). To further explore the applicability of Bergmann’s rule, we examined patterns across orders and families within terrestrial vertebrate lineages (Fig. 2 and Table S3). Bergmann’s rule was valid in 22% (9/41) and 15.4% (8/52) of families in amphibians and reptiles, and 41.9% (13/31) and 21.1% (4/19) of orders in birds and mammals (Fig. 2 and Table S3). Consequently, support for Bergmann’s rule was stronger in endotherms (34%) compared to ectotherms (18%) when analysed at the family or order level ( X 2 = 4.434, df = 1, p = 0.035). Additionally, ectotherms demonstrated a higher percentage of species that reversed Bergmann’s rule compared to endotherms (20% vs. 4%; X 2 = 7.406, df = 1, p = 0.007), with 14.6% (6/41) in amphibians, 25% (13/52) in reptiles, 3.2% (1/31) in birds, and 5.3% (1/19) in mammals. The high heritability of the models (most with ℎ² > 0.9) suggested that much of the variation in body size could be explained by phylogenetic relationships (Table 1). Despite substantial variation in the strength and direction of body size–latitude relationships across taxonomic groups, we observed stronger support for Bergmann’s rule in endotherms compared to ectotherms. At the species level, body size was negatively correlated with temperature and precipitation along latitudinal clines in birds and mammals but not in amphibians and reptiles (Fig. S1–S2; Tables S1–S2). We also identified several ecological traits driving Bergmann’s rule (Table S4, Fig. S3). Bergmann’s rule was more strongly supported in non-tropical than tropical species and in terrestrial than aquatic species. Non-hibernating mammals and migratory or open-habitat birds followed Bergmann’s rule more consistently than their relatives. Other life-history traits, including diet and activity time, were not significant predictors for Bergmann’s rule in all vertebrates, although herbivorous birds and mammals, as well as amphibians and birds with longer activity times, followed Bergmann’s rule (Table S4, Fig. S3). Overall, these findings suggested that Bergmann’s rule was likely influenced by a combination of ecological factors rather than a single predictor. Testing Bergmann’s rule at the population level We used a meta-analysis framework to evaluate general trends of body size across populations in species with data from more than five populations. Among ectotherms, 14.8% of amphibians and 23% of reptiles followed Bergmann’s rule, while 15.4% and 9.5% of these groups, respectively, exhibited trends consistent with the reverse of Bergmann’s rule (Fig. 3). In endotherms, a similar proportion of bird and mammal species (~22%) followed Bergmann’s rule ( X 2 = 1.363, df = 1, p = 0.243), whereas 22.7% of birds and 28.4% of mammals exhibited trends consistent with the reverse Bergmann’s rule (Fig. 3). Additionally, support for Bergmann’s rule was not stronger at the population level than at the species level, in terms of the percentage of species (20.7%, 163/786) and families/orders (23.7%, 34/143) following Bergmann’s rule ( X 2 = 0.668, df = 1, p = 0.414; Tables S3 and S5). At the population level, body size was negatively correlated with temperature in 16.5% of species (n = 130) and with precipitation in 19.3% of species (n = 152), but no significant correlation was observed in the remaining species (Tables S6–S7, Fig. S1-S2). Species adhering to Bergmann’s rule (the “follow” group) exhibited a higher proportion of non-tropical species compared to those reversing the rule (the “reverse” group) or showing no significant latitudinal trends (the “non-significant” group) in amphibians and reptiles, but not in birds and mammals (Tables S8–S9). Additionally, the ”follow” group included a higher proportion of carnivorous species in reptiles and birds, as well as a greater proportion of species inhabiting dense habitats in birds, compared to the ”reverse” and ”non-significant” groups. Furthermore, the “reverse” group comprised a higher proportion of nocturnal species compared to the “follow” and “non-significant” groups (Tables S8–S9, Fig. S4). However, the three groups did not differ significantly in species proportions concerning other ecological traits, including ecosystem type, reproductive mode, foraging strategy, sex determination, migration, and hibernation (Tables S8–S9, Fig. S4). Overall, our analysis showed results similar to those at the species level and suggested that Bergmann’s rule of body size is likely influenced by a combination of ecological factors rather than a single predictor. Discussion Our analysis of body size–latitude correlations across nearly all terrestrial vertebrate species reveals that Bergmann’s rule is not universally applicable. While the rule holds for endothermic species, including birds and mammals, it is not valid for ectothermic species such as amphibians and reptiles (Table 1 and Fig. 1). Additionally, the percentage of orders or families following Bergmann’s rule is significantly higher in endotherms (34%) than in ectotherms (22%) (Fig. 2), but only slightly higher in endotherms (22%) than in ectotherms (18%) at the population level (Fig. 3). Overall, we observed stronger support for Bergmann’s rule in endotherms compared to ectotherms. These findings challenge the traditional interpretation of Bergmann’s rule as a broadly applicable biogeographic principle and highlight the importance of considering taxonomic groups and ecological context ( Bergmann 1847; Meiri 2011; Steudel et al. 1994 ). The weaker support for Bergmann’s rule in ectotherms than in endotherms likely stems from differences in thermoregulatory mechanisms ( Rodríguez et al. 2006 ). Endotherms rely on metabolic heat production, making larger body sizes advantageous for heat retention in colder climates ( Bergmann 1847; Rodríguez et al. 2006 ). In contrast, ectotherms depend on external heat sources, and larger body sizes may be disadvantageous in cold environments due to slower heat acquisition, which can hinder feeding, mating, and predator avoidance ( Meiri et al. 2020; 29. Pincheira-Donoso et al. 2007; Tracy et al. 2005; Vidan et al. 2017 ). This thermoregulatory constraint may explain why many ectotherms exhibit patterns opposite to Bergmann’s rule. More broadly, our findings suggest that Bergmann’s rule may not universally apply to all animals in the context of ongoing rapid environmental changes ( Bellard et al. 2012; Gardner et al. 2011; Parmesan & Yohe 2003; Stephenson et al. 2010 ). The differential sensitivity of endotherms and ectotherms to environmental changes has profound implications for their ability to adapt to climate change. For example, numerous studies have documented body size shrinkage in birds and mammals in response to global warming, a phenomenon consistent with the heat retention mechanism (Sheridan & Bickford 2011; Zheng et al. 2023). These adaptive responses may enhance their resilience to changing climates, allowing them to maintain fitness and survival in warmer environments. In contrast, amphibians and reptiles, with their weaker or reversed latitudinal body size clines, appear to be more sensitive to temperature changes (Fig. 2 and 3). Our findings at the order level further support this: while some ectothermic lineages (e.g., Caudata) show limited adherence to Bergmann’s rule, the majority exhibit no significant trends or reverse patterns (Fig. 2 and 3). Moreover, ectotherm families demonstrate a higher percentage of species that reverse Bergmann’s rule (14.6–25%) compared to endotherm orders (3.2–5.3%). These results highlight their limited adaptive capacity and heightened vulnerability to climate change, which may increase their risk of extinction. Indeed, amphibians and reptiles are among the most threatened vertebrate groups, with climate change being a major driver of their declines ( Huey et al. 2010; Liao et al. 2016; Mi et al. 2023; Sinervo et al. 2010; Wake & Vredenburg 2008 ). This underscores the urgent need for conservation strategies that address the unique vulnerabilities of ectotherms in the face of global climate change. While our study provides a comprehensive assessment of Bergmann’s rule across terrestrial vertebrates, several limitations should be acknowledged. First, our analyses rely on available species-level and population-level data, which may not fully capture the ecological and genetic diversity within lineages. Second, due to insufficient data, we cannot currently assess the impact of climate change or seasonality on changes in animal body size, which could have significant effects ( Bro-Jørgensen 2008; Calder 1974; Sheard et al. 2020; Sheridan & Bickford 2011). Third, while we identified several ecological traits associated with Bergmann’s rule (Fig. S3-S4), the mechanisms underlying these associations remain poorly understood. Future studies should incorporate high-resolution environmental data and experimental approaches to better elucidate the drivers of body size variation. In conclusion, our findings demonstrate that Bergmann’s rule is not a universal principle but rather a context-dependent pattern shaped by thermoregulatory mechanisms, ecological traits, and phylogenetic history. Endotherms, particularly birds and mammals, exhibit stronger adherence to Bergmann’s rule, reflecting their capacity for rapid morphological responses to environmental changes. 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Summary of statistics from Bayesian phylogenetic generalized linear mixed models predicting the body size by species range latitude and temperature Amphibian All (latitude) 4217 0.0008 [-0.0012, 0.0029] 0.06 0.9187 All (temperature) 4217 -0.0017 [-0.0037, 0.0002] 0.12 0.9170 Reptile All (latitude) 4840 -0.0002 [-0.0017, 0.0009] 0.66 0.9550 All (temperature) 4840 0.00004 [-0.0017, 0.0018] 0.92 0.9390 Aves All (latitude) 4541 0.0078 [ 0.0068, 0.0088] 0.01 0.8386 All (temperature) 4541 -0.0099 [-0.0117, -0.0085] 0.01 0.8386 Mammal All (latitude) 6425 0.0062 [0.0049, 0.0072] 0.01 0.7863 All (temperature) 6425 -0.0070 [-0.0093, -0.0048] 0.01 0.7870 Abbreviations: CI - credible interval; h2 (heritability) - the proportion of residual variance attributable to phylogenetic relationships; n - sample size; pMCMC - p values from Bayesian phylogenetic generalized linear mixed models; β - posterior mean slope estimates. Fig. 1 Relationships between body size and latitude across terrestrial vertebrates. a, Amphibians, b, Reptiles, c, Birds, d, Mammals: relationships between log-transformed body size and absolute latitude. SVL - snout-vent length; BL - body length. Shaded areas represent credible intervals. Animals’ icons were created in BioRender (https://BioRender.com/o63a116). Fig. 2 Posterior slope estimates and percentages for body size−latitude relationships across terrestrial vertebrate orders/families. Dots represent the mean slope estimates for the body size−latitude correlations with 95% credible intervals. The solid line represents the location where the posterior mean slope estimates equal zero. Different colours indicate different patterns of body size changes along latitudes: yellow – species conforming to Bergmann’s rule (larger body size with increasing latitude), red – species not conforming to Bergmann’s rule (smaller body size with increasing latitude), and cyan – species with no statistically significant relationship. Animals’ icons were created in BioRender (https://BioRender.com/o63a116) Fig. 3 Summary of population-specific body size changes by latitude across terrestrial vertebrate groups. a, Amphibians (n = 149), b, Reptiles (n = 126), c, Birds (n = 282), and d, Mammals (n = 229). Different colours indicate different patterns of body size changes along latitudes: yellow – species conforming to Bergmann’s rule (larger body size with increasing latitude), red – species not conforming to Bergmann’s rule (smaller body size with increasing latitude), and cyan – species with no statistically significant relationship. Animals’ icons were created in BioRender (https://BioRender.com/o63a116) Information & Authors Information Version history V1 Version 1 18 April 2025 Peer review timeline Published Ecology Version of Record 18 Feb 2026 Published Copyright This work is licensed under a Non Exclusive No Reuse License. Keywords bergmann's rule interspecific intraspecific latitude temperature Authors Affiliations Oleksandra Oskyrko 0000-0003-0092-4193 Institute of Zoology Chinese Academy of Sciences View all articles by this author Jiajia Liu 0000-0002-1923-5964 Fudan University View all articles by this author Weiguo Du 0000-0002-1868-5664 [email protected] Fudan University View all articles by this author Metrics & Citations Metrics Article Usage 522 views 224 downloads .FvxKWukQNSOunydq8rnd { width: 100px; } Citations Download citation Oleksandra Oskyrko, Jiajia Liu, Weiguo Du. Is Bergmann's rule valid for terrestrial vertebrates?. Authorea . 18 April 2025. 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