An inverse latitudinal gradient in individual niche specialization: the case of pinnipeds

Latitudinal gradients in species richness are well documented across taxa, yet how intraspecific ecological diversity varies across broad spatial scales remains unclear. We compiled stable isotope data from individuals across 21 pinniped populations spanning 15°S to 70°N to test whether individual diet specialization follows a latitudinal gradient. Using a standardized framework that partitions total niche width into within- and between-individual components, we show that pinniped populations exhibit a latitudinal gradient of individual specialization: individuals become more specialized toward the poles. This pattern was driven by increased between-individual variation rather than within-individual niche expansion, without leading to changes in total population niche width. Our findings challenge the expectation that greater ecological opportunity in the tropics promotes stronger individual specialization. Instead, prey evenness and productivity pulses may facilitate greater intraspecific niche divergence at higher latitudes. By revealing a novel macroecological pattern of intraspecific diet specialization, our study underscores the importance of integrating individual-level processes into explanations of large-scale biodiversity gradients and food-web dynamics.

Latitudinal gradients of biodiversity have long been documented in terms of the number of species in local communities or biogeographic regions (Willig et al. 2003, Hillebrand 2004, Grenyer et al. 2006, Barboza and Defeo 2015) . However, much less is known about how intraspecific ecological diversity varies from the poles towards the Equator. Because intrapopulation niche variation is prevalent across diverse taxa and shapes biodiversity patterns at higher levels of biological organization (Costa-Pereira et al. 2019, Arroyo‐Correa et al. 2023), understanding the macroecology of individual specialization can offer new insights into the historical debate about the causes and consequences of latitudinal gradients of biodiversity (Villegas-Amtmann et al. 2011, Araújo and Costa-Pereira 2013). The niche width of any population (total niche width, TNW) is a result of the combination of two facets of intraspecific resource use variation: niche variation within individuals (within-individual component, WIC) and among individuals (between-individual component, BIC) (Roughgarden 1972). Thus, the degree of individual specialization in a population is high when BIC contributes largely to TNW relative to WIC, which can have significant ecological and evolutionary impacts, influencing community dynamics and species interaction (Bolnick et al. 2011). From a macroecological perspective, intraspecific variation in resource use across populations tends to increase towards lower latitudes, suggesting a strong contribution of the ecological opportunity (i.e., diversity of available resources) to the latitudinal gradient of intraspecific diversity (Araújo and Costa-Pereira 2013). However, it remains unclear whether latitudinal patterns in individual specialization are consistent across different ecosystem types (e.g., marine vs. terrestrial), and which ecological and environmental factors contribute most to variation in individual specialization at broad spatial scales. Tropical and subtropical regions generally have higher resource diversity and habitat heterogeneity, which create opportunities for individuals to diverge in their niches by adopting distinct foraging strategies, thereby increasing the population niche width via increased individual diet specialization (Tittensor et al. 2010, Araújo et al. 2011, Stuart-Smith et al. 2013). In turn, toward the poles, environments generally show limited prey diversity, which is expected to constrain individual diet specialization (Araújo et al. 2011, Araújo and Costa-Pereira 2013). Predictions become more complex when we consider that multiple ecological factors, in addition to environmental prey diversity, also vary latitudinally and may interactively determine the degree of individual variation (Araújo et al. 2011, Costa‐Pereira et al. 2018). For example, as species richness tends to increase in tropical areas, interspecific competition is expected to intensify toward the tropics (Schemske et al. 2009, Hargreaves 2024). Because the effects of these ecological factors on individual specialization may not be consistent across taxa with different feeding modes, examining how intraspecific niche diversity changes with latitude across populations and species from the same taxonomic family can help reveal when and how we can predict patterns of individual variation in broad geographic contexts. Pinnipeds (i.e., seals, fur seals, sea lions and walrus) are top predators in marine ecosystems that integrate multiple energy pathways with a broad latitudinal range(McCann et al. 2005, Rooney et al. 2006, Arim et al. 2010, Keppeler et al. 2021). These species exhibit a wide range of intraspecific trophic strategies, with populations comprising generalist or highly specialized individuals (Kernaleguen et al. 2012, Cárdenas-Alayza et al. 2022, de Lima et al. 2022). Moreover, pinnipeds often breed in dense colonies and frequently occur in sympatry (Kernaleguen et al. 2012, Franco-Trecu et al. 2014), which is expected to intensify ecological interactions. We hypothesize two opposing scenarios for how individual specialization varies with latitude. Based on known environmental drivers of individual specialization (Araújo et al. 2011), we expected pinnipeds to exhibit higher levels of specialization in tropical regions due to increased ecological opportunity. Alternatively, the relatively lower productivity of many tropical marine ecosystems (Tittensor et al. 2010, Chakraborty et al. 2020) could intensify interspecific competition, thereby reducing the degree of individual specialization in pinniped populations.

We compiled raw stable isotope data from wild pinniped populations with repeated measures per individual (i.e., sequential whisker sections of each or measures of multiple tissues of each individual), using publicly available repositories. We used longitudinal stable isotope data to quantify individual diet specialization in δ¹³C and δ¹⁵N values, which are widely used to quantify variation in resource use in pinnipeds (Kernaléguen et al. 2015, Garrido‐de León et al. 2025), reflecting the origin of primary production and trophic level, respectively (Newsome et al. 2007). Our search initially identified 189 studies containing potential raw datasets (see Supplementary Material for details on data acquisition and database preparation). After reviewing the pre-selected literature and applying our inclusion criteria, we retained only studies with publicly available datasets. Finally, we compiled stable-isotope data from 21 pinniped populations across 10 species, spanning a broad latitudinal range from 15.3°S to 70.7°N (Fig. S1, Supplementary material). The raw isotopic dataset compiled to address the research questions of this study is available at Material Supplementary (’ Raw isotope data’ ). We quantified the degree of individual trophic specialization (ITS) as WIC/TNW with the RInSp R package using continuous resource data (Zaccarelli et al. 2013). Following Roughgarden’s variance-partitioning framework, we estimated the TNW (i.e the sum of WIC and BIC) as the total variance of isotopic values at population-level (Roughgarden 1972). WIC was estimated as the mean variance of individuals’ repeated measures, and BIC was subsequently derived as TNW – WIC. Thus, WIC captures within-individual variability around each individual’s mean, whereas BIC reflects differences among individuals in their average isotopic values To make the measure more straightforward to interpret, we expressed ITS as a V index = 1 - ITS, where V represents diet variation in resource use (Bolnick et al. 2007). This index ranges from 0 to 1, with higher values reflecting populations with a larger proportion of specialized individuals (Table S1, Supplementary Material). To examine how total niche components and individual specialization vary latitudinally, we regressed V index, TNW, WIC, and BIC on absolute latitude. We fitted phylogenetic generalized least squares (PGLS) regression models, which accounts for shared evolutionary history and species-level heterogeneity, thereby controlling for non-independence among species (Grafen 1989). We used a maximum likelihood approach to estimate the phylogenetic signal in the residuals (λ) (Pagel 1999). (See methodological details in Table S2). We obtained the pinniped phylogeny from VertLife (http://vertlife.org/), which provides ultrametric phylogenetic trees generated from molecular data and time-calibrated birth–death models (Jetz 2014). All analyses were performed in R software (R Development Core Team 2024).

We compiled data from 469 individuals [(mean per population (SD) = 29 individuals (27)] and 7,742 isotopic measures [(mean per individual = 22 measures (16)]. We found a positive linear relationship between absolute latitude and V index for both stable isotopes (Fig. 1a, b; Table 1, estimate = 0.01, p-value = 0.02 for δ¹³C; estimate = 0.005, p-value = 0.002 for δ¹⁵N). This means that individual specialization within pinniped populations increases from the equator to the poles. We found no evidence of phylogenetic signals in the regression models for either isotope (Table S2, Supplementary material), indicating that residuals were distributed independently of the phylogenetic relationships among the analyzed species. Moreover, we found a significant effect of absolute latitude on BIC for δ¹³C (Fig. 1c), with BIC increasing toward higher latitudes (Table 1; estimate = 0.02, p-value = 0.01) and showing no evidence of phylogenetic signal (Table S2). In contrast, no significant latitudinal effect was observed for TNW or WIC for either δ¹³C or δ¹⁵N, nor for BIC in δ¹⁵N (Table 1).

The latitudinal gradient in species richness and diversity has a long history in the intersection of ecology, evolution and biogeography (Rohde 1992, Gaston 2000). Going beyond species-level patterns, we explored whether individual diet specialization in pinnipeds varies predictably with latitude. While previous studies have documented diet specialization at local scales for this group (Franco-Trecu et al. 2014, Drago et al. 2015, de Lima et al. 2019, Cárdenas-Alayza et al. 2022), integrative efforts to examine macroecological patterns within a consistent analytical framework remain scarce (Franco‐Trecu et al. 2022). We found that individuals exhibit greater individual diet specialization through niche divergence (i.e., increased between-individual variation) in temperate and polar ecosystems. These results suggest that lower productivity in tropical oceans may increase interspecific competition, potentially leading to reduced individual specialization (Araújo et al. 2011). Therefore, our study provides insights into how individual-level dietary variation contributes to a latitudinal gradient in intraspecific niche specialization, potentially reflecting underlying geographic gradients in resource availability and species interactions. More than a decade ago, Araújo and Costa-Pereira (2013) compiled data from the literature across diverse taxa and found that populations contain more trophically diverse individuals towards lower latitudes, an intraspecific macroecological pattern that mirrors the classic latitudinal gradient of species richness. However, here we found the opposite trend across pinniped populations. This inverse latitudinal gradient of individual specialization suggests that, rather than increased ecological opportunity in the tropics, as suggested by Araújo and Costa-Pereira (2013), the latitudinal variation of other environmental drivers of intraspecific niche variation may explain the stronger individual specialization towards the poles in pinnipeds. Although species richness typically declines toward the poles, temperate and polar marine regions often exhibit higher prey evenness and local abundance, accompanied by strong, seasonal pulses in marine productivity (Villegas-Amtmann et al. 2011, Stuart-Smith et al. 2013). A more even prey community may increase the potential for intrapopulational niche diversification, particularly as pinnipeds exploit a wide range of prey types throughout the productivity cycle in temperate waters. In contrast, tropical systems exhibit relatively higher taxonomic richness, resulting in greater ecological opportunity. However, marine prey communities are often dominated by a few abundant species, resulting in low evenness and lower total biomass (Stuart-Smith et al. 2013). Despite the definition of ecological opportunity as a driver of individual specialization that deals with the diversity of available resources sensu (Araújo et al. 2011), our results suggest that prey evenness can also play a role in shaping the variety of foraging opportunities individuals experience in marine ecosystems. Therefore, we suggest that quantifying ecological opportunity as a driver of individual specialization may be more complex than initially thought, as prey species richness per se does not necessarily measure the scope for niche variation within populations, which limits our ability to understand and predict individual specialization across large geographic scales. In terrestrial ecosystems, the number of trophic levels —as inferred from community level variation in δ¹⁵N— has been shown to increase with latitude (Vulla et al. 2009). Thus, polar environments may exhibit longer food webs, resulting in higher scope for intraspecific variation in δ¹⁵N in generalist predators (Bonin et al. 2020). If a similar pattern applies to marine ecosystems, it may help explain why individual specialization in trophic resource use increases toward higher latitudes. However, marine food webs are characterized by pronounced seasonal dynamics that generate substantial isotopic variability at the base of the food web. For example, intra-annual variation in phytoplankton δ¹³C of up to 10–12‰ has been reported at high and mid-latitudes, compared with much narrower ranges (0–2‰) in tropical and subtropical regions (Magozzi et al. 2017). This pattern is driven by seasonal pulses in primary productivity and shifts in phytoplankton community composition (Villegas-Amtmann et al. 2011). Such baseline variability expands the isotopic space available to consumers and therefore needs to be considered when interpreting latitudinal patterns of isotopic variation. Although elevated isotopic dispersion at higher latitudes is consistent with increased individual-level differentiation, disentangling baseline-driven variability from behavioral niche divergence remains a key challenge in large-scale comparative analyses. Importantly, whiskers integrate dietary information over annual to multi-annual timescales (Hirons et al. 2001, Cherel et al. 2007), such that individuals repeatedly experience the full range of seasonal and interannual baseline variability characteristic of their environment. Under this temporal-integration framework, persistent isotopic divergence among individuals is unlikely to arise solely from baseline heterogeneity and instead points to consistent differences in how individuals exploit temporally and spatially variable trophic resources. Our results show that increased individual specialization towards the poles is achieved through a higher contribution of the between-individual component of niche variation (BIC) to the total population-level variation in δ¹³C and marginal significance for δ¹⁵N. However, surprisingly, this latitudinal increment in interindividual niche divergence did not result in a higher total niche width for the population. This pattern suggests a non-consistent variation in WIC along the latitudinal gradient that offsets the expected expansion of the population niche. Also, it contradicts predictions from theory and previous empirical studies (Bolnick et al. 2007, Costa et al. 2008, Maldonado et al. 2017), indicating that the variation in BIC does not necessarily reflect population niche expansion. The latitudinal increase in BIC may be associated with more diverse foraging strategies and habitat use for pinniped consumers. For instance, in the Antarctic population of leopard seals ( Hydrurga leptonyx ), individual specialization was primarily driven by divergence among individuals (BIC) rather than by niche expansion within individuals (Sperou et al. 2025). This example aligns with our findings and suggests that variation in foraging behavior and habitat use may underlie increased intraspecific trophic diversity towards higher latitudes. Therefore, our results stimulate future studies investigating the underlying mechanisms shaping the observed higher intraspecific foraging diversity towards the poles. Together, our findings reveal an inverse latitudinal gradient of individual diet specialization in pinnipeds, highlighting that this emergent phenomenon within populations can exhibit large-scale, macroecological patterns that vary across the tree of life and ecosystem types. Our results also provide new insights into the intraspecific diversity of functional roles within species of top predators across marine ecosystems. Mobile predators link distinct trophic pathways (e.g., pelagic and benthic) and stabilize food web dynamics (McCann et al. 2005, Rooney et al. 2006). However, high individual specialization may limit this role, as specialist individuals exploit narrower niches and may move less across habitats (Quevedo et al. 2009, Matich et al. 2011), reducing their ability to integrate energy pathways and influence community structure at the level of individual predators (Rooney et al. 2006, Bolnick et al. 2011). Placing individual specialization in a biogeographic context can thus expand our understanding of how top predator populations and individuals act as agents of change in marine food webs. Ultimately, understanding the links between individual specialization, trophic position, and community processes is key to predicting food web resilience under global environmental change.

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Estimates (in log scale) and SE in brackets are shown for each parameter. *** and * denotes p <0.001 and significant terms, respectively. | Stable isotope | Model | Intercept | Latitude | | δ¹³C | log ( V index) ~ latitude | -1.69 (0.33) p = *** | 0.01 (0.00) p = 0.02* | | log (BIC) ~ latitude | -2.54 (0.66) p = 0.001* | 0.02 (0.01) p = 0.06 | | | log (WIC) ~ latitude | -1.51 (0.56) p = 0.01* | -0.006 (0.01) (0.01) p = 0.61 | | | log (TNW) ~ latitude | -2.12 (0.60) p = 0.002* | 0.02 (0.01) p = 0.06 | | | δ¹⁵N | V index ~ latitude | 0.19 (0.12) p = 0.12 | 0.005 (0.002) p = 0.02* | | log (BIC) ~ latitude | -2.5 (0.66) p = 0.001* | 0.02 (0.01) p = 0.06* | | | log (WIC) ~ latitude | -0.95 (0.58) p = 0.12 | -0.004 (0.01) p = 0.67 | | | log (TNW) ~ latitude | -0.86 (0.58) p = 0.15 | 0.008 (0.01) p = 0.46 | Information & Authors Information Version history Copyright This work is licensed under a Non Exclusive No Reuse License.

Authors Metrics & Citations Metrics Article Usage 191views 104downloads Citations Download citation Federico Garrido-de León, Raul Costa-Pereira, Valentina Franco-Trecu. An inverse latitudinal gradient in individual niche specialization: the case of pinnipeds. Authorea. 23 December 2025. DOI: https://doi.org/10.22541/au.176650222.29680129/v1 DOI: https://doi.org/10.22541/au.176650222.29680129/v1 If you have the appropriate software installed, you can download article citation data to the citation manager of your choice. Simply select your manager software from the list below and click Download. For more information or tips please see 'Downloading to a citation manager' in the Help menu.