Individual Specialization in a Generalist Apex Predator: The Leopard Seal

Abstract

Apex predators are typically considered dietary generalists, which often masks individual variability. However, individual specialization-consistent differences among individuals in resource use or ecological role-is common in apex predators. In some species, only a few specialized individuals can significantly impact prey populations. Leopard seals (Hydrurga leptonyx) are apex predators important to the structure and function of the Southern Ocean ecosystem. Though broadly described as generalists, little is known about their trophic ecology at the population or individual level. We analyzed δ¹³C and δ¹⁵N profiles in whiskers (n = 46) from 34 leopard seals in the Western Antarctic Peninsula to assess trophic variation. We also evaluated individual consistency across years using repeat samples from 7 seals over 2-10 years. We compared population and individual isotopic niche space and explored drivers of intraspecific variation in leopard seal trophic ecology. We find that leopard seals have a broad trophic niche (range: 6.96%-15.21‰) and are generalists at the population level. However, most individuals are specialists (59% for δ¹⁵N and δ¹³C), with only a few generalists (13% for δ¹⁵N, 6% for δ¹³C). Individuals also specialize at different trophic levels. Most variation in trophic ecology is driven by individual specialization, but sex and mass also contribute. We also find that some seals specialize over time, consistently foraging at the same trophic level, while others switch within and between years. This suggests some seals may disproportionately impact prey, especially when specialists consistently target specific species. Long-term specialization by a few leopard seals likely contributed to the decline of the local Antarctic fur seal population. Our findings show the importance of examining individual specialization in leopard seals across their range to understand their impact on other prey populations. This approach should be applied to other apex predator populations, as a few specialists can significantly impact ecosystems.

Keywords: foraging strategies; individual variation; intraspecific competition; marine mammal; niche variation; specialist.

FIGURE 1

Conceptual model showing four different population‐level patterns of dietary specialization based on isotope signatures of individual diets (δ¹⁵N) over time adapted from (Vander Zanden et al. 2010). Circles represent individuals and their δ¹⁵N value for a layer of inert tissue (e.g., whisker, baleen, and claws) reflecting diet. Arrows track changes in individual δ¹⁵N values through time. (A) A specialist population with a small isotopic niche width composed of four specialist individuals with overlapping δ¹⁵N values; (B) A generalist population with a large isotopic niche width composed of four different δ¹⁵N specialist individuals; (C) A generalist population with a large isotopic niche width composed of four generalist individuals; (D) A generalist population with a large isotopic niche width composed of two generalist and two specialist individuals.

FIGURE 2

Isotopic analysis of leopard seal whiskers. (A) Population‐level isotopic space (δ¹⁵N and δ¹³C) for leopard seals color‐coded by δ¹⁵N specialization category: High trophic‐level specialist (H‐Specialist; blue), medium‐to‐low trophic‐level specialist (ML‐Specialist; green), intermediate (orange), and generalist (yellow). Each point represents the average isotopic value for an individual's whisker(s). Ellipses show standard isotopic ranges for each δ¹⁵N specialization category: Dashed for 75% and solid for 50% of the data. A pie chart shows the proportion of the population in each δ¹⁵N specialization category. (B) Representative plots of δ¹⁵N signatures for a leopard seal whisker of each δ¹⁵N specialization category (HL‐Specialist: Seal 84; ML‐Specialist: Seal 144; Intermediate: Seal 37; Generalist: Seal 111). Leopard seal art 2024 Roger Hall inkart.net. Leopard seal photo by Renato Borras‐Chavez.

FIGURE 3

Individual‐level isotopic space (δ¹⁵N and δ¹³C) for leopard seals color‐coded by δ¹⁵N specialization category: High trophic‐level specialists (H‐Specialists; blue variations), medium‐to‐low trophic‐level specialists (ML‐Specialists; green and purple variations), intermediates (orange and red variations), and generalists (yellow and pink variations). Ellipses represent the standard isotopic ranges encompassing 75% of the data for each individual's whisker(s), while points indicate the average isotopic value for each individual's whisker(s). The dotted lines represent the δ¹⁵N range for each category. Leopard seal art 2024 Roger Hall inkart.net.

FIGURE 4

Comparison of isotopic consistency between a long‐term δ¹⁵N H‐S for specialist (A; Seal 397) and a variable specialist (B; Seal 12) across different years. Bivariate plots of isotopic space (δ¹⁵N and δ¹³C) for Seal 397 (C), showing consistency in δ¹⁵N values across the years, and for Seal 12 (D), showing variation in δ¹⁵N values across the years. Each point represents an individual whisker segment. The dark gray polygon shows the individual's total isotopic space. The light gray polygon shows the population's total isotopic space. The colors represent the δ¹⁵N specialization category assigned each year: High δ¹⁵N specialists (H‐Specialists) in blues, medium‐to‐low specialists (ML‐Specialists) in green, and generalists in yellow. Box and whisker plots of δ¹⁵N values for Seal 397 (E) and Seal 12 (F) show examples of consistency (E) in δ¹⁵N isotope values across different years compared to variability associated with prey switching (F). In both plots, horizontal bars represent the mean concentrations for each individual and the ends represent the range. Leopard seal photos by Dan Costa.

FIGURE 5

Yearly trends in δ¹⁵N values for leopard seals (2012–2023). Top panel shows the yearly δ¹⁵N values with the fitted trend (blue line) and the 95% confidence interval (shaded area). Bottom panel shows the slope of the seasonal rate of change in the predicted δ¹⁵N values based on the first derivative of the fitted GAM shown in the upper panel. The bottom panel displays temporal windows where significant linear increases or decreases of isotopic values occurred in time, with the red line representing the fitted trend. Blue and red shaded areas indicate non‐significant and significant effects, respectively, at the 95% confidence level.