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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. Leopard seals are apex predators in the Southern Ocean. The species is broadly described as generalists; however, our study shows that most individual leopard seals are dietary specialists. This specialization has led to niche partitioning among individuals. Individual specialization, drives trophic variation, and we find differences between individuals based on sex and body size. We also find that some leopard seals forage consistently at one trophic level, while others switch trophic levels. In our study location, long‐term specialization by a few large adult female leopard seals has likely contributed to the decline of their prey, Antarctic fur seals. Our study highlights the ecological significance of individual variability in apex predators.

Evaluating physiological responses in the context of a species’ life history, demographics, and ecology is essential to understanding the health of individuals and populations. Here, we measured the main mammalian glucocorticoid, cortisol, in an elusive Antarctic apex predator, the leopard seal ( Hydrurga leptonyx ). We also examined intraspecific variation in cortisol based on life history (sex), morphometrics (body mass, body condition), and ecological traits ( δ 15 N, δ 13 C). To do this, blood samples, life history traits, and morphometric data were collected from 19 individual leopard seals off the Western Antarctic Peninsula. We found that adult leopard seals have remarkably high cortisol concentrations (100.35 ± 16.72 μg/dL), showing the highest circulating cortisol concentration ever reported for a pinniped: 147 μg/dL in an adult male. Leopard seal cortisol concentrations varied with sex, body mass, and diet. Large adult females had significantly lower cortisol (94.49 ± 10.12 μg/dL) than adult males (120.85 ± 6.20 μg/dL). Similarly, leopard seals with higher isotope values (i.e., adult females, δ 15 N: 11.35 ± 0.69‰) had lower cortisol concentrations than seals with lower isotope values (i.e., adult males, δ 15 N: 10.14 ± 1.65‰). Furthermore, we compared cortisol concentrations across 26 closely related Arctoid taxa (i.e., mustelids, bears, and pinnipeds) with comparable data. Leopard seals had the highest mean cortisol concentrations that were 1.25 to 50 times higher than other Arctoids. More broadly, Antarctic ice seals (Lobodontini: leopard seal, Ross seal, Weddell seal, crabeater seal) had higher cortisol concentrations compared to other pinnipeds and Arctoid species. Therefore, high cortisol is a characteristic of all lobodontines and may be a specialized adaptation within this Antarctic-dwelling clade. Together, our results highlight exceptionally high cortisol concentrations in leopard seals (and across lobodontines) and reveal high variability in cortisol concentrations among individuals from a single location. This information provides the context for understanding how leopard seal physiology changes with life history, ecology, and morphology and sets the foundation for assessing their physiology in the context of a rapidly changing Antarctic environment.

Species-specific welfare indicators are important in promoting positive welfare for zoo animals. Reptiles are a notoriously understudied group in regards to behavior, welfare needs, and husbandry requirements. Using opportunistically obtained samples, we evaluated how blood lactate is affected by variation in manual capture and restraint in crocodilians. Lactate is an indicator of anerobic metabolism in reptiles. It offers a relatively simple and indirect way to assess physiological stress. Determining the best way to reduce struggling during capture and associated stress is of great importance to these species’ overall welfare. Blood samples (N = 53) were collected from 10 different species of crocodilians. It was found that age class was a significant predictor variable of lactate levels after capture, and longer handling time seemed to cause an increase in lactate. Finally, draining enclosure pools for a small number of the captures was associated with higher lactate levels compared to other capture factors that were recorded. This study showed that welfare of captive crocodilians could be improved by shortening the duration of physical restraint events when possible. Developing objective measures of welfare and establishing baseline recommendations for care and handling of crocodilians will ultimately promote and improve their wellbeing, along with that of other understudied reptiles in zoos.

Little is known about the reproductive biology of the leopard seal (Hydrurga leptonyx), a Southern Ocean predator. Here we observed sexual behavior in wild leopard seals in Laguna San Rafael, Chile during a 2 h courtship interaction between a female and male. The female was hauled out on ice, mostly lying still (69% of the time) or moving (19%). The male was mostly under water (87%) or at the water’s surface (11%). The female made seven in-air calls (i.e., thump pulse, noseblast, blast, growl). The male produced 65 underwater calls (i.e., low- and high-double trills, unidentified trills). The underwater calls appeared to be directed toward the female. After the primary male vocalized for an hour, one or two unidentified leopard seals briefly swam near the female. After leaving the area, we heard underwater calls for another 8 h. The next day, the primary male was hauled out on ice with a swollen genital opening. The male was bleeding from a laceration caudal to the preputial opening, suggesting the male attempted to mate and that the female, or another seal, was responsible for the injury. Together, we find that leopard seal courtship involves a suite of behavioral and acoustic behaviors by both sexes, both in air and under water. This is the first description of leopard seal sexual behavior in the wild. Our study also provides the first evidence that leopard seals mate in South America.