| Summary: | As large-bodied predators, pinnipeds (seals, fur seals, sea lions, and walruses) play essential roles in the structure and function of marine ecosystems through consumer-prey interactions. Solid knowledge of their foraging ecology and trophic niche is critical to investigating changes in the structure of ecosystems over time. Diverse studies have shown that extant pinnipeds display divergent foraging strategies and habitat preferences. However, it is not well understood what mechanisms underlie these different strategies, nor how they have evolved and affected community structure. In this dissertation, I use stable isotope and morphometric analyses on museum specimens to examine the foraging ecology, niche segregation, and ecomorphology of extant pinnipeds and to uncover how extinct pinnipeds partitioned their ancient communities and how these dynamics compare to modern ones.In this dissertation, I show that sympatric otariids across the North Pacific and the Southern Hemisphere display distinct foraging modes, consistent with energetic tradeoffs associated with their body size disparities. Fur seals predominantly feed offshore and on pelagic food webs, whereas sympatric sea lions rely on nearshore and benthic resources, resulting in comparable niche segregation patterns across regions and stressing the driving role of body size in otariids' foraging behavior and niche partitioning. I also conducted an exhaustive examination of sympatric otariids from the eastern North Pacific, the most diverse otariid community in the world. I demonstrate that California sea lions (Zalophus californianus), Steller sea lions (Eumetopias jubatus), northern fur seals (Callorhinus ursinus), and Guadalupe fur seals (Arctocephalus townsendi) display significant differences in their size, feeding morphology, and foraging. Body size and feeding morphology are not related to foraging at the intraspecific level in a consistent way. However, when species and sex identity are excluded, foraging is significantly related to body size and ...
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