Feeding morphology and body size shape resource partitioning in an eared seal community

Abstract

Body size and feeding morphology influence how animals partition themselves within communities. We tested the relationships among sex, body size, skull morphology and foraging in sympatric otariids (eared seals) from the eastern North Pacific Ocean, the most diverse otariid community in the world. We recorded skull measurements and stable carbon (δ¹³C) and nitrogen (δ¹⁵N) isotope values (proxies for foraging) from museum specimens in four sympatric species: California sea lions (Zalophus californianus), Steller sea lions (Eumetopias jubatus), northern fur seals (Callorhinus ursinus) and Guadalupe fur seals (Arctocephalus townsendi). Species and sexes had statistical differences in size, skull morphology and foraging significantly affecting the δ¹³C values. Sea lions had higher δ¹³C values than fur seals, and males of all species had higher values than females. The δ¹⁵N values were correlated with species and feeding morphology; individuals with stronger bite forces had higher δ¹⁵N values. We also found a significant community-wide correlation between skull length (indicator of body length), and foraging, with larger individuals having nearshore habitat preferences, and consuming higher trophic level prey than smaller individuals. Still, there was no consistent association between these traits at the intraspecific level, indicating that other factors might account for foraging variability.

Keywords: body size; community structure; foraging ecology; functional morphology; marine mammals; niche partitioning.

Figure 1.

Distribution of otariids. Circles represent the location of major breeding colonies and their size, the number of coexisting species. Communities from sub-Antarctic islands are not depicted. Inset illustrates the distribution range of species inhabiting the eastern North Pacific Ocean. Modified from [24].

Figure 2.

(a) Community-wide relationship between δ¹³C and δ¹⁵N values showing enrichment in larger skull sizes. Correlation between the skull length and δ¹³C (b) and δ¹⁵N values (c). Black lines represent the linear correlation model.