Prey capture and processing in otariid pinnipeds with implications for understanding the evolution of aquatic foraging in marine mammals

Abstract

The behaviours used by mammalian predators to capture and process their prey play a pivotal role in determining hunting success when targeting different species, or prey in different foraging environments. This is especially true in aquatic environments, where marine mammals like pinnipeds encounter a wide range of prey species while hunting both in open water and near the sea floor. However, due to difficulties faced when observing behaviour in diving mammals, little is known about the mechanics of prey handling in most pinnipeds species. The aim of this thesis was to remedy this by performing captive feeding trials that explored how foraging behaviours varied under different conditions for otariid species that have not been studied before now. When capturing small fish in open water, Australian fur seals (Arctocephalus pusillus doriferus) used raptorial biting. Rather than simply snapping at prey with the jaws, this involved combined use of biting and suction, where they used suction to draw prey within range of the teeth before the jaws snapped shut. In contrast, when capturing prey concealed within a feeding device, fur seals used strong suction alone to draw prey into the mouth. Australian and subantarctic fur seals (Arctocephalus tropicalis) swallowed small prey whole after the initial capture, while large prey was first brought to the surface and processed into smaller pieces by shaking. By chewing on prey between shakes, the fur seals created weak points that made it more likely to break when shaken further. Australian sea lions (Neophoca cinerea) were also able to chew and shake food; however, unlike fur seals, they also tore food held between their forelimbs and teeth. By directly observing how prey capture and processing occurs in otariid pinnipeds, we are able to begin to draw broader comparisons between these species and the other main clades of pinnipeds. Phocine phocids (the northern seals) also use biting and suction when capturing large vs. small prey, but unlike fur seals they are able to grip prey in clawed forelimbs while processing it with their teeth. Monachine phocids (the “southern” seals) have forelimbs intermediate in morphology between the clawed phocines and highly derived otariids. Those that have been studied or observed seem to display similar behaviours to the fur seals from our studies. Australian sea lions are unusual in being similar to phocine seals that hold prey in their forelimb flippers, but rather than using claws to secure prey, they squeezed prey between flat palms. These patterns highlight key differences in behaviour that may relate to differences in forelimb morphology that stem from the degrees to which each clade has adapted its anatomy for aquatic locomotion. By making direct observations of behaviour for a wide range of pinniped species we can therefore identify patterns that provide a better understanding of the role that foraging behaviour played throughout pinniped evolution. These results also provide insights into how prey capture and processing behaviours influence the foraging choices made by seals hunting at sea, providing insights into what makes pinnipeds such successful marine predators.