Pinnipeds are a clade of secondarily aquatic arctoid carnivorans, including 34 extant species dispersed across most of the world's oceans. Extant species are separated into three families (Figure 12.1): Odobenidae (walruses, 1 species), Phocidae (seals, 19 species), and Otariidae (sea lions and fur seals, 14 species) and display a wide range of ecological diversity (Reeves et al., 2002). Predominantly, pinnipeds are generalist feeders. They are opportunistic, and their diets may vary annually, between colonies and between individuals within a colony (King, 1983; Sinclair and Zeppelin, 2002; Williams et al., 2007). However, several species have evolved more specialist feeding techniques: (1) Odobenus rosmarus is a suction feeder, using powerful facial musculature to produce forces large enough to extract molluscs from their shells (Adam and Berta, 2002); Erignathus barbatus (Phocidae) also uses suction feeding (King, 1983; Marshall et al., 2008); (2) Lobodon carcinophagus (Phocidae) is a filter feeder; it uses multicuspidate teeth to sieve out krill as water is expelled from the mouth; (3) Hyrdrurga leptonyx (Phocidae) feeds on large, warm-blooded prey such as penguins and seal pups (Adam and Berta, 2002).
Reproductive strategies of the pinnipeds are also diverse. Otariids are universally dimorphic with large harems. Their young are weaned over long periods of up to 2 years whilst learning to forage (Kovacs and Lavigne, 1992; Schulz and Bowen, 2004). On the other hand, phocid young are relatively precocial (4–50 days weaning) and learn foraging skills after leaving their mothers. Phocids also show a diversity of mating strategies and degree of dimorphism (Schulz and Bowen, 2004). It has been hypothesised that this shorter time spent on land has allowed phocids to exploit a broader range of habitats, including polar regions (Kovacs and Lavigne, 1992; Schulz and Bowen, 2005). Odobenids show extremely long lactation times of three years. During this period, young walruses often accompany mothers on foraging trips.