YEATES, L.C.; WILLIAMS, T.M.; STAEDLER, M.M.; Univ. of California Santa Cruz; Univ. of California Santa Cruz; Monterey Bay Aquarium: Diving and foraging energetics in the sea otter (Enhydra lutris): Is small size a disadvantage in water?

Among mammals, sea otters represent the most recent lineage to reenter the marine environment. One key adaptation in transitioning from a terrestrial to marine lifestyle is an increase in metabolic rate, and thus daily energy requirements. To assess the daily energetic needs of sea otters, we calculated activity budgets of 18 wild male sea otters in Monterey Bay, CA, and measured the cost of individual behaviors in two captive male otters. An open flow respirometry system with a metabolic dome placed on the surface of a 10 m water tower was used to assess diving costs. We found that wild otters spent 28% of the day feeding, 39% resting and 22% performing other behaviors (grooming and swimming). Metabolic rates ranged from a mean resting rate of 14.9 mLO₂min^-1kg^-1 &plusmn 2.0 SD (n = 33) to a mean grooming rate of 29.4 mLO₂min^-1kg^-1 &plusmn 2.6 SD (n = 8). Maximum measured metabolic rate recorded was for post absorptive grooming (32.9 mLO₂min^-1kg^-1), 6.5 times the basal metabolic rate of a terrestrial mammal of similar size. Diving metabolic rates ranged from 15.5 to 23.0 mLO₂min^-1kg^-1. Although the energetic cost of individual dives is low in comparison to other activities such as swimming or grooming, the daily cost is considerable. For example, wild male sea otters perform 29 &plusmn 14.5 SD dives hr^-1 during a typical foraging bout and may spend 50% of the day foraging. The resulting daily energetic requirement for an adult sea otter in Monterey Bay is 3317 Kcal. Although this value is similar to the predicted field metabolic rate of marine mammals, it is approximately 2 times the value for a similarly sized terrestrial mammal.