As the smallest and one of the most recently evolved marine mammals, sea otters face physiological challenges rarely encountered by larger, more derived aquatic species. To examine the effect of these challenges on foraging costs and resultant daily energy budgets, we measured the energetics of resting, grooming, diving and foraging for adult, male sea otters. The energy expended for these different behaviors as determined from open flow respirometry was then standardized across activity budgets measured for wild sea otters to estimate field metabolic rates (FMR). We found that the metabolic rate of captive otters performing single dives ranging in duration from 40 to 192 s was 17.6+/-0.5 ml O(2) kg(-1) min(-1) and only 1.3 times resting rates. This rate increased significantly if the animals foraged during submergence. The cost of a foraging dive for sea otters was nearly twice that predicted for phocid seals, which was attributed in part to elevated locomotor costs associated with buoyancy and swimming style. Our behavioral studies indicate that wild sea otters spend the greatest proportion of the day feeding and resting, with the largest daily energy expenditure (6.1+/-1.1 MJ day(-1)) associated with foraging. The resulting mean FMR for wild sea otters based on the energy expended for all behaviors was 15.7+/-2.7 MJ day(-1) and matched predicted FMR values based upon a regression of known FMR values for other marine mammals across a range of body sizes. This was achieved by counterbalancing elevated foraging costs with prolonged periods of rest on the water surface.