Energetic cost of foraging in free-diving emperor penguins

Hypothesizing that emperor penguins (Aptenodytes forsteri) would have highe r daily energy expenditures when foraging for their food than when being ha nd-fed and that the increased expenditure could represent their foraging co st, we measured field metabolic rates (FMR; using doubly labeled water) ove r 4-d periods when 10 penguins either foraged under sea ice or were not all owed to dive but were fed fish by hand. Surprisingly, penguins did not have higher rates of energy expenditure when they dove and captured their own f ood than when they did not forage but were given food. Analysis of time-act ivity and energy budgets indicated that FMR was about 1.7 x BMR (basal meta bolic rate) during the 12 h d(-1) that penguins were lying on sea ice. Duri ng the remaining 12 h d(-1), which we termed their "foraging period" of the day, the birds were alert and active (standing, preening, walking, and eit her free diving or being hand-fed), and their FMR was about 4.1 x BMR. This is the lowest cost of foraging estimated to date among the eight penguin s pecies studied. The calculated aerobic diving limit (ADL(C)), determined wi th the foraging period metabolic rate of 4.1 x BMR and known O-2 stores, wa s only 2.6 min, which is far less than the 6-min ADL previously measured wi th postdive lactate analyses in emperors diving under similar conditions. T his indicates that calculating ADL(C) from an at-sea or foraging-period met abolic rate in penguins is not appropriate. The relatively low foraging cos t for emperor penguins contributes to their relatively low total daily FMR (2.9 x BMR). The allometric relationship for FMR in eight penguin species, including the smallest and largest living representatives, is kJ d(-1) = 1, 185 kg(0.705).