Energy reserve allocation in fasting Northern Elephant Seal pups: inter-relationships between body condition and fasting duration

1. Organisms are forced to make trade-offs when allocating energy reserves during sustained periods of fasting. 2. For most animals, lipid catabolism is the preferred source of energy to safeguard important protein sources. However, marine mammals also have a compounding pressure to conserve some lipid stores. The main site of lipid storage in these animals is the blubber layer, which is not only an important energy source during fasting, but is also the primary thermal barrier when at sea. 3. To explain how the allocation of protein and lipid reserves during fasting are influenced by body condition (body mass and percentage lipid of total body mass), a dynamic state variable model that takes into account fitness consequences of different allocation strategies was developed. 4. This model was parameterized with respect to conditions faced by weaned Northern Elephant Seal (Mirounga angustirostris Gill) pups. It incorporates the independent effects of body mass and protein mass (the two state variables) on survival and the costs of utilizing either lipid or protein during the postweaning fast. 5. Predictions of lipid and protein allocation by the model were not significantly different from measurements on wild seals. Finally, the model showed that body lipid content and fasting duration both influence allocation of energy reserves.