MASS CHANGE IN FASTING IMMATURE MALE NORTHERN FUR SEALS

Immature male northern fur seals (Callorhinus ursinus) lost an estimat ed 20-30% of their body mass while fasting during the breeding season on St. Paul Island, Alaska. This is somewhat less than the mass loss r eported for territorial adult male pinnipeds. During fasting, absolute rates of mass change (averaging -0.68 kg.day(-1)) were positively cor related with initial mass. Mass-specific rate of change (averaging 2%. day(-1)) did not vary with age or initial mass. Although they lose lar ge amounts of mass while fasting, immature male fur seals achieve a ne t increase in mass during the breeding season by making feeding trips to sea. We postulate that the capability for prolonged fasting in imma ture males facilitates the development of social and sexual skills tha t are Likely to increase future reproductive success.