CHANGES IN BODY-MASS AND FEEDING-BEHAVIOR IN MALE HARBOR SEALS, PHOCA-VITULINA, IN RELATION TO FEMALE REPRODUCTIVE STATUS

Loss of body mass has been used as an index of the cost of reproductiv e effort in mammals. We studied changes in body mass of male harbour s eals, Phoca vitulina, during the breeding season on Sable Island, Nova Scotia, Canada. Individually marked subadult (n=21) and adult (n=22) males were captured at approximately six-day intervals throughout the breeding season. Adult males weighed an average of 108 kg+/-5.6 (S.E.) at initial capture, whereas subadults weighed an average of 76+/-6.6 kg. The rate of mass loss by adult males did not differ from zero duri ng the premating period (i.e. the period without receptive females), b ut differed significantly from zero (-0.91+/-0.007 kg/day) during the mating period (i.e. the period with receptive females). By contrast, t he rate of mass change of subadults did not differ from zero in either period, indicating that food availability was unlikely to be responsi ble for the observed changes in adults. Adult males lost up to 24% of body mass during the breeding season. Examination of sera for the pres ence of chylomicrons (i.e. evidence of recent feeding) also suggested that adults stopped feeding during the mating period, while subadults did not. These results suggest that reproduction represents a signific ant energetic cost to adult male harbour seals.