Mechanisms for sex-based segregation in foraging behaviour by a polygynous marine carnivore

Sexual segregation in foraging is often attributed to constraints arising from sexual size dimorphism, such as differing physiological abilities and energy requirements, or to reproductive commitments including nutritional requirements and behavioural limitations such as parental care. In species with sexual size dimorphism and a polygynous mating system, there are sex differences in both body size and reproductive commitments, so distinguishing the mechanism underlying sexually segregated foraging is particularly difficult. We investigated movements of the sexually dimorphic, polygynous Australian fur seal Arctocephalus pusillus doriferus, and compared foraging strategies of juveniles to adults in order to investigate foraging constraints associated with differences in body size separately from those also associated with reproduction. Foraging trip duration, range, core area, and distance to core area from the breeding site were similar between the sexes in juveniles and adults, but juveniles were overall more constrained than adults. After foraging trips, juveniles and adult females were more likely to return to the breeding site than to other sites, probably because of physiological limitations (juveniles) or reproductive commitments (adult females). Freedom from reproductive commitments allowed adult males to reside in areas away from the breeding site and thereby meet the higher energy requirements arising from their larger body size, without competing with adult females. Consequently, intra-species differences in foraging strategies of Australian fur seals are shaped first by foraging constraints associated with body size, and to a lesser extent by reproductive behaviour.