Reproductive effort of male northern elephant seals: estimates from mass loss

Abstract

The energetic component of reproductive effort of male northern elephant seals, Mirounga angustirostris, was estimated from mass loss over the breeding season and correlated with dominance rank and age. Fifty-four unrestrained bulls were weighed on a platform scale by luring them with a model of a female seal or moving them with a tarpaulin and using playback of male aggressive vocalizations. Adult males weighed up to 2300 kg upon arrival at the breeding rookery. Mean rate of mass loss during the breeding season was 7.1 ± 1.5 (SD) and 4.6 ± 0.8 kg per day for 17 adults and 13 subadults, respectively. Rate of mass loss was positively correlated with body size (mass or length) for both age-classes. Mass-specific rate of mass loss did not differ between age-classes but increased with increasing dominance rank among adult males. Reproductive effort, expressed as percentage of body mass lost over the 3-month breeding season, was greater for high-ranking bulls (mean 41.4%) than for low-ranking adults (33.8%), but was not related to age-class or body size. High-ranking males experienced higher mating success and expended more energy than subordinate males. Comparison with a previous study on conspecific females indicates that mass-specific energetic investment in reproduction is similar for both sexes, despite marked sex differences in reproductive strategy and duration of effort.