Harp and hooded seals–a case study in the determinants of mating systems in pinnipeds

Abstract

The similarities and differences that occur in the reproductive behaviour and energetics of harp and hooded seals make these two species an interesting case study for refining our understanding of the determinants of mating systems in pinnipeds. Both species breed in whelping patches that form in pack-ice regions in the North Atlantic. This parturition environment provides virtually unlimited space, but strict temporal limits. The lactation strategies employed by these species are similar in that the nursing period of each species is short, pup growth is fast and energy transfer is efficiently accomplished through the production of fat rich milk. Harp seal pups are born weighing 9.9±1.7 kg and grow at a rate of 2.3±0.5 kg/day during the 12.9±2.5 day nursing period. Mass loss by lactating female harp seals averages 27% (38 kg) of their total body mass (139.0±17.0 kg). Hooded seal pups are born weighing 24.4±2.6 kg and grow at a rate of 6.9±1.2 kg/day during the 3.8±0.5 day nursing period. The concomitant mass loss by their mothers is 16% (38 kg) of their parturition mass (236.5±35.5 kg). The duration of lactation, in addition to the degree of synchrony among females in birthing, means that females are only available for male breeding attempts for a short period of time in either species. This places a high premium on the choice of mating strategies for males of both species. However, male harp seals and male hooded seals face quite different regimes with respect to female spacing patterns in relation to water and to other females. Females harp seals give birth near open water with a nearest neighbour distance of 5–10 m. Female hooded seals give birth far from open water with an average of 50 m between females. The differences in female habitat choice and mobility have profound effects in the determination of what defines a successful strategy for males. Male monopolization of females is not possible in harp seals and males must mate opportunistically, possibly with scramble or sperm competition determining success. Male hooded seals are able to mate guard to a greater degree and display a form of polygyny that involves serially attending a number of females during a reproductive season.