Studies on the southern elephant seal, Mirounga leonina (L.). III. The annual cycle in relation to age and sex

Abstract

The annual cycle of the southern elephant seal, Mirounga leonina (L.), at Macquarie and Heard Islands, where there are large undisturbed populations, is described from records of branded seals up to 9 years old, and from classifmd counts. Each age and sex has its own regular sequence of seasonal activities, and the onset of breeding largely replaces the autumn-winter haul-out of immature individuals. The moult is described, and the possible significance of the autumn-winter haulout is discussed. The pupping period in spring is constant and well synchronized annually and regionally. Weaning anticipates the summer rise of zooplankton in surface waters. There is no difference in parturition date between primiparous and multiparous cows of all ages, no matter when the moult was completed. An environmental timer of implantation is postulated as the means of ensuring seasonal constancy of breeding.

Food does not appear to be limiting at any season, and population movements are confined to partial dispersal of immatures. Few cows are recorded breeding more than a short distance from their birthplace, but local changes of location are common. Ten marked individuals are known to have moved from one subantarctic island to another, and two from Heard Island to the Antarctic continent.

The seasonal cycle is compared with other Pinnipedia; solitary and gregarious breeding, harem formation and polygyny, and aquatic and terrestrial mating are discussed. As efficiency on land deteriorated owing to aquatic adaptation, it is suggested that social grouping by cows in exposed breeding habitats, probably for protection of pups, and early terrestrial mating provided the opportunity for harem formation, the hierarchy of dominance among bulls, and polygyny.