Social effects of space availability on the breeding behaviour of elephant seals in Patagonia

The Patagonian breeding colony of southern elephant seals, Mirounga leonina, of Peninsula Valdes, Argentina is characterized by long, continuous, homogeneous and spacious beaches. Reproduction in this colony occurs at the lowest group densities reported for the species. The availability of ample space resulted in the dispersion of reproductive females along 160 km of coastline, with a range of 0-300 reproductive individuals per km. Females grouped together in small harems (median=11 females, range 2-122, N=432), and individual distance between females was one to two female body lengths. Body contact within the harem was rare. Consequently, agonistic interactions (AIs) between females and female aggression (Ag) towards alien pups occurred infrequently (median rate=4 AIs/female per 100 h and 1 Ag/pup per 100 h, respectively). Pup mortality rate was low for an elephant seal rookery (3.5% for 3487 pups). Most dead pups were stillborn, and only three of 38 pups whose cause of mortality could be determined died from trauma or starvation following mother-pup separation. Female dispersal allowed many males to have access to females. About 45% of all males of reproductive size in the colony (about 1000 individuals) had a harem. Most adult males mated with receptive females. Low-density breeding conditions benefited female reproductive success through an increase in pup survival. Female dispersal resulted in small harem size, however, and limited the potential for polygyny. (C) 1996 The Association for the Study of Animal Behaviour