MARINE MAMMAL SCIENCE, 8(4):387-396 (October 1992) 0 1992 by the Society for Marine Mammalogy GROWTH AND DISTRIBUTION OF A SOUTHERN ELEPHANT SEAL COLONY CLAUDIO CAMPAGNA AND MIRTHA LEWIS Centro Nacional Patagonico, 9 120 Puerto Madryn, Chubut, Argentina Wildlife Conservation International, New York Zoological Society, Bronx Zoo, Bronx, New York, New York 10460 ABSTRACT In contrast with most southern elephant seal, Mirounga leonina, colonies, births at Peninsula Valdes, Argentina, increased from 7,455 in 1982 to 9,636 in 1990. Colony size during the 1990 breeding season, including pups, was estimated at 19,000. Colony growth may respond to abundant food resources and lack of competitors. The range of distribution of elephant seals in Patagonia has not changed since at least 1972 but the spatial distribution of births along the coastline of Peninsula Valdes has varied. In 1982, 58% of the births occurred in the northeast portion of the Peninsula versus 36% in 1990. Females may prefer to give birth on broad, sandy beaches. In 1982, 56% of the females gave birth on pebble beaches and 44% on sandy beaches. In 1990, 24% bred on pebbles versus 76% on sand. Sand substrate may be preferred by females because it may confer thermoregulatory advantages in relieving heat stress. Key words: elephant seals, Mirounga leonina, demography, population growth, Peninsula Valdes. The recovery of the northern elephant seal, Mirounga angustirostris, after its virtual extinction (Bartholomew and Hubbs 1960, Antonelis et al. 1981) con- trasts with a sharp population decline of the southern species, M. leonina, at several breeding places in subantarctic and antarctic waters over the past 50 years (Skinner and van Aarde 1983, McCann 1985, Burton 1986, Hindell and Burton 1987). Contrary to this downward trend, preliminary data on the south- ern elephant seal colony of Peninsula Valdes, Argentina, suggest it could be the only one that is expanding (Scolaro 1976, Vergani et al. 1987). The Valdes colony has ecological, demographic and geographic features that differentiates it from others of the same species and from its northern counterpart (Le Boeuf and Petrinovich 1974). It is the northernmost large seal breeding colony in the Southern Hemisphere. It is located on a continent rather than on an island, in temperate rather than antarctic or subantarctic waters, and 550 km away from deep water, where elephant seals apparently forage (Hindell 1990). Animals are widely distributed along 200 km of coastline (Vergani 387