THERMOREGULATORY BEHAVIOR OF SOUTHERN SEA LIONS AND ITS EFFECT ON MATING STRATEGIES

Southern sea lions, Otaria byronia, breed during the Austral summer on flat rocky shelves containing tidepools and boulders, or on wide, uniform pebble beaches lacking tidepools and shade. The first substrate provides breeding animals with relief from heat stress, the second does not. A three-year study of two rookeries at Peninsula Valdes, Argentina, from 1985 to 1987 revealed major differences in mating behaviour and mating success of males, associated with temperature, solar radiation and rookery topography. At Puerto Piramide, males defended territories containing tidepools throughout the breeding season. Thermoregulatory requirments caused females to concentrate in tidepool areas, and males completed aggressively for these recources. Seven males that defended territories with large, water-renewable tidepools copulated a mean of 11.6 .+-. 4.2 times during three weeks, versus a mean of 1.4 .+-. 1.4 times for ten males that defended territories without tidepools. The latter lost their females and went to sea when air temperature rose over 30.degree. C. At Punta Norte, the homogeneous pebble substrate forced animals to breed in a narrowly confined area along the high tide mark, where the substrate remained wet. Only those males holding females in this narrow zone copulated. All of 32 marked males that associated with females near the high tide mark copulated. Males breeding outside of this narrow strip of beach deserted and lost females after 1-3 hrs of air temperatures .gtoreq. 30.degree. C, and solar radiations .gtoreq. 1.2 cal/cm2/min. Rookery toporgraphy and associated thermoregulatory constrains are critical variables shaping breeding behaviour in southern sea lions. The thermal and physical environment is a determinant of the social behaviour of otariids and has been important in shaping their social system.