Social organization of sperm whales off the Galapagos Islands, February–April 1985

Abstract

Between February and April 1985 sperm whales (Physeter macrocephalus) were tracked visually and acoustically in the waters west of the Galapagos Islands. Individuals were identified and measured photographically. Using statistical criteria the female and immature whales encountered were clustered into 13 groups with closed membership during the study period. These groups had a median estimated population of 19.5 animals each, and associated with one another at different times. The groups showed no significant immigration into or emigration from the study area during the study. Seven large males were identified. These associated with each other and the groups of females and immatures for periods lasting approximately 6 h. Individual males were found with different groups of females and immatures at different times. A simple model suggests that a male employing a "searching" strategy should encounter more oestrous females than the traditionally accepted "harem" holder when the interval between encountering groups of females is less than the duration of the females' oestrus. Neither the groups of females nor the mature males appeared to possess territories or preferred ranges within the study area. The number of large males encountered was considerably less than that expected if all large males were present on the breeding grounds, suggesting that males may not breed every year.