Temporal and geographic variation in the social structure of female sperm whales

The social structure of female Physeter macrocephalus was examined from data collected off the Galapagos Islands in 1985, 1987, and 1989, off the Seychelles Islands in 1990, and off mainland Ecuador in 1991. "Units', permanent associations between individuals, showed no significant deviation from a mean size of 13 individuals. "Groups', temporary associations between units lasting a few days, had a mean size of 24 individuals. Group size was (non-significantly) lower in 1987 (an "El Nino' year) than in 1985 or 1989. "Aggregations' of groups in the same general area lasting periods of hours were signifiantly larger in 1985 (mean of 77 individuals) than in 1987 (47 individuals). There are significant differences between areas in aggregation sizes but not in group sizes. Groups appear to aggregate frequently off the Galapagos Islands, occasionally off mainland Ecuador, and very rarely off the Seychelles Islands. -from Authors