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

• Published in: Canadian Journal of Zoology
• Main Authors: Whitehead, Hal, Kahn, Benjamin
• Format: Article in Journal/Newspaper
• Language: English
• Published: Canadian Science Publishing 1992
• Subjects: Animal Science and Zoology; Ecology, Evolution, Behavior and Systematics; Physeter macrocephalus
• Online Access: http://dx.doi.org/10.1139/z92-289; http://www.nrcresearchpress.com/doi/pdf/10.1139/z92-289

The social structure of female sperm whales (Physeter macrocephalus) was examined from data collected during studies of identified individuals off the Galápagos Islands in 1985, 1987, and 1989, off the Seychelles Islands in 1990, and off mainland Ecuador in 1991. Three levels of social organization were examined in the Galápagos Islands studies. "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 lower in 1987 (an "El Niño" year) than in 1985 or 1989, but this difference was not statistically significant. "Aggregations" of groups in the same general area lasting periods of hours were significantly larger in 1985 (mean of 77 individuals) than in 1987 (mean of 47 individuals). There are significant differences between areas in aggregation sizes but not in group sizes. Groups appear to aggregate frequently off the Galápagos Islands, occasionally off mainland Ecuador, and very rarely off the Seychelles Islands. These temporal and geographic differences in sizes of groups and aggregations in sperm whales are probably caused by variations in the distribution and abundance of their food, but responses to recent exploitation may also be a factor.