Aggregations of mature male sperm whales on the Galapagos Islands breeding ground

The distribution and behavior of mature (12.3-16.3 m) male sperm whales (Physeter macrocephalus) were studied on the Galapagos Islands breeding ground from April to June 1995. In contrast to previous research seasons when males were observed only in close spatial and temporal proximity to mixed schools of females and immature animals, in 1995 males were sighted in loose aggregations, separated by hours to days from our vessel's encounters with mixed schools. Only one of ten identified males was observed in spatial proximity to a mixed school. Aggregations consisted of two to four (minimum estimates) mature males travelling within a range of a few kilometers and were characterized by consistency of heading among individuals. Aggregations moved over time. During encounters, one to three males were observed at the surface at the same time, with interindividual distances of less than 1,000 m. Synchrony of heading was apparent between spatial associates, and its extent appeared to be related to interindividual distance. Clustering (two or more individuals within 100 m) was observed on only two occasions. No agonistic behaviors were seen. Functions of mature male aggregation on a breeding ground remain unclear. Possible explanations for our observations are local prey abundance, or some form of sociality mediated over spatial scales of hundreds to thousands of meters.