ИСТИНА

Baird’s beaked whales are deep-divers, usually living far from shore, making them hard to observe, and their social structure is poorly understood. Here we summarize our observations of individually identified whales over the span of seven years (2008-2014) around the Commander Islands (Northwestern Pacific, Russia) to provide insights on their long-term social structure. We analyzed the photographs of 154 individually recognized Baird's beaked whales and identified the presumed sex based on the number and types of scars. We assumed that whales with a large number of conspecific scars were adult males, while the animals with many cookie-cutter shark (round) scars and few conspecific scars were adult females. Whales with few scars were considered immature individuals or young mature females and were not sexed. For 13 whales, the presumed sex was confirmed by the genetic analysis of biopsy samples. We then analyzed the group composition using the data on presumed sex of individuals. We found three types of groups: groups consisting exclusively of the presumed males (mean group size = 8.64), mixed groups (mean group size = 14.8) and groups consisting of the presumed females with calves and young animals (mean group size = 6.0). We also analyzed the distribution of sexes in stable alliances that were observed over the study period. The alliances observed over the longest time span consisted of the presumed males. The maximum association coefficient was larger in presumed males, than in presumed females (mean for males = 0.82, mean for females = 0.67, Mann-Whitney test p<0.05). We suppose that Baird's beaked whales live in a fission-fusion society where individuals group together according to common interests, and that "nurseries" containing females with young are less stable than the male alliances.