Unusual use of shallow habitats may be evidence of a cultural tradition in Baird's beaked whales.

Socially transmitted behavioural patterns (i.e. cultural traditions) have been observed in many whale species from large baleen whales to small dolphins. However, no cultural traditions have been described so far in beaked whales, an elusive and poorly studied group of toothed whales. In this study we report a local population of Baird's beaked whales, Berardius bairdii , in the Commander Islands regularly using a shallow area with depths of less than 300 m which is uncharacteristic for this species. We analysed the distribution of this behaviour and mtDNA haplotypes among individual whales to explore whether it represents a cultural tradition and whether it is transmitted vertically within separate maternal lineages or horizontally among nonrelated whales. We found that the whales that frequently visited the study area, and therefore were familiar with the underwater landscape, often used the shallow areas of the shelf slope, while transient whales, unfamiliar with the local conditions, mostly adhered to the depth range typical of their species. Only those transient whales that maintained social bonds with residents were sometimes observed in shallow areas. Order-of-acquisition diffusion analysis showed that social transmission of knowledge about the shallow habitat was significantly more likely than individual asocial learning. Two mtDNA haplotypes were shared between whales that used the shallow areas and those that did not, suggesting that these categories did not represent separate maternal lineages. We conclude that knowledge of the shallow areas is transmitted horizontally through social learning, and therefore this is an example of a local cultural tradition. • We report on the first possible cultural tradition in beaked whales. • Some Baird's beaked whales used a shallow area, which is atypical for this species. • Resident whales familiar with the local conditions often used the shallow area. • Only those transients that had social bonds with residents occurred in the shallows. • The knowledge of the shallow area is transmitted through social learning. [ABSTRACT FROM AUTHOR]