Dialect variation in Mediterranean sperm whales shows evidence of cultural evolution in an isolated population

ABSTRACT

Population isolation and bottlenecks can affect cultural evolution. This has been shown in bird vocal dialects but is less studied in mammals. The isolated and weakly structured population of sperm whales (Physeter macrocephalus) in the Mediterranean Sea provide an opportunity to address this knowledge gap. We studied dialect variation in this population using the species’ primary social vocalization, stereotyped patterns of clicks called codas. We analysed 5291 codas recorded over 112 days between 2003 and 2021 from both the western and eastern deep-water basins of the Mediterranean Sea. We confirmed previous studies showing the dominance of codas with a 3+1 pattern but also identified two distinct dialect groups. The 2+1 and slow 3+1 coda types identified the western dialect while the 8I and fast 3++1 coda types identified the eastern dialect. Notably, groups recorded in the east occasionally produced the ‘western’ slow 3+1 coda types but the converse was not true, suggesting a directionality in dialect formation. The similarities between the apparent effects of isolation on sperm whale dialects and isolation effects reported in studies of the cultural evolution of both bird song dialects and human languages suggest convergences in vocal dialect evolution across social taxa.