Spatial and temporal variation in sperm whale coda vocalizations: stable usage and local dialects

Abstract

Variation in vocal repertoires within species can result from various processes, from genetic drift to cultural evolution. Studying the detailed nature of such variation over time and space can provide insight into these underlying processes. The temporal stability or otherwise of vocal variants is important information in assessing the possibility of interaction between vocal variation and genetic evolution, while the geographical scale of variation can give useful clues in detailing population structure, thus providing management information. We studied variation in the vocal output of sperm whales, Physeter macrocephalus, which use short stereotyped click sequences called codas in social contexts, over timescales of up to 6 years and spatial scales of up to 10 000 nautical miles. We found no evidence from either correlation or regression analyses for change over time in the coda output of known social units. We did, however, find evidence for reduced similarity over distances of 200–1000 km. The apparent temporal stability of coda output suggests the possibility of evolutionary interactions. The scale of spatial variation is broadly similar to estimated home range sizes for this species, and the potential for interbreeding over such distances suggests differences between areas may represent local dialects rather than geographical variation through isolation.