Overlapping and matching of codas in vocal interactions between sperm whales: insights into communication function

Many animals engage in dyadic vocal exchanges. Studying the patterns of vocal output and spatial arrangement of individuals in these interactions can often reveal information concerning their function. Sperm whales, Physeter macrocephalus, frequently exchange short sequences of clicks, termed codas, in social contexts. We analysed the coda vocalizations of sperm whale social units encountered in two different oceans to test hypotheses about how coda exchanges are organized. We also used a dynamic recording array to estimate the spatial scale of these vocal interactions. Coda production was influenced by the timing and types of codas produced by other unit members, resulting in the production of duet-like sequences of coda exchanges between pairs of whales. Codas were more likely to be made within 2 s of another coda than expected by chance, and whales were more likely to match previously produced codas than expected by chance, although matching appeared to be largely a result of the matching of one particular coda type within each social unit. Patterns of overlapping and matching exchanges did not seem to be correlated with relatedness or social affiliation. These exchanges occurred over a range of spatial scales, and are thus likely to be functional both between whales that are near and between those that are comparatively far from one another. The context of these exchanges, reciprocity in coda overlapping, and the sequencing of exchanges into duet-like chains all suggest that coda overlapping and matching function to reinforce social bonds between whales.