Bottlenose dolphins' broadband clicks are structured for communication

Abstract

Bottlenose dolphins broadband click vocalizations are well studied in the literature with respect to their echolocation function. Their use for communication among conspecifics has long been speculated, but not conclusively established so far. In this study we categorize dolphins click productions into types on the basis of their amplitude contour and analyze the distribution of individual clicks and click sequences against their duration and length. We demonstrate that the repertoire and composition of clicks and click sequences follow three key linguistic laws of efficient communication, namely, Zipfs rank-frequency law, the law of brevity and Menzerath-Altmann law. Conforming to the rank-frequency law suggests that clicks may form a linguistic code that is subject to selective pressures for unification, on the one hand, and diversification, on the other. Conforming to the other two laws also implies that dolphins use clicks in accord with the compression criterion, or minimization of code length without loss of information. Our results furnish novel evidence for conformity to the linguistic laws in this type of dolphins signal and in the realm of animal vocalizations more generally.