Different modes of acoustic communication in deep-diving short-finned pilot whales (<i>Globicephala macrorhynchus</i>)

Toothed whales use a pneumatic sound generator to produce echolocation and communication sounds. Increasing hydrostatic pressure at depth influences the amplitude and duration of calls but not of echolocation clicks. Here we test the hypothesis that information transfer at depth might be facilitated by click-based communication signals. Wild short-finned pilot whales (27) instrumented with multisensor DTAGs produced four main types of communication signals: low- and medium-frequency calls (median fundamental frequency: 1.7 and 2.9 kHz), two-component calls (median frequency of the low and high frequency components: 2 and 9 kHz), and rasps (burst-pulses with median interclick interval of 21 ms). Rasps can be confused with foraging buzzes, but rasps are shorter and slower, and are not associated with fast changes in body acceleration nor reduced acoustic output of buzzes, characteristic of prey capture attempts. Contrary to calls, the energy flux density of rasps was not significantly affected by depth. This, and a different information content, may explain the observed increase in the relative occurrence of rasps with respect to calls at depth, and supports the hypothesis that click-based communication signals may facilitate communication under high hydrostatic pressure. However, calls are produced at depth also, indicating that they may carry additional information relevant for deep-diving animals, including potential communication among whales diving at the same time in this highly social deep-diving species