Acoustic multi-tasking in beaked whales: The use of alternating echolocation regimes during the descent phase of deep foraging dives

Abstract

Beaked whales use echolocation to navigate and hunt at extreme depths, and the timing of their echolocation clicks while actively searching for prey is generally characterized by a stable inter-click interval (ICI). However, analysis of autonomous, long-term passive acoustic data revealed a markedly different pattern during the vocal portion of their descent towards the seafloor. Diving beaked whales were found to alternate between two ICI regimes, while also gradually increasing their overall click rate. This strategy is presumably used to simultaneously monitor two different target ranges as they approach their preferred foraging depth. One ICI regime likely corresponds to the two-way-travel time of sound to the seafloor, and by examining the rate at which the time interval between clicks decreased in this seafloor-tracking ICI regime, we calculated estimates of dive descent rate. The second ICI regime was consistently more rapid, and represents a shorter search range that is likely used by the whale to inspect the nearby water column for the presence of prey and other features. This unique echolocation behavior was identified in acoustic encounters of four different species of beaked whales recorded at a variety of sites in the North Pacific Ocean and the Gulf of Mexico.