Acoustic behaviour of echolocating porpoises during prey capture

Abstract

SUMMARYPorpoise echolocation has been studied previously, mainly in target detection experiments using stationed animals and steel sphere targets, but little is known about the acoustic behaviour of free-swimming porpoises echolocating for prey. Here, we used small onboard sound and orientation recording tags to study the echolocation behaviour of free-swimming trained porpoises as they caught dead, freely drifting fish. We analysed porpoise echolocation behaviour leading up to and following prey capture events, including variability in echolocation in response to vision restriction, prey species, and individual porpoise tested. The porpoises produced echolocation clicks as they searched for the fish, followed by fast-repetition-rate clicks (echolocation buzzes) when acquiring prey. During buzzes, which usually began when porpoises were about 1-2 body lengths from prey, tag-recorded click levels decreased by about 10 dB, click rates increased to over 300 clicks per second, and variability in body orientation (roll) increased. Buzzes generally continued beyond the first contact with the fish, and often extended until or after the end of prey handling. This unexplained continuation of buzzes after prey capture raises questions about the function of buzzes, suggesting that in addition to providing detailed information on target location during the capture, they may serve additional purposes such as the relocation of potentially escaping prey. We conclude that porpoises display the same overall acoustic prey capture behaviour seen in larger toothed whales in the wild, albeit at a faster pace, clicking slowly during search and approach phases and buzzing during prey capture.

Acoustic monitoring and prey association for beluga whale, Delphinapterus leucas, and harbor porpoise, Phocoena phocoena, off two river mouths in Yakutat Bay, Alaska

“…Previous studies have reported the occurrence of click trains during porpoise foraging behavior, with progressively decreasing ICI's ending in a high rate terminal "buzz", with ICI's close to or less than 2 ms during the fi nal prey capture (Verboom and Kastelein, 2003;DeRuiter et al, 2009;Miller, 2010;Wisniewska et al, 2012). This same echolocation behavior has been suggested for feeding belugas (Roy et al, 2010;Castellote et al, 2013).…”

“…Time intervals between clicks in echolocation click trains, termed interclick intervals (ICI), have been used as a behavioral indicator in several odontocetes, including harbor porpoise (DeRuiter et al, 2009;Verfuss et al, 2009) and beluga (Roy et al, 2010;Castellote et al, 2013). Previous studies have reported the occurrence of click trains during porpoise foraging behavior, with progressively decreasing ICI's ending in a high rate terminal "buzz", with ICI's close to or less than 2 ms during the fi nal prey capture (Verboom and Kastelein, 2003;DeRuiter et al, 2009;Miller, 2010;Wisniewska et al, 2012).…”

Directional escape behavior in allis shad (Alosa alosa) exposed to ultrasonic clicks mimicking an approaching toothed whale

“…If the ultrasound detector is an energy detector, the threshold can be reached by slow clicking with high sound pressure or fast clicking at lower sound pressure. That feature will accommodate the fact that toothed whales click slowly and with high source levels at long ranges from their prey, but switch to a very fast repetition rate buzzing of lower sound pressure just before prey capture when they are within a body length of the prey (Deruiter et al, 2009;Madsen et al, 2005). With the estimated response energy thresholds one can use the passive sonar equation (Urick, 1983) to estimate at what range an allis shad will be able to detect a toothed whale, for instance a bottlenose dolphin.…”

“…To mimic the acoustic exposures from toothed whales at different ranges, we played simulated echolocation click trains with a total duration of 1s at four different repetition rates of 1, 20, 50 and 250clickss -1 (Fig.2C). Approximately 250clickss -1 are emitted from at toothed whale in the final prey capture phase, when it is within a few metres of the prey (Miller et al, 1995;Madsen et al, 2005;Deruiter et al, 2009). Repetition rates of 50clickss -1 and 20clickss -1 mimic a delphinid toothed whale in the approach phase during a prey capture when it would be approximately 10-30 m from the prey (Au, 1993) and a single click could be from scanning toothed whales, that has not locked the biosonar on the prey.…”

“…Another possible cue to the proximity of the predator is the rate at which a toothed whale is clicking (Astrup, 1999;Astrup and Møhl, 1997). Like echolocating bats, toothed whales produce echolocation clicks at a higher repetition rate when they approach their prey, and most prey capture attempts are terminated with a buzz phase during which the repetition rate is up to several hundred clicks per second (Miller et al, 1995;Madsen et al, 2002;Madsen et al, 2005;Deruiter et al, 2009;Verfuss et al, 2009). Thus, if ultrasound detection in Alosinae is used as a way to avoid predation from toothed whales, we hypothesize that Alosinae will exhibit negative phonotaxis, i.e.…”