Buzzing during biosonar-based interception of prey in the delphinids Tursiops truncatus and Pseudorca crassidens

Abstract

Echolocating bats and toothed whales probe their environment with ultrasonic sound pulses, using returning echoes to navigate and find prey in a process that appears to have resulted from a remarkable convergence of the two taxa. Here, we report the first detailed quantification of echolocation behaviour during prey capture in the most studied delphinid species, a false killer whale and a bottlenose dolphin. Using acoustic DTAGs, we demonstrate that just prior to prey interception these delphinids change their acoustic gaze dramatically by reducing inter-click intervals and output >10-fold in a high repetition rate, low output buzz. Buzz click rates of 250-500 Hz for large but agile animals suggest that sampling rates during capture are scaled with the whale's manoeuvrability. These observations support the growing notion that fast sonar sampling accompanied by a low output level is critical for high rate feedback to inform motor patterns during prey interception in all echolocating toothed whales.

Amazon river dolphins (Inia geoffrensis) modify biosonar output level and directivity during prey interception in the wild

“…Following the arrival of target echoes, botos make use of lag times around 10 ms at the shortest target ranges while employing increasingly longer lag times for longer target ranges. This finding differs from some previous studies using stationed and actively swimming captive bottlenose dolphins where lag times at comparable target ranges have been reported as being fairly constant at around 20 ms (Morozov et al, 1972;Au, 1980), but agrees with the other observations indicating no support for constant lag time usage in freely swimming animals approaching prey (Wisniewska et al, 2014).…”

“…Interestingly, we observed that botos, which in general click much faster than similarly sized marine toothed whales (Ladegaard et al, 2015), at rates comparable to other river-dwelling species (Jensen et al, 2013), decreased their ICIs from approach to buzzing only gradually from roughly 30 to 10 ms (Figs 2 and 3). In contrast, similarly sized marine species may downregulate ICI by more than an order of magnitude, ending with buzz ICIs as short as ∼2 ms when catching prey (DeRuiter et al, 2009;Wisniewska et al, 2014). This bears a striking resemblance to the observation that harbour porpoises exposed to a clutter situation use click rates higher than normal during the approach phase, but decrease click rate during buzzing (Miller, 2010).…”

“…3), which compares to the buzz phase that characterises prey interception in other toothed whales (DeRuiter et al, 2009;Wisniewska et al, 2014;Fais et al, 2016). The observation that botos also buzz during prey capture may underscore a fundamental trait about echolocation in toothed whales; even though botos have spent >10 million years adapting to life in a remarkable habitat of rivers and flooded jungles while evolving in parallel with marine toothed whales (Hamilton et al, 2001;Martin and da Silva, 2004), these different species still seem to share a basic biosonar framework that calls for comparable biosonar adjustments and high sampling rates during the critical phase of prey capture.…”

Support for the beam focusing hypothesis in the false killer whale

“…If there is a maximum curvature possible for the melon as a result of anatomical limitations, this would impose a minimum FR for the transmitted sonar waveform. Many odontocete species produce low-amplitude, high-repetition clicks (termed 'buzzes') when prey are within one body length (DeRuiter et al, 2009;Johnson et al, 2004;Verfuss et al, 2009;Wisniewska et al, 2014). These buzzes are distinctly different from the clicks made when prey are at greater distances, and their rapid production rate may eliminate any need for focusing.…”

On doing two things at once: dolphin brain and nose coordinate sonar clicks, buzzes, and emotional squeals with social sounds during fish capture

“…As they hunt for fish, the complex, asymmetrical, muscular nose makes brief, high-peak frequency click trains that are focused through the fatty melon-shaped forehead (Au, 1993;Jensen et al, 2009;Ridgway et al, 2014;Wisniewska et al, 2014). The clicks bounce off fish, returning echoes to the dolphin's ear (Au, 1993) where the cochlea converts the echoes into nerve impulses (McCormick et al, 1970(McCormick et al, , 1980.…”