Multi-echo processing by a bottlenose dolphin operating in “packet” transmission mode at long range

Abstract

Bottlenose dolphins performing echolocation tasks at long ranges may utilize a transmission mode where bursts, or "packets," of echolocation clicks are emitted rather than single clicks. The clicks within each packet are separated by time intervals well below the two-way travel time, while the packets themselves are emitted at intervals greater than the two-way travel time. Packet use has been shown to increase with range; however, the exact function of packets and the advantages gained by their utilization remain unknown. In this study, the capability for dolphins to utilize multi-echo processing within packets of echoes was investigated by manipulating the number of available echoes within each packet as a dolphin performed a long-range echolocation task. The results showed an improvement in detectability with an increase in the number of echoes in each packet and suggest that packet use is an adaptation to allow multi-echo processing at long ranges without introducing range ambiguity.

Discrimination of fast click series produced by tagged Risso's dolphins (Grampus griseus) for echolocation or communication

“…Assuming that dolphins do not emit the next click until the echo from the target has been received and its information has been processed (Au, 1993), the mean ICI of Grampus click trains in the present study (0.15 s) suggests a maximum detection range of ICI/2×1500 m s −1 =112 m. The aforementioned assumption that dolphins do not click until they have processed information from the previous echo may be questioned, given evidence of multi-echo processing in bottlenose dolphins (Ivanov, 2004;Finneran et al, 2014). However, this detection range is similar to the approximately 100 m range estimated based on Grampus click source properties .…”

Dolphin echolocation behaviour during active long-range target approaches

“…The number of clicks per packet increases with target range for some animals (Ivanov, 2004;Finneran, 2013), and for one dolphin the target detection threshold improved 3 dB for every doubling of the number of returning echoes following packet emission (Finneran et al, 2014). Although click packets can improve detection performance, the use of packets does not appear to be triggered by a decrease in received echo amplitude with increasing target range, but instead by an increase in echo delay (Finneran, 2013;Finneran et al, 2014). In a recent study on mine hunting capabilities, Ridgway et al (2018) observed that two bottlenose dolphins occasionally produced click packets when returning to the operator's boat having completed their tasks, whereas packets did not appear to be related to echolocation streams emitted during the search for mines.…”

“…The use of click packets has mainly been reported under laboratory conditions for stationary animals comprising a beluga whale (Delphinapterus leucas) (Turl and Penner, 1989) and a few bottlenose dolphins (Tursiops truncatus). These animals were engaged in 'go/no go' long-range target detection and discrimination experiments using either real (Ivanov and Popov, 1978;Turl and Penner, 1989;Ivanov, 2004) or phantom targets (Finneran, 2013;Finneran et al, 2014). Click packet emission occurred when echolocating towards static targets at ranges exceeding 75-120 m, with packet use being common beyond 200 m (Turl and Penner, 1989;Ivanov, 2004;Finneran, 2013;Finneran et al, 2014).…”

“…These animals were engaged in 'go/no go' long-range target detection and discrimination experiments using either real (Ivanov and Popov, 1978;Turl and Penner, 1989;Ivanov, 2004) or phantom targets (Finneran, 2013;Finneran et al, 2014). Click packet emission occurred when echolocating towards static targets at ranges exceeding 75-120 m, with packet use being common beyond 200 m (Turl and Penner, 1989;Ivanov, 2004;Finneran, 2013;Finneran et al, 2014). The number of clicks per packet increases with target range for some animals (Ivanov, 2004;Finneran, 2013), and for one dolphin the target detection threshold improved 3 dB for every doubling of the number of returning echoes following packet emission (Finneran et al, 2014).…”

“…Click packet emission occurred when echolocating towards static targets at ranges exceeding 75-120 m, with packet use being common beyond 200 m (Turl and Penner, 1989;Ivanov, 2004;Finneran, 2013;Finneran et al, 2014). The number of clicks per packet increases with target range for some animals (Ivanov, 2004;Finneran, 2013), and for one dolphin the target detection threshold improved 3 dB for every doubling of the number of returning echoes following packet emission (Finneran et al, 2014). Although click packets can improve detection performance, the use of packets does not appear to be triggered by a decrease in received echo amplitude with increasing target range, but instead by an increase in echo delay (Finneran, 2013;Finneran et al, 2014).…”