Recognition of Frequency Modulated Whistle-Like Sounds by a Bottlenose Dolphin (Tursiops truncatus) and Humans with Transformations in Amplitude, Duration and Frequency

Abstract

Bottlenose dolphins (Tursiops truncatus) use the frequency contour of whistles produced by conspecifics for individual recognition. Here we tested a bottlenose dolphin’s (Tursiops truncatus) ability to recognize frequency modulated whistle-like sounds using a three alternative matching-to-sample paradigm. The dolphin was first trained to select a specific object (object A) in response to a specific sound (sound A) for a total of three object-sound associations. The sounds were then transformed by amplitude, duration, or frequency transposition while still preserving the frequency contour of each sound. For comparison purposes, 30 human participants completed an identical task with the same sounds, objects, and training procedure. The dolphin’s ability to correctly match objects to sounds was robust to changes in amplitude with only a minor decrement in performance for short durations. The dolphin failed to recognize sounds that were frequency transposed by plus or minus ½ octaves. Human participants demonstrated robust recognition with all acoustic transformations. The results indicate that this dolphin’s acoustic recognition of whistle-like sounds was constrained by absolute pitch. Unlike human speech, which varies considerably in average frequency, signature whistles are relatively stable in frequency, which may have selected for a whistle recognition system invariant to frequency transposition.

Human auditory discrimination of bottlenose dolphin signature whistles masked by noise: Investigating perceptual strategies for anthropogenic noise pollution

“…Prior work has shown that humans are capable of discriminating among dolphin whistlelike sounds and among object echoes ensonified with dolphin clicks (Au & Martin, 1989;Branstetter, DeLong, Dziedzic, Black, & Bakhtiari, 2016b;DeLong, 2017;DeLong, Au, Harley, Roitblat, & Pytka, 2007a;DeLong, Au, & Stamper, 2007b;DeLong, Heberle, Wisniewski, & Mercado, 2014;Fish, Johnson, & Ljungblad, 1976;Gorman & Sawatari, 1985;Helweg, Roitblat, Nachtigall, Au, & Irwin, 1995). Such comparisons are possible due to several similarities in human and dolphin hearing.…”

“…Prior research has compared the performance of human and dolphin listeners on the same listening task (Au & Martin, 1989;Branstetter et al, 2016b;DeLong, 2017;DeLong et al, 2007a;DeLong et al, 2007b;DeLong et al, 2014;Gorman & Sawatari, 1985). Human listeners, used to model dolphin hearing and processing, have been used to better understand the auditory processing of dolphins because humans are able to explain their strategies verbally.…”

“…Familiar individuals' vocalizations ought to contain voice cues (e.g., timbre) that could distinguish them from the vocalizations of unfamiliar conspecifics, but these differences do not seem to be salient. Given that dolphins seem able to use frequency contour to identify signature whistles, but not voice cues, it is likely that a signature whistle's meaning is derived primarily, if not solely, by its frequency contour (Branstetter et al, 2016b;Janik et al, 2006;Sayigh et al, 1999;Sayigh et al, 2017). Since frequency contour is so important for a dolphin's understanding or recognition of signature whistles, it is likely that the most detrimental types of anthropogenic noises will be those which mask frequency contour more effectively.…”

“…When researching dolphin auditory processing, humans often serve as a valuable model organism, as humans and dolphins share some similarities in auditory processing, and humans are able to give verbal explanations and feedback on their strategies in auditory discrimination (Au & Martin, 1989;Branstetter et al, 2016b;DeLong, 2017;DeLong et al, 2007a;DeLong et al, 2007b;DeLong et al, 2014). In these studies, humans and dolphins are often exposed to the same auditory stimuli (e.g., echoes).…”

“…Branstetter and colleagues (2016b) investigated both species' ability to discriminate sounds similar to natural dolphin whistles. Many of these studies have also involved interviews or questionnaires for the human participants, which have asked participants what elements of the sounds they were attending to when performing the discrimination tasks (Au & Martin, 1989;Branstetter et al, 2016b;DeLong, 2017;DeLong et al, 2007a;DeLong et al, 2007b;DeLong et al, 2014;Gorman & Sawatari, 1985). By analyzing human feedback and comparing performance between humans and dolphins (specifically error patterns), researchers have been able to generate informed hypotheses about strategies dolphins may use in their own auditory processing.…”