The newly described Araguaian river dolphins,Inia araguaiaensis(Cetartiodactyla, Iniidae), produce a diverse repertoire of acoustic signals

Abstract

The recent discovery of the Araguaian river dolphin (Inia araguaiaensis) highlights how little we know about the diversity and biology of river dolphins. In this study, we described the acoustic repertoire of this newly discovered species in concert with their behaviour. We analysed frequency contours of 727 signals (sampled at 10 ms temporal resolution). These contours were analyzed using an adaptive resonance theory neural network combined with dynamic time-warping (ARTwarp). Using a critical similarity value of 96%, frequency contours were categorized into 237 sound-types. The most common types were emitted when calves were present suggesting a key role in mother-calf communication. Our findings show that the acoustic repertoire of river dolphins is far from simple. Furthermore, the calls described here are similar in acoustic structure to those produced by social delphinids, such as orcas and pilot whales. Uncovering the context in which these signals are produced may help understand the social structure of this species and contribute to our understanding of the evolution of acoustic communication in whales.

The Social Role of Vocal Complexity in Striped Dolphins

“…Biphonation is a phenomenon-occurring in the repertoire of several mammalian species. Among the odontocetes, it was identified for Tursiops truncatus (Reiss, 1988;Bojanowski et al, 2000;Kriesell et al, 2014;Papale et al, 2015), Orcinus orca (Filatova et al, 2009), Stenella frontalis (Herzing, 1996;Evans-Wilent and Dudzinski, 2013;Kaplan et al, 2018), Grampus griseus (Corkeron and Van Parijs, 2001), and Inia araguaiaensis (Melo-Santos et al, 2019).…”

Repeated downsweep vocalizations of the Araguaian river dolphin, Inia araguaiaensis

“…They were identified during only 10 of 82 Inia encounters, which comprised over 43 hours of recordings across a wide variety of behavioral contexts and locations. Furthermore, downsweeps were not detected by Melo- Santos et al (2019) in 15 hours of recordings of a socializing/feeding group of humanhabituated dolphins in the lower reaches of the Tocantins River. This suggests such sounds are used in more specific behavioral contexts.…”

“…Nevertheless, their rare occurrence in our recordings makes it unlikely that they are required to maintain group cohesion as in delphinids (Janik, 2009). More common signals like pulsed calls (Melo Santos et al, 2019) likely play a more important role in social cohesion than downsweeps.…”

Vocal sequences in narwhals (Monodon monoceros)

“…For example, the "patterned burst pulses" in northern right whale dolphins are the only other series that appear to be rhythmically repeated, and the "burst pulse sequences" produced by dusky dolphins rarely had more than two subunits. Calves of Araguaian river dolphins (Inia araguaiaensis) produce calls similar to the narwhals' burst pulse series, although it remains to be tested whether they are combined into sequences of series (Melo-Santos et al, 2019).…”