![]() ![]() River dolphins are known as “evolutionary relics”, and their evolutionary history still a matter of debate. These acoustic complexities are thought to carry information on group identity and maintaining social cohesion, and even an individual’s identity or emotional state. The non-linear nature of the call recordings, including sub-harmonics and biphonation, is similar to that of toothed whales such as long-finned pilot whales ( Globicephala melas). “There are a lot of obstacles like flooded forests and vegetation in their habitat, so this signal could have evolved to avoid echoes from vegetation and improve the communication range of mothers and their calves,” she says. May-Collado speculates that the riverine habitat has been a driver in the evolution of the intermediate frequency. The pitch of calls was also unique, falling in between the high-frequency clicks and whistles of toothed whales and dolphins, and the low-frequency calls of baleen species. “It’s exciting marine dolphins like the bottlenose use signature whistles for contact, and here we have a different sound used by river dolphins for the same purpose,” adds May-Collado. The most common sound made was a short two-component call, produced by baby dolphins when they approached their mothers after a short separation. “Their vocal repertoire is very diverse.” “We found that they do interact socially and are making more sounds than previously thought,” she says. The findings, published in the journal PeerJ, reveal that rather than being mostly silent as previously assumed, vocal communication in Araguaian dolphins is on par with that of their talkative marine counterparts.Įven though the team captured 727 good quality acoustic signals resulting in 237 sound types, May-Collado believes that the sample size was not enough to capture the entire repertoire. ![]() Sounds were categorised according to duration and the presence of non-linear phenomena such as subharmonics or biphonation. All whistles and pulsed calls were selected for further examination using a program called ARTwarp. More than half of the studied animals were able to be identified as individuals.īack in the lab, the sound recordings were first subjected to a spectrographic analysis. Information such as sex, age and natural marks on the dorsal and ventral parts of the body were recorded, as well as specific behaviours. Underwater recordings were taken at the same time as video footage, resulting in 15.7 hours of acoustic material. The river dolphin genus has been little studied, so the researchers expected to find more acoustic signals than those previously recorded – but, even so, the results were a big surprise. The study, led by Laura May-Collado from the University of Vermont, US, combined acoustic technology with underwater behavioural observations as the dolphins came to the market to be fed by locals and tourists. Now, a unique opportunity has allowed researchers to study a small group of the mammals, which regularly visits a fish market in the Araguaia-Tocantins river system in Brazil. ![]()
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