Brazil’s Amazon rainforest is home to a dizzying array of life, from colorful poisonous frogs to deadly jaguars. One resident is the Araguaian river dolphin, which was only recently discovered by scientists in 2014. Unlike playful ocean dolphins, the Araguaian river dolphin is relatively solitary. Mother dolphins interact frequently with their baby calves, but rarely do adults interact with each other. Because of the river dolphins’ preference for solitude, scientists thought that they didn’t make as many sounds to communicate as ocean dolphins.

Recent research demonstrated that these dolphins do, in fact, have a large “vocabulary.” A team of scientists from Brazil, the United Kingdom, and the United States recorded dolphins at a fish market where the dolphins often visited to grab a meal. They recorded the length and tone of noises made by the dolphins for 15 hours over the course of a month. Using software that turned the sounds into visual curves, they found that the dolphins made 237 “types” of sound. Most of the sounds were short and used two different relatively deep tones.  This contrasts with ocean dolphin vocalizations, who use more whistling tones.

Both river and ocean dolphins vocalize when trying to communicate with their calves. However, it appears ocean dolphins vocalize to socialize with nearby dolphins, while the river dolphins are more frequently warning others to stay away.  The river dolphins’ deeper vocalizations might be due to their environment. The rivers are filled with plants, which can hinder the dispersal of sound waves between dolphins. The scientists propose that the reason river dolphins use shorter and deeper tones to limit how much their vocalizations are muffled by the dense vegetation. The researchers propose that studying the vocalizations of other species of dolphins that aren’t accustomed to human contact will allow us to understand how vocalization behaviors appear in different dolphin populations, how they help dolphins survive in different environments, and how the sounds can elucidate the evolutionary path of different dolphin species.


Managing Correspondent: Emily Kerr

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

Press article: Rare footage of river dolphins helps scientists crack mystery of marine mammal communication

Image Credit: Michelle Bender, Amazon river dolphin.jpg, wikicommons

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