The echolocation clicks of toothy whales and dolphins typically encounter few obstacles at sea. Amazon river dolphins, on the other hand, live in shallow channels and flooded forests alongside dense vegetation – confined environments where sonar operations might result in high levels of clutter and reverberation. According to new findings published in the Journal of Experimental Biology, these dolphins rely on a high-frequency, short-range biosonar. 

Previous studies found that body size plays an important role in the evolution of toothed whale echolocation. Aarhus University’s Michael Ladegaard and colleagues wanted to see if habitat shaped the evolution of their biosonar as well. They recorded the echolocation clicks of wild Amazon river dolphins (Inia geoffrensis, also called botos) in three locations in the Amazon during October of 2013: near São Tomé in Brazil, at the confluence of Rio Negro and Rio Solimões, and in the Mamirauá Sustainable Development Reserve. The dolphins were recorded from small aluminum-hulled boats, and an array of seven hydrophones were deployed vertically as the team drove slowly ahead of the animals. The researchers recorded almost 35,000 echolocation clicks, of which 268 were recorded head-on and within 21 meters (70 feet) of the equipment. 

These river dolphins, the researchers discovered, produce soft, high-pitched echolocation clicks that lasted 14.1 microseconds with a brief interval of 35 microseconds between the clicks.

By increasing the frequency of their clicks, these freshwater dolphins could direct their sonar better than their ocean faring cousins. With soft, lower amplitude clicks, echoes only return from nearby objects. That means all of the echoes that they need to interpret return within milliseconds, Inside JEB explains, allowing them to produce high rates of about 30 clicks a second, while limiting reverberations.

Low-amplitude, highly directional biosonar systems, the team argues, are advantageous in riverine habitats because they simplify the auditory scene and help with target detection in cluttered, acoustically complex spaces.