Amazon river dolphins in the Rio Negro, Brazil. Photo credit: kevinschafer.com.
Whales and dolphins cruising the high seas encounter few obstacles to confound their echolocation systems. However, a select group of cetaceans that have made their homes in rivers and estuaries must contend with the bewildering array of confusing echoes produced by their confined environment. And for Amazon river dolphins – also known as botos – the situation could become even more perplexing when their river home bursts its banks and floods the surrounding forest. Michael Ladegaard from Aarhus University, Denmark, explains that bats suffer similar challenges when echolocating in caves and forests, so he and his colleagues, Frants Havmand Jensen, Mafalda de Freitas, Vera Ferreira da Silva and Peter Madsen decided to find out more about the echolocation clicks to see how well they are adapted to the confined conditions.
The team travelled to three locations in the Amazon river basin to record the echolocation clicks in the white silty water of the Rio Solimões and then to São Tomé to record in the dark waters of the Rio Negro and where the waters mingle at the confluence of the two rivers. As the botos had to be perfectly in line with the underwater microphones (hydrophones), Ladegaard and his colleagues recorded almost 35,000 clicks with a string of seven hydrophones suspended vertically in the water beneath a boat before identifying 268 clicks that had been produced when the animals were head-on and within 21 m of the hydrophone array. Analysing the click frequency spectra and intensity, and the width of the beam of sound, the team found that the animals produce narrow (half-power beamwidth of 10.2 deg) soft (peak–peak source level of 191 dB re. 1 μPa) high-pitched (101 kHz) echolocation clicks that lasted 14 ms with a brief interval of 35 ms between clicks.
Comparing the click's acoustic parameters with those of marine dolphins, the team explains that the botos’ echolocation system is well adapted to their cluttered surroundings. They click softly so that echoes only return from nearby objects. This in turn means that all of the echoes that they need to interpret return within milliseconds, allowing the animals to produce high click rates of around 30 clicks per second while limiting the reverberations produced in the confined space of shallow rivers. The team also noticed that the botos could change the width of the beam of sound, which may allow them to adjust their acoustic view depending on the complexity of their surroundings.
However, the river dolphins’ soft echolocation clicks buck the relationship between click frequency and intensity that has been found for marine cetaceans. According to Ladegaard, large marine cetaceans produce softer clicks with more low-frequency vibrations than smaller species. He also explains that the increase in the low-frequency spectrum means that the softer clicks are more diffuse and the beam of sound is wider, which is fine when echolocating for lunch in the open ocean. However, botos live in a far more complex environment, necessitating better-directed echolocation clicks, so the river dolphins adopted a different tactic. They raised the pitch of their echolocation clicks, allowing the freshwater animals to produce soft echolocation clicks that can be well directed in their cluttered homes.
So, Amazon river dolphins’ echolocation clicks are well suited to their surroundings, and Ladegaard concludes, ‘Habitat in addition to size may play an important role in the evolution of toothed whale echolocation’.
