In the often murky underwater world of river dolphins, light cannot travel far, and this makes it difficult for them to see what is happening around them. However, in contrast to light, water is an ideal medium for sound. Underwater sound travels four times faster compared to air. Dolphins have adapted well to this. They have a good sense of hearing and can produce various types of sounds, which they use to orient themselves, to locate food, and to communicate with each other.
Because of the elusive nature of river dolphins, it’s not always easy to study them visually, but since river dolphins make sounds, analysing these sounds can help to obtain a better understanding of their behaviour and interactions with the environment. For example, we can learn what the effects are of human-made sounds, such as boat engines. What we know from studies of marine dolphins is that human-made noise can mask the communication sounds of dolphins. It can also affect/disrupt essential behaviours such as feeding, resting, and mating when they move away to avoid disturbance of sounds. And in the worst case, very intense sounds can also be so harmful that it results in hearing damage.
The sound repertoire of most dolphins can be divided into three major categories:
echolocation clicks, whistles, and burst-pulse sounds.
Here you find some examples of these sounds we recorded of Bolivian river dolphins.
The images are a visual representation (spectrogram) of a sound clip. In the spectrogram, the vertical scale represents the frequencies, and the horizontal scale represents the time (duration). The amplitude (or energy or “loudness”) of a particular frequency at a specific time is represented by the different colours. The blues corresponding to low amplitudes and brighter green colours correspond to stronger (or louder) amplitudes.
Echolocation clicks are short, high-pitched sounds which are not audible to the human ear. From the echoes they receive back, dolphins can make a ‘sound-image’. They obtain information such as how far is the object, its size, and if is it moving or not.
The recording is delayed so that it is audible to us.