Many animals use urine as a signal. We are probably most familiar with this in dogs, who ‘scent-mark’ their local surroundings to give information to other dogs.
The sort of information urine contains differs by species, but it can tell others about a range of personal qualities, such as physical health, social dominance and reproductive availability.
While urine is known to be a common way to share information in terrestrial (land-living) mammals, whether it serves the same function in marine mammals isn’t very well known. A recent study on botos, also known as Amazon river dolphins or pink river dolphins, offers clues that urine may indeed be an important method of information-sharing in this marine mammal species.
Aerial urination in botos
Researchers set about observing wild botos in the Tocantins River, central Brazil, in order to better understand their general behaviour.
While watching the botos, they noticed something curious. Sometimes, male dolphins would roll onto their backs, so that they were belly-up, and urinate. Their urine stream would arc into the air and land in the water somewhere near their heads. While peeing, they swam in a linear, circular or zig-zag fashion.
Often, another male (the ‘receiver’) would swim to where the urine was landing in the water and stick his snout into the urine stream. Sometimes, he would even appear to chase the stream.
The researchers saw 'aerial urination' happen many times, and both the urinator and receiver were always males. Because of this, they think that aerial urination may have a social function in this species.
“This behaviour will require more research,” Dr Claryana Araújo-Wang, lead author of the study, tells BBC Wildlife, “however, we hypothesise that aerial urination helps in advertising male quality in terms of social position or physical condition.”
Boto snouts (known as rostrums) have many bristles on them. It is thought these bristles help the dolphin’s pick up signals from their environment, and the scientists suggest that they might be helping the dolphins ‘decode’ the messages in other botos’ urine.
While much is still to be learned about aerial urination, the scientists have some ideas about how this behaviour spreads between individuals. They suggest that because the behaviour has only ever been observed in males, it may be socially learned – i.e., it is picked up through observation of other peeing male botos.
The team plan to continue studying boto behaviour, and are keen to dig further into exactly what information males are sharing and receiving during these aerial urination events.
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