Male Amazon river dolphins have been documented rolling upside down and firing a stream of urine into the air. As if that isn鈥檛 bizarre enough, other males will usually seek out the urine as it arcs back down to the water, possibly to receive social cues in a similar way to how land mammals use scent marking.
at CetAsia Research Group in Ontario, Canada, and her colleagues documented the unusual behaviour while studying Amazon river dolphins (Inia geoffrensis), or botos, in the Tocantins river in central Brazil.
鈥淥n the first occasion, we saw a male flip his belly up out of the water, expose his penis and then proceed to urinate into the air,鈥� says Ara煤jo-Wang. 鈥淲e were really shocked, as it was something we had never seen before.鈥�
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Aerial urination has rarely been documented in cetaceans. The team had heard a few reports from other researchers who had seen Amazon river dolphins doing this on rare occasions, but the behaviour hadn鈥檛 been studied before.
Ara煤jo-Wang and her colleagues spent around 219 hours observing the dolphins and saw the males fire urine into the air 36 times, with the streams often reaching 1 metre high before landing 1 metre in front of the animal.
What鈥檚 more, they discovered that on two-thirds of occasions, another male was present. Intriguingly, the other male would approach, and sometimes pursue, the urine stream with its snout, or rostrum. Other times, they waited in the area where the stream of urine landed in the water.
The researchers鈥� best guess to explain this behaviour is that bristles on the dolphin鈥檚 rostrum might act as chemical sensors to detect urine and interpret social cues. Hormones in urine might signal a male鈥檚 quality, in terms of social position or physical condition, says Ara煤jo-Wang.
A 2022 study by at Stephen F. Austin State University in Texas and his colleagues found that bottlenose dolphins (Tursiops truncatus) can use their tongues to identify different individuals underwater by the taste of their urine.
鈥淚t would be fascinating to see if this behaviour from the river dolphins relates to this ability,鈥� says Bruck. 鈥淚t seems unlikely that botos are processing complex social cues using just the hairs on their rostrum.鈥�
But it also remains unclear why Amazon river dolphins fire urine into the air instead of directly into the water. Bruck speculates that because river dolphin vision is poor, the sound of the urine hitting the water could be an acoustic signal to let them know where the urine is to be sampled.
鈥淲e still don鈥檛 know what other information can be gleaned from urine, including sex or reproductive status, so chemical signalling represents an exciting new frontier in dolphin research,鈥� says Bruck.
Behavioural Processes
Article amended on 3 February 2025
We corrected the scientific name of the bottlenose dolphin.
