Dolphins have pulled off some bizarre stunts, but this one is something else. Picture this: a male Amazon river dolphin swims belly-up to the surface. He tilts his body just right, and suddenly — a thin golden arc sprays through the air before splashing back down.
But wait, it gets weirder. Another male dolphin, seemingly entranced, rushes forward. He stretches his snout (or rostrum) to the very spot where the urine is falling. He lingers, perhaps analyzing something, before casually swimming away as if nothing happened.
So… what’s actually going on here?
A strange pissing contest
Amazon river dolphins (also called pink river dolphins or boto) are a quirky bunch. They’re more curious than their ocean-dwelling cousins, often playing with foreign objects and engaging in unexpected antics. And as it turns out, they also have a rather peculiar interest in peeing.
A group of Canadian biologists investigating them recorded 36 cases of what seems to be a strange, almost ritual behavior involving urine.
“Aerial urination starts with a boto slowly positioning itself upside down, exposing its penis above water, and ejecting a stream of urine into the air. When a ‘receiver’ male is present, it either approaches the urine stream with its rostrum (sometimes pursuing it) or stays where the stream contacts the water,” the researchers note.
“We were really shocked, as it was something we had never seen before,” study author Claryana Araújo-Wan recounted to New Scientist.
Which begs the question: why are they doing this?
Communicating through urine?
The short answer is we’re not really sure.
Scientists have long known that animals use urine for more than just waste disposal. Urine spraying can function as scent marking, mate selection, territory marking, predator defense, and communication among conspecifics. Chemical communication through urine is far less effective in the aquatic environment, but examples do exist, the researchers note.
“For instance, dominant males of African cichlids use urine pulses in both reproductive and territorial contexts while the narrow-clawed crayfish use urine to signal aggressive behavior.”
Sometimes, single males do it by themselves. Most of the time, however, there was at least one other male dolphin around, which hints at a social component. Sometimes, the “receiver” would position themselves directly under the urine stream.
It’s possible that this is also a form of communication, maybe signaling friendship or lack of aggressivity. Most dolphins lack functional olfactory organs, which means traditional scent-marking behaviors wouldn’t make much sense for them. But the boto’s strange aerial urination habit suggests an alternative: a kind of “haptic” smell — felt, rather than sniffed.
Feeling liquid communication
Botos have tiny bristles on their rostrums — barely noticeable but surprisingly sensitive. These could serve as chemical detectors, allowing them to “read” the chemical composition of their companion’s urine as it drizzles down into the water. Instead of sniffing, they might be absorbing chemical signals through their skin and specialized nerve endings.
If this were simply a byproduct of needing to relieve oneself, it would happen randomly, or at least include females. Instead, aerial urination seems to serve a very specific social function.
Could it be a male bonding ritual, strengthening alliances in a world where competition for mates is fierce? Or perhaps it’s a display of dominance, a show of virility akin to the way a male elk urinates in its own wallow to impress potential mates?
To a human observer, the behavior is undeniably comical, but for the Amazon dolphins, it could have a specific and important role.
The study “Aerial urination suggests undescribed sensory modality and social function in river dolphins” was published in the journal Behavioural Processes.
