A rare behaviour in cetaceans: the urine jets of botos intrigue researchers

Researchers have documented a previously unknown behaviour in Inia geoffrensis: the male Amazon river dolphin rolls onto its back at the surface and projects a jet of urine into the air, a behaviour described as aerial urination. The phenomenon was recorded 36 times between 2014 and 2018, over 218.9 hours of observation on the Tocantins River, Brazil.

In about 67% of cases, another male, called the “receiver”, is present. It positions itself where the jet lands, or follows the jet with its rostrum, apparently attracted by the flow of urine.

The mean duration of the jets is about 11.6 seconds (standard deviation ± 9.6 s), with extremes ranging from 1 to 41 seconds.

Sensory capacities and the role of the vibrissae

The authors suggest that the sensory hairs (vibrissae) of the rostrum play a part in detecting molecules present in the urine, making up a sensory modality that may still be undescribed, sometimes called “haptosense”.

This mechanism would serve to perceive hormonal or physiological compounds indicating an individual’s body condition or social status.

Social setting and context of the events

All the sessions involve males only, both urinating and receiving. Aerial urination occurs mainly in a social context rather than for simple elimination.

The receiver appears to actively seek out the jet. In some cases it follows the flow with its rostrum.

Hypotheses on the communicative function

• Transmission of information on the physical quality, the social status or the reproductive state of the emitting male.
• A chemical signal working as an aquatic analogue of urine marking in terrestrial mammals.
• Use of acoustic cues: the sound of urine striking the water surface could act as an auditory marker in an environment of reduced visibility.

Distinctive features of botos and the Amazonian environment

Botos live in turbid waters where vision is limited. This ecological constraint could favour the emergence of chemical or tactile signals to make up for the visual deficit.

The social structure of males in this species, with competitive interactions over access to resources or to mates, could also account for a communication system based on the chemical detection of individual qualities.

Key technical data

Item	Value/status
Number of events observed	36
Total observation time	218.9 h
Proportion with a receiver present	~67%
Mean duration per event	11.6 s
Participants	Males only
Study area	Tocantins River, Brazil
Research institutions	Botos do Cerrado Project, CetAsia Research Group

Future research directions

It would be useful to measure the chemical composition of the urine jets in order to identify the molecules involved. Correlating these data with physiological or behavioural markers would make it possible to test the hypothesis of a signal linked to reproductive state or social status.

Experiments targeting the vibrissae could test their sensitivity to chemical stimuli, and so support the idea of a previously unrecorded tactile-chemical sensory mode.

Further observations in different populations of Inia geoffrensis would make it possible to determine whether this behaviour is socially learned or innate, and whether it is specific to certain age classes or environmental contexts.

Source of this article: https://doi.org/10.1016/j.beproc.2025.105149

Photo credit: A male Amazon river dolphin (Inia geoffrensis) urinating into the air – Claryana Araújo-Wang / Botos do Cerrado Research Project / CetAsia Research Group