Marking Water

Scientists studying Amazon river dolphins in central Brazil about a decade ago noticed some odd behavior.

The male dolphins would roll on their backs, expose their genitals above the water, and urinate in the air. Then, another male would put his face in the area where the urine landed.

The research team from Canada’s CetAsia Research Group then conducted a study to better understand why the dolphins – also known as botos or pink river dolphins – were doing this.

They conducted nearly 219 hours of observations and recorded 36 instances of males performing “aerial urination,” Popular Science reported.

They explained that one male would swim in a linear, circular, or zig-zag fashion while doing it. The other male – dubbed the “receiver” – would then swim to the landing spot and stick its snout into the stream. Sometimes the males would also chase the stream.

The perplexing findings provide some new insights into the role of urine in aquatic animals, the researchers noted.

Many land-based creatures, such as cats, dogs, and rats, use urine as a way to mark their territory, but also communicate other information, such as physical health and reproductive availability.

However, urine has been less studied in marine creatures. Still, previous research has detected instances of communication: For example, dominant male African cichlid fish interpret urine pulses for both reproductive and territorial information.

Lead author Claryana Araújo-Wang and her colleagues believe some form of communication is happening with the cetaceans.

“This behavior will require more research,” she told BBC Wildlife. “However, we hypothesize that aerial urination helps in advertising male quality in terms of social position or physical condition.”

The boto snouts have many bristles that could help them pick up signals and “decode” messages from their fellow dolphin’s urine.

Since it has only been observed in male botos, the researchers believe it may be a socially learned behavior.