2 male dolphins were seen playing with an anaconda while sexually aroused in a perplexing encounter captured by researchers

• Researchers in Bolivia captured a rare encounter between Bolivian river dolphins and Beni anacondas.
• At one point, two adult male dolphins held onto the snake and swam in unison while sexually aroused.

Two Bolivian river dolphins were seen swimming around and playing in the Tijamuchi River in Bolivia. The unusual part? They were carrying a Beni anaconda, an apex predator, in their mouths.

Researchers captured the rare encounter in August 2021 in photos and described it in a paper published last month in the journal Ecology.

The research team spotted a group of dolphins immediately upon arriving to the site and began taking photos. Only when they reviewed their first images did they realize the dolphins were carrying the snake, according to the study.

The researchers said it became clear the dolphins were playing with the snake rather than trying to eat it, in part because the interaction lasted for at least seven minutes.

At one point, they observed the adult males each holding onto the anaconda and swimming in unison. Upon reviewing the photos later, the researchers realized the dolphins had erect penises, which also supported the idea that it was a playful interaction.

Playing is a well-documented behavior in mammals and dolphins generally, but the encounter was the first-ever recorded between a Bolivian river dolphin and a Beni anaconda.

The researchers said many questions remained and offered possible alternative explanations for the behavior, including predation.

Beni anacondas are large semi-aquatic snakes, generally reaching more than 6 feet long, and typically have no known predators. Other than one record of cannibalism, there's no published record of an animal eating a Beni anaconda.

Because the snake was submerged for much of the encounter, the researchers said it most likely died.

The researchers said it was also possible the adult male dolphins were teaching the juvenile dolphins that were present about the Beni anaconda, or that the dolphins were engaged in an attempt at courtship. They said male Amazon river dolphins have been observed in the past carrying objects in what appeared to be an attempt aimed at females.

Diana Reiss, a marine mammal scientist at Hunter College who was not part of the study, told The New York Times the dolphins may have been sexually stimulated by the anaconda: "It could have been something to rub on."