Here’s something you probably never expected to see: A team of researchers documented a group of Bolivian river dolphins holding an anaconda from their mouths as they swam. The unexpected sighting took place in August 2021, near the Tijamuchi River in Bolivia. The phenomenon was so startling that it warranted the publication of a research paper dubbed, “A case of playful interaction between Bolivian River Dolphins with a Beni Anaconda” in the journal Ecology.

The study’s authors do not know exactly why the dolphins were messing with the big constrictor snake but do not think they were preying on it. “It became clear they were playing with the snake rather than trying to eat it,” they write. “One river dolphin was firmly holding the snake behind its head, while the other was holding onto the first third of its body. [The dolphins were] swimming together in a synchronized manner, with similar depth and speed.”

The study’s authors say that at least six dolphins were observed playing with the snake. There were also juvenile dolphins around and the adult dolphins appeared to be showing off their quarry to them. The interaction went on for at least seven minutes—a length of time that indicated to the scientists that the phenomenon was an instance of play, not predation. Dolphins in captivity are known to be playful, but this sort of behavior has not often been observed in the wild. Though anacondas are semi-aquatic and can hold their breath, it’s unlikely that the snake survived the encounter. The researchers did not witness the dolphins eating the serpent.

“This is the first time I’ve heard of dolphins playing with a large snake,” Dr. Sonja Wild, a behavioral ecologist at Max Planck Institute of Animal Behavior in Germany, who did not participate in the study, told the New York Times. Wild added that she thought it was possible that the snake had been injured or dead before the dolphins found it.

Following the encounter, the researchers realized something unexpected from analyzing the pictures they’d gathered: The dolphins appeared to be sexually aroused. “It is unclear whether this display is a part of courtship,” the study’s authors write. “It is entirely plausible that the observation reported in our study could be a sexual-social behavior.”

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Beni anacondas are endemic the Bolivian Amazon. They can grow to lengths of more than 6 feet and are considered opportunistic predators that typically prey on fish, birds, and small predators. Bolivian river dolphins are large, freshwater mammals. Compared to ocean dolphins, scientists know relatively little about the behavior of river dolphins because murky water tends to prevent extended observation. According to the study, Bolivian river dolphins are facing population declines due to overfishing and habitat loss.