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Bolivian river dolphins sometimes dangle anacondas in their mouths and scientists don’t know why

Seriously, dolphins?

Tibi PuiubyTibi Puiu October 2

Edited and reviewed by Zoe Gordon

Not all dolphins live in the oceans. Some can be found swimming in major rivers, especially in South America and Asia.

The Bolivian river dolphin (Inia geoffrensis boliviensis), one of the few such species, is pretty elusive (and pretty endangered). Biologists are particularly interested in them because their populations serve as indicators for the overall health of the basins where they live, but usually, they can only spot their fins or tails.

Imagine the excitement of biologists from the Noel Kempff Mercado Museum of Natural History during one seemingly uneventful day in August 2021. That day, they saw not one, but two Bolivian river dolphins with their heads above the river. But that was really nothing compared to what they noticed next.

Bolivian river dolphins anaconda

You definitely don’t see this every day. Credit: Omar M. Entiauspe Neto, Steffen Reichle, Alejandro dos Rios.

Taking out their cameras to snap some quick pictures of this extraordinarily rare sighting, the researchers could now see that the two dolphins were holding an anaconda in their beaks, handling it like it was a plush toy.

This was absolutely shocking to the members of the teams. It’s safe to say something like this had never been witnessed, or at least this is the first time it was documented.

Bolivia’s Beni anacondas are apex predators, meaning no one messes with them in their ecosystem. Apart from a single case of cannibalism, no one had ever seen Bolivian anacondas getting killed or eaten by any other wild animal.

But why were the dolphins doing this? That’s the million-dollar question that scientists can’t yet seem to answer definitively.

The Bolivian river dolphin is one of four freshwater river dolphins, locally known as bufeos, swimming in South America. They can grow up to 2.8 meters (9 feet) long and have many teeth that they use to munch on fish and crabs. However, snakes are not part of their typical diet.

An Unusual Habit

Writing in the journal Ecology, Omar Entiauspe-Neto and colleagues believe the likeliest explanation for this bizarre event is play at the snake’s expense. For both oceanic and river dolphins, play is a critical aspect of their lives, which provides opportunities for calves to practice and perfect locomotive skills, including those involved in foraging and mating.

The snake probably had much less fun than the frolicking dolphins. Beni anacondas are semi-aquatic creatures that can hold their breath underwater for extended, albeit limited periods of time. Some of the six Bolivian river dolphins counted by the researchers at Tijamuchi River during this event held and submerged the anaconda underwater for at least seven minutes, likely killing the snake.

There were also some dolphin juveniles witnessing the scene, which introduces another possibility: the snake handling was meant to be instructional. However, the juveniles could have just been curious all the same.

Another wild although not entirely implausible explanation is that the dolphins felt a bit frisky. Dolphins are some of the most sexually active and sexually aggressive animals on the face of the Earth. They will just stick it wherever it fits.

Scientists have previously observed a male bottlenose dolphin wrap a live wriggling eel around its penis. Another male was seen attempting to penetrate the blowhole of a pilot whale in an aquarium.

Since the photos of the dolphins showed they had erected penises, it’s possible the exciting experience of handling a huge snake with their beaks may have been sexually stimulating for them. The sensation of rubbing against the snake may have also been sexually enthralling for the dolphins. It may even have been possible that the male dolphins tried to penetrate the snake — that wouldn’t have been surprising.

A Habitat Under Pressure

When it comes to the Amazon, the environmental discussion is usually aimed at the rainforest, but the river itself is in trouble.

A 2023 study in the Peruvian Amazon tracked multiple dolphins via satellite tags and found their movements overlapped with heavy fishing zones, dredging sites and proposed dam sites. The study concluded that those human disturbances (fishing gear, dredging, infrastructure) were significant threats to those dolphins.

During the severe drought of 2023, lakes in the Amazon such as Lake Tefé experienced mass mortality events of river dolphins: more than 200 dolphins died in one lake amid extremely high water temperatures and low water levels. In 2024, it got even worse.

A field programme led by Omacha Foundation, the National Marine Mammal Foundation and other partners carried out health checks on freshwater dolphins (members of the Inia geoffrensis family) in Colombia and Peru in early 2025. They found high mercury levels and drug-resistant bacteria in the dolphins.

We know much more about ocean dolphins than their river-dwelling counterparts, partly because the latter are more elusive and the waters they swim in are murky. But they are every bit as fascinating and interesting. There’s much we don’t know about their intelligence, and if they play with anacondas, who knows what other fascinating things they do?

This article was originally published in 2023 and has been edited to include more recent information.

Tibi Puiu

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