by SANDRINE CEURSTMONT
Grass snakes (Natrix natrix) PHOTO/Sven Zacek/naturepl.com
We used to assume that male snakes were in charge and females were largely passive, but that has proved to be spectacularly wrong
When Jesús Rivas removed a female anaconda from a snake orgy in order to examine her, he got a surprise.
The anaconda’s swollen body suggested she was full of food, so Rivas waited for her to throw up: snakes often vomit after a meal if they have over-eaten or are stressed, to make themselves lighter so they can flee. But instead of a typical prey, like a capybara, a reptilian tail started emerging from her mouth.
“It was an anaconda,” says Rivas, a herpetologist from New Mexico Highlands University in Las Vegas. “And the hemepenis was there, so it was a male.”
The female anaconda had eaten one of her most recent sexual partners, a phenomenon known as sexual cannibalism. Her startling action is part of a growing body of evidence that we have misunderstood how snakes have sex.
In anacondas, sexual cannibalism only goes one way: the female consumes the male.
Previously, scientists had assumed that female snakes are submissive during courtship and mating, but it is now clear that they have a prominent role. “There was the interpretation that females had no say in the mating process,” says Rivas. He thinks that assumption stems from bias by early researchers, who were predominantly male.
In fact female snakes are physically imposing, so it is not surprising that they can overpower – and even swallow – their mates. In many animals, males are larger than females, but for most snakes the opposite is true.
In anacondas, females are on average 4.7 times larger than males. That is the biggest size difference between sexes in any land-living vertebrate. “I was surprised,” says Rivas. “The difference is drastic.”
The reason males are so often larger than females is that it helps them secure a mate.
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