A South American black widow spider starts biting, wrapping up and eating her willing partner before they have finished mating – and then mates with (and eats) another male.

“Usually there are some advantages to the male for being eaten during mating, like longer copulations as well as decreased female receptivity to future males,” says Luciana Baruffaldi at the University of Toronto, Scarborough, Canada. “In this case, though, we don’t yet know how the male benefits from sexual cannibalism.”

Despite a reputation for eating their mates, female widow spiders don’t always engage in such behaviour.

However, scientists have identified three species in which widow females do often practise sexual cannibalism – eating a mate – during copulation. In two of the species – the redback spider (Latrodectus hasselti) and the brown widow spider (Latrodectus geometricus) – the males actually initiate the cannibalism by flipping themselves over to “offer” their abdomens to the female, Baruffaldi says.

Curious about the behaviour of the third species – the mirabilis widow (Latrodectus mirabilis), native to South America – Baruffaldi and her colleague Maydianne Andrade collected dozens of juvenile spiders from the wild in Uruguay.

They kept the arachnids in plastic cages at the Clemente Estable Institute of Biological Research in Montevideo, Uruguay, until they had moulted, meaning they had reached sexual maturity. The scientists then placed each female into a larger “mating arena” cage. Once each female had built a web, the researchers added a male to the arena.

Read more: Some male spiders tie up females before mating to avoid being eaten

All 20 females accepted males that “courted” them by vibrating the webs. During copulation, 14 of the females cannibalised the males – which put up no resistance. The females bit the males’ legs and bound them up in silk while pulling the males’ abdomens up to their mouths to start eating them.

Significantly, males appeared to sacrifice themselves in this way even though it doesn’t necessarily guarantee they will father the female’s offspring. In a second lab experiment, the researchers discovered that most of the females that had mated with (and eaten) a male within the last month were perfectly willing to mate with a second male – before eating him as well.

The males’ passive acceptance of their fate suggests they must find some benefits to sexual cannibalism, says Baruffaldi. “They’re just there like, ‘Hello, I’m here, eat me if you want’,” she says.

It is possible that sexual cannibalism might keep female spiders “entertained” while the male transfers large quantities of sperm and other substances related to reproduction, she says. It might also give the male a better chance to insert his “plug,” a part of his reproductive organ that he sometimes leaves within one or both of the female’s two genital openings. This plug might help the male boost his chances of fathering offspring even if the female mates again.

What’s more, a recent study also showed that, at least in some species, widow offspring grow faster and are more likely to survive to adulthood when the female eats the male during mating.

It is also possible a male considers self-sacrifice worth it even if they have a low probability of siring offspring, because the alternative is to not sire offspring at all. “A lot of males get killed even trying to find a female,” says Baruffaldi. “So maybe when they do find one, they’re like, ‘Okay, this is my only chance,’ and they just invest everything they’ve got into that one chance.”

Journal reference: Behavioural Processes, DOI: 10.1016/j.beproc.2021.104406