A male Darwin’s bark spider binds an older female in silk to restrain her before mating. Image: Evolutionary Zoology Lab

Spiders are known for their extreme and bizarre sexual behaviors — think sexual cannibalism and genital mutilation — but one species may just be the kinkiest of all.

Male Darwin’s bark spiders apparently perform oral sex, and lots of it, on their female counterparts — something rarely observed in non-mammalian species. A study published in Scientific Reports found that the males salivate onto female genitalia before, after, and during copulation, as many as 100 times in a single sexual encounter.

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It doesn’t stop there, either. According to the researchers, Darwin’s bark spiders have a “rich sexual repertoire,” which also includes restraining older females with silk binds, chewing off their own genitals while mating, and mating while the female is in the process of molting.

Scientists hypothesize that males are forced to get freaky due to the extreme size difference between sexes. Female Darwin’s bark spiders can grow up to four times larger than and 195 times heavier than males. As with other spiders that display this kind of sexual size dimorphism, like widow spiders and orb weavers, males often face intense competition and have adopted certain behaviors to increase their chances of reproduction.

Perhaps the marathon oral sex sessions are a way to prove they’re worthy mates. Or the explanation may lie solely in chemistry — the enzymes in the male’s saliva could give that individual an advantage over a rival male’s sperm, the researchers say.

“Cunnilingus-like behaviors” have been observed among bonobos and fruit bats, but rarely spiders. Image: Rob Bixby

Without further research, we can only guess why they engage in what is a relatively rare behavior in the animal kingdom. Female-on-male oral sex contact has been observed among macaques, lemurs, bonobos, hyenas, cheetahs, lions, dolphins and bats, but “cunnilingus-like behaviors” are less common and rarely seen outside of mammals.

Other than their fascinating sex lives, Darwin’s bark spiders are also known for producing the toughest biomaterial on the planet. Their silk is more than 10 times tougher than a piece of Kevlar the same size and they create the largest webs on Earth — spanning up to 30 square feet!