Imagine, if you will, the plight of a tiny male spider trying to mate with a female twice his size and 14 times heavier — a female likely to eat him before he even finishes the deed, then keep on mating with as many males as she can when she’s done so that he might not even father any offspring with her. That’s just life for the male Darwin’s bark spider (Caerostris darwini), a Madagascar native famous for making the world’s strongest web silk.
Male spiders have a few tricks to ensure they survive a sexual encounter and manage to pass their genes on. Sometimes males bind females in silk before mating, or mate with them just after they’ve molted, as females can’t eat while their new skin is settling in. Some males — including the Darwin’s bark spider — even chew off their own genitals while mating, so that the female is “sealed off” from mating with other males.
But the Darwin’s bark spider goes the extra mile, doing something quite rare in the invertebrate world. Every time males engage in sex, they salivate on their mate’s genitals.
“In this spider species, the male first hooks his fang to female genitals, and then salivates into them. He repeats this behavior before, between and after copulation, up to 100 times in one mating,” says Matjaž Gregorič of the Slovenian Academy of Sciences and Arts, who led the new study.
Gregorič isn’t sure what motivates the behavior, which has only been recorded in two other invertebrates: fruit flies and widow spiders. The practice hasn’t been well-documented in widow spiders, so their bedroom habits don’t make the darwini's proclivity for oral activities any less mysterious.
Outside of the invertebrate world, several non-human mammals — lemurs, hyenas, lions and a few other species — engage in oral sex. But in these cases, males are always on the receiving end of things, so to speak. The reverse is pretty rare. It is only known to occur in fruit bats, bonobos and one bird species. Bonobos are known for using sexual pleasure to keep their super-friendly society running smoothly, but in general, oral sex behavior provides some sort of immediate advantage, mostly for the giving partner.
In other words, it’s actually a pretty selfish act.
In the case of the Darwin’s bark spider, one theory is that oral sex allows males to broadcast their high quality as mates to females they encounter. They could also be trying to make the females calmer to avoid being eaten. But Gregorič favors another idea that has to do with the chemistry of sex. “We find it the most likely that salivating into female genitals might give one male's sperm a chemical advantage over rival males' sperm. Likely, that is why they do it as much as they can,” Gregorič says.
Karl Gruber is an evolutionary biologist and freelance science and health writer based in Perth, Western Australia. He is currently pursuing a PhD at the University of Western Australia and the Western Australian Museum, where he investigates the genetic diversity of a group of Australian trapdoor spiders.