The females of many spider species have evolved giant sizes, and they’re known for extreme sexual behaviors – ranging from mate binding and plugging to sexual cannibalism, genital mutilation, and emasculation. With a recently described species of Madagascan spider called Darwin’s bark spider (Caerostris darwini), the females are several times bigger and over a dozen times heavier than their male mates. According to new findings published in Scientific Reports, they have a rich sexual repertoire that includes oral sexual contact. The males routinely salivate onto female genitalia before, during, and after copulation.
Oral sexual encounters are rarely described in the animal kingdom, except in mammals of course. Macaques, lemurs, bonobos, hyenas, cheetahs, lions, dolphins, and bats are all known to engage in fellatio-like behaviors. Cunnilingus-like behaviors, on the other hand, are ever rarer: It’s been observed in bonobos, fruit bats, and fruit flies, and some male birds peck out rival male’s sperm. Until now, the only spiders known to exhibit oral sexual encounters are the widows.
Darwin’s bark spider produces giant webs with the world’s toughest silk, and they’re increasingly becoming models in silk research. Their behaviors, however, haven’t really been studied. So, a team led by Matjaž Gregorič from the Scientific Research Centre of the Slovenian Academy of Sciences and Arts spent two weeks monitoring a 100-meter (328-foot) transect in Madagascar’s Andasibe-Mantadia National Park. The team analyzed the bark spiders’ sexual size dimorphism levels and sex ratios and documented their interactions. Back at the lab, they conducted mating trials with 17 virgin females who were older (that is, three to 10 days after maturation). Four of these females were used in a total of nine re-mating trials.
All 29 of the males performed oral sex acts on the females. The male typically hooked one of his fangs to the female’s copulatory opening, turned his body perpendicular to her, and then secreted fluids from his mouth into her copulatory openings.
“Oral sexual contact seems to be an obligate sexual behavior in this species as all males did it before, in between, and after copulations, even up to 100 times,” Gregorič said in a statement. You can watch a video of their oral sexual encounters here.
The adaptive significance of oral sexual contact in spiders is still unknown. It’s possible that it signals the quality of the male or perhaps it reduces sperm competition with chemicals that favor one male’s sperm over that of another.