Spiders are known for some of the strangest mating behavior in the animal world. Aside from some female spiders choosing to eat their male counterparts, some male wall crawlers also castrate themselves while they do the deed with their partners. Other spider species even engage in some form of insect bondage to avoid getting eaten after copulations.

Now, scientists have discovered another quirk in spider mating that can be compared to the act of having oral sex.

In a new study featured in the journal Scientific Reports, researchers from the Slovenian Academy of Sciences and Arts found that male Darwin's bark spiders (Caerostris darwini) in Madagascar often salivate onto the genitalia of their female counterparts as part of their sexual repertoire.

While the creatures are known to engage in genital mutilation and sexual cannibalism as part of their mating ritual, this is the first instance that the spiders were observed to carry out cunnilingus-like behavior.

Study co-author Matjaz Gregoric said that this form of oral sex appears to be an important part of the bark spiders' sexual behavior as all males typically do it before, during and after mating. Some male spiders even go as far as doing it 100 times during copulations.

Engaging in oral sex is a very rare occurrence in the animal kingdom. Mammals are the only ones that have shown such tendencies, with lemurs, macaques, bonobos, bats, lions, cheetahs, hyenas and dolphins having been observed to engage in fellatio-like behavior. Cunnilingus-like acts, such as the one observed in Darwin's bark spiders, are even rarer.

According to the researchers, it is likely that male bark spiders engage in such oral sexual contact as a way to boost their chances of producing an offspring by letting their female partners know about their quality.

It could also be their way of establishing a chemical environment that would help their sperm appear more favorable compared to those of their rivals. The enzymes found in the male spider's saliva could be used to destroy the sperm left over by the female spider's previous mating partner.

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