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Scientists were stunned to find a giant female spider that forces males to perform 'oral sex' up to 100 times in one session.

Oral sex is rare in the animal kingdom, apart from in mammals, where macaques, lemurs, bonobos, hyenas, cheetahs, lions, dolphins and bats are known to engage in fellatio. Yet cunnilingus is even rarer.

Now scientists in Solvenia have found that the Madagascan Darwin's bark spiders do it too.

The females are several times larger and heavier than males who tend to be a third smaller.

The males have to contend with being eaten after sex or even having their genitals mutilated by rivals.

Read more:False widow spider bite leaves man with gruesome 3cm-deep hole in leg

(Image: YouTube)

Scientists found to their surprise that males routinely salivate onto female genitalia before, during and after sex.

They believe the arachnids perform oral sex oral to boost its chances of mating by either showing its quality as a 'lover', or by creating a chemical environment that would favour its sperm over a rival's.

The spider, which was only discovered in 2009 and named in honour of Charles Darwin on the 150th anniversary of the publication of The Origin of Species, produces one of the largest known orb webs of up to 30 sq ft.

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Its silk is also the toughest biological material ever studied, over ten times tougher than a similarly-sized piece of Kevlar.

Arachnologist Dr Matjaz Gregoric, a research associate at the Scientific Research Centre of the Slovenian Academy of Sciences and Arts said: "Several clades [groups] of spiders whose females evolved giant sizes are known for extreme sexual behaviours such as sexual cannibalism, opportunistic mating, mate-binding, genital mutilation, plugging, and emasculation.

"However, these behaviours have only been tested in a handful of size-dimorphic spiders.

(Image: Youtube/EZlab ZRC SAZU)

"Here, we bring another lineage into the picture by reporting on sexual behaviour of Darwin's bark spider, Caerostris darwini.

"This sexually size-dimorphic Madagascan species is known for extreme web gigantism and for producing the world's toughest biomaterial.

"Our field and laboratory study uncovers a rich sexual repertoire that predictably involves cannibalism, genital mutilation, male preference for teneral females, and emasculation.

"Surprisingly, C. darwini males engage in oral sexual encounters, rarely reported outside mammals.

"Irrespective of female's age or mating status males salivate onto female genitalia pre-, during, and post-copulation.

"While its adaptive significance is elusive, oral sexual contact in spiders may signal male quality or reduce sperm competition.

"Oral sexual contact seems to be an obligate sexual behaviour in this species as all males did it before, in between, and after copulations, even up to 100 times."

The study was published in Scientific Reports.