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Most of us would prefer a cuddle - or a nap - after a bout of passion with our other half.

But the coin spider has a somewhat more unusual post-coital routine: he bites off his own genitals.

This self-emasculation was previously considered a peculiar evolutionary phenomenon, with unknown function.

Lost your appetite? Scientists didn't understand why male coin spiders (Herennia ornatissima, large female, small male, pictured) behaved so oddly by eating their genitals after mating with a female until now

But now scientists have found out why they do it.

By removing the excess baggage, the spider becomes lighter on his feet and better able to keep rivals away from his freshly impregnated mate.

The male spider (Herennia multipuncta, pictured), by removing the excess baggage, becomes lighter on his feet and better able to keep rivals away

This is important for spiders because several males can fertilise the same batch of eggs. 

So keeping his mate away from others is the only way the coin spider secures paternity of resulting offspring.

And, for extra insurance, he damages his genitals while still inside his mate, in order to block up the access route to her reproductive organs.

He then chews the genitals completely off so he can leave them inside the female and fight off others males.

Researcher Majaz Kuntner, from the Slovenian Academy of Sciences and Arts, said: 'The small spider male only produces enough sperm to fertilise the eggs of a single large female.

'Since he faces huge competition from other males, he chooses to monopolise a female by mutilating his genitals in the female genitals, thereby blocking access.

'He then chews off his mutilated organs, in order to become lighter and more agile, and defend his female better.'

The study was published in the journal Behavioural Ecology and Sociobiology. 

ARE SPIDERS GETTING BIGGER? MILD SUMMER GAVE ARACHNIDS A FEAST

Experts have warned that homes may be set for an invasion of larger than normal spiders who have feasted on an abundance of prey in the last few months.

That's because this year the warm summer has allowed certain spiders to eat more than usual and grow to their upper limits.

Say 'hi' to your housemate! A warm summer has let spiders grow to their upper limits

And it could mean we'll see more and more large spiders in our homes in the coming months. 

The mild summer has meant the eight-legged creatures have had plenty to eat and very few have perished.

With temperatures set to fall, experts from Sydney University have said the larger-than-usual house spiders will be heading indoors in the coming weeks to find a mate.

Professor Adam Hart of the University of Gloucestershire agreed with their predication and said: 'This year has been seemingly a good one for the invertebrates which spiders feed on, and it's quite mild out there.'

Both sexes stay in their webs until the autumn when the males become nomadic and search for females.

Mr Lawrence Bee of the British Arachnological Society tells MailOnline that people often notice larger spiders this year as the cold weather drives them inside, with males hunting for females.

But he agrees that the particularly mild summer we've had, not too hot and not too cold, will have given spiders access to more prey.