Old sperm is bad news, By Melissa Hogenboom

The longer it is stored in the testes, the lower its quality. And offspring created from older sperm can suffer detrimental health impacts.

So keeping things a bit fresh down there is important.

Many mammals have mechanisms for renewing their sperm stores. For example, some biologists have proposed that male masturbation in humans serves this exact purpose, releasing sperm before its quality depreciates.

In mammals such as mice though, different tactics must come into play.

In order to understand how sperm quality changes when there is little or no sexual activity, researchers have been analysing the sperm of house mice.

They compared those which had not been sexually active for about two months to those which had a much more recent sexual encounter.

It came as a complete surprise to the team that the sperm quality of these two different groups was the same.

The rested males versus the active males had equivalent numbers of sperm, of equal motility and the number of DNA mutations did not differ dramatically either.

So what's going on?

The mice are able to keep sperm quality high by using a number of tactics when they have no access to a mate. These include spontaneous ejaculations and losing sperm in urine. There may even be molecular ways of keeping sperm young to protect it while in storage.

"The idea of spontaneous ejaculations is going to occur in different mammals too," explains lead author Renee Firman of the University of Western Australia. The research is published in the Journal of Evolutionary Biology.

The team also looked at the differences in levels of "sperm competition" among males.

This is where a female on heat may mate with several males in one night, receiving sperm from each. This sperm then competes inside the female, with the male with the best sperm fathering her offspring.

Males in these competitive circumstances had better quality sperm and larger tests, says Firman.

"We think that's because those males had been under intense selection to produce high quality sperm," she says.

This conforms to the "raffle principle": the more tickets you have in a raffle, the more likely you will win. The same is true of sperm.

In mice at least, competition is clearly no bad thing. It keeps things fresh.