A species of cricket has the largest testes in relation to its body weight of any known creature in the world, stunned scientists have reported.

The tuberous bushcricket (Platycleis affinis) has testes that amount to 13.8 per cent of its body mass - the rough equivalent of a man hauling around tyre-sized testicles weighing 10 kilos.

"We couldn't believe the size of these organs. They seemed to fill the entire abdomen," said Karim Vahed, a behavioural ecologist at Britain's University of Derby.

The tuberous bushcricket beat all rivals in a testicular comparison of 21 species of bushcrickets, also known as katydids.

But for all its gonad grandstanding, the insect did not produce more sperm per ejaculation than others and this offers an intriguing challenge to evolutionary theory.

Testes tend to be larger in species where females are more promiscuous.

The assumption behind this is that the male which produces the most sperm has an advantage over his rivals in love.

But the tuberous bushcricket may cause this theory to be revisited, say the authors.

Its large reserves of sperm enable it to ejaculate, in tiny amounts, with a bigger number of females, thus boosting its chances of reproductive success.

"Traditionally it has been pretty safe to assume that when females are promiscuous, males use monstrously-sized testicles to deliver huge amounts of sperm to swamp the competition," said researcher James Gilbert of Cambridge University.

"Our study shows that we have to rethink this assumption. It looks as though the testes may be that big simply to allow males to mate repeatedly without their sperm reserves being exhausted."

Said Vahed: "Extra large testes in bushcrickets allow males to transfer relatively small ejaculates to a greater number of females. Males don't put all their eggs - or rather sperm - in one basket."

The research appears in Biology Letters, a journal of Britain's Royal Society.

- AFP