Bush-crickets have the world's biggest testicles (relatively speaking)S

A species of bush cricket is in possession of nature's most ridiculously over-sized testicles - these balls are 14% of the bug's body mass. The crickets need all that extra sperm storage just to ensure they'll have offspring.

The tuberous bush-cricket is the new record-holder for the world's largest testicles relative to body size. At 14%, it easily beats the old champion, a species of fruit fly with testes that took up 10.6% of its total size. To put that in some perspective, my quick calculations suggest human testicles account for only about .05% of the average male's body size. If humans possessed testicles of similar relative size, they would be nearly the size of a couple of basketballs.

Chief researcher Dr. Karim Vehad explains why the insects need such ridiculously large testicles:

"We couldn't believe the size of these organs, they seemed to fill the entire abdomen. We are also interested in the reason why they are so large. An almost universal evolutionary rule appears to be that such variation in relative testes size is linked to female mating behaviour; testes tend to be larger in species where females are more promiscuous, as has been demonstrated in various species in fish, birds, insects and mammals. But at least two hypotheses could account for this pattern – sperm competition on the one hand and male mating rate on the other. Yet our study appears to be the first study to show that, in the case of the Tuberous Bushcricket, bigger testes don't necessarily produce more sperm per ejaculate."

It appears that the testicles are there to allow the male bush-crickets to mate with lots of different partners. In this type of species, multiple males are competing for a female's unfertilized eggs, so one might expect the bush-crickets to use their large sperm resources to provide a lot of genetic material for one particular female and drive away the competition. But that doesn't appear to be the strategy the bush-crickets use.

Dr. Vehad and his colleague Dr. James Gilbert explain what the bush-crickets are actually doing, making it quite clear in the process that, yes, they're enjoying discussing this as well:

Dr. Gilbert: Traditionally it has been pretty safe to assume that when females are promiscuous, males use monstrously-sized testicles to deliver huge numbers of sperm to swamp the competition - even in primates. 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."

Dr Vahed: "This strongly suggests that 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 researchers suggest that large testicles are an evolutionary benefit for species where everyone is expected to take on multiple partners. That means males can't risk running out of sperm - and when adulthood only lasts for a couple of months, that means they need to carry as much of it with them as they can.

[Biology Letters]