A tiny hopping insect has the largest testicles of any species on the planet compared to its body mass, British researchers report.

"We couldn't believe the size of these organs. They seemed to fill the entire abdomen," Karim Vahed, a behavioural ecologist at the University of Derby, said in a statement about the testicles of the tuberous bushcricket. Bushcrickets, known in North America as katydids, are related to grasshoppers and true crickets.

Vahed is the lead author of a paper published in the journal Biology Letters on Wednesday that found the tuberous bushcricket's gigantic testes make up 14 per cent of the male's body mass.

He and his co-authors, Darren Parker at the University of St. Andrews and James Gilbert at the University of Cambridge, measured the mass of the testes in 21 species of bushcricket collected in Europe. They compared that to the mass each species ejaculated, the sperm count per ejaculation and the number of males that each female mates with on average, data that have mostly been published in previous studies.

In general, animals' testicles are larger in species where females mate with more than one male, since a male that produces more sperm may out-compete other males. Even though their adult lifespan is just two months, female tuberous bushcrickets manage to mate with up to 23 different males, the researchers noted.

But they were surprised to find that the enormous testicles didn't release proportionately large volumes of sperm each time the males ejaculated.

"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," Gilbert said in a statement.

"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."