Scientists say the record-breaking testicles may allow the male cricket to mate with several females in quick succession

The animal kingdom has gained a new record holder after scientists declared a species of cricket to possess the largest known testes of any creature in relation to its body mass.

Delicate measurements of the tuberous bush cricket, Platycleis affinis, showed that its testes accounted for 14% of the insect's body weight. If the same proportion were applied to a man, his testicles would weigh the equivalent of six bags of sugar each.

Scientist at the universities of Cambridge and Derby examined the testes of 21 species of bush cricket as part of an investigation into the evolutionary consequences of the insects' sexual habits.

"These really are quite phenomenal testes. They take up nearly the whole of the bush cricket's abdomen. It just shows how competitive reproduction is for some species. If you can't spread your genes, that's it in terms of evolution," said Dr Karim Vahed, a behavioural ecologist at Derby, who led the study.

Writing in the journal Biology Letters, Vehad and his colleagues explain that larger testes are usually found in animals where the female mates repeatedly with different males. By evolving larger testes, males can produce more sperm and outcompete the others and so boost the chances of passing on their genes.

The effect of such "sperm competition" is apparent in chimps, where a female typically mates with all of the males in her group. To compete, male chimps have developed the largest testes of all the great apes. By comparison, male gorillas have exclusive access to a harem of females and have very small testicles.

Vahed's team found that different rules apply to bush crickets. Tests on the insects showed that, as expected, testes size was larger in species where the females mated multiple times. Yet those crickets with the largest testes produced the least sperm.

The scientists believe that rather than trying to flood females with more sperm, male crickets have evolved an alternative strategy.

"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," said Dr James Gilbert, a co-author on the study at Cambridge.

The theory, while speculative, has some evidence to support it. After mating with one female, tuberous bush crickets were ready to mate again within an hour, while other species with smaller testes took as long as five days to be ready.

The tuberous bush cricket wrests the record for largest testes in relation to body weight from a tiny species of fruit fly, Drosophila bifurca, whose testes account for 10.6% of its bodyweight.