“Jiminy Cricket!” is the exclamation that comes to mind after learning that British scientists have just identified the species with the world’s largest testicles in relation to body weight.

Researchers from the University of Derby and the University of Cambridge have determined that the Tuberous bushcricket (Platycleis affinis) produces testes which are 14 percent of the male body mass, setting a world record.

(Karim Vahed holding a specimen of a male Tuberous bushcricket together with its testicles; Credit for images: University of Derby)

The species with the second largest testicles in relation to body weight is thought to be Drosophila bifurca, a fruit fly whose testes to body weight ratio has been recorded as 10.6 percent. This fly still may hold the world’s record for longest sperm, however. Sperm from the fly, when uncoiled, measures more than 2 inches, which is 1,000 times longer than human sperm.

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Regarding the bushcricket testes, lead researcher Karim Vahed of the department of Behavioral Ecology at the University of Derby, said, “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,” he added. “An almost universal evolutionary rule appears to be that such variation in relative testes size is linked to female mating behavior; 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,” he continued. “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.”

(A Tuberous bushcricket)

He and colleagues Darren Parker and the University of Cambridge’s James Gilbert compared relative testes size across 21 species of bushcricket. They discovered that testes were proportionately larger in species where females mated with more males. Female Tuberous bushcrickets mate with up to 23 different males in their two-month adult life.

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The male Tuberose bushcricket mates, however, did not produce more sperm. It was just the opposite. They produced less voluminous ejaculates.

“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. “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.”

Vahed added, “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.”