Scientists have found a bushcricket species with testicles that account for up to 14% of its body weight. It is the animal with the biggest testicles in relation to its body weight, they write in Biology Letters. In a study of their mating strategies, the team found they release only small amounts of sperm at each mating. That suggests the big testes are for mating with many females, not producing competitive volumes of sperm for each encounter.
Significant research across the animal kingdom has shown that male testicle size is correlated to the degree of promiscuity within a given species. The more partners a female has, the larger the male's testicles are likely to be. Larger testicles produce more sperm, and a long-standing assumption has been that a kind of numbers game is played out within the female to fertilise her eggs. The male that provides a higher amount of sperm in that scenario has a higher likelihood of fathering offspring. Many experiments in vertebrate species - including in our closest primate relatives - have borne out that idea.
However, an alternative hypothesis is that larger testicles permit a higher total number of mates, rather than a higher amount of sperm allocated to a single mate. Karim Vahed of the University of Derby and his team began their studies by measuring the testicle size of 21 species of bushcrickets (also known as katydids). While proportional testicle size ranged widely across the species, the team found that one species - Platycleis affinis - far surpassed the previous record for the proportionally largest testicles. Their testes make up some 14% of their body weight - roughly equivalent to a male human with testicles weighing five kilograms (11lbs) each.
However, in studying the amount of sperm that the crickets produce each time they mate, they found a surprising result. "Males with bigger testicles are actually producing smaller amounts of ejaculate," Dr Vahed said. "This very much favours the alternative hypothesis: that it's about the number of different females the male can fertilise, rather than getting a greater success per female." Dr Vahed said that although this is in contrast to results from vertebrates, it breaks down an inherent bias in science to favour theories about animals most similar to us. "One important message is that we shouldn't expect that exactly the same rules and situations apply across all species; it can be an either-or."