MALES that boast the largest testicles also have the smallest brains, a study of bats has revealed.

Although not quite proof of the charge that men tend to think with their genitals, the discovery offers the first firm evidence that males make an evolutionary trade-off between intelligence and sexual prowess.

The large amounts of energy needed to support large testicles that pump out vast quantities of sperm, and big brains that support a more advanced intellect, mean that male bats struggle to do both, the findings suggest.

So in species where females are promiscuous — creating an evolutionary advantage for large testes and lots of sperm — the males tend to grow big testicles at the expense of smaller brains.

The biggest-endowed of the flying mammals have testicles that make up as much as 8.4 per cent of their body mass — compared with between 0.02 per cent and 0.75 per cent for primates.

Brain size, however, falls as the bats’ testicles get larger. The silver-tipped myotis bat, Myotis albescens, is estimated to invest about twice as much energy in its genitalia as in its brain.

In the study, a team led by Scott Pitnick of Syracuse University, in New York State, looked at testicle and brain size in 334 different species of bat. They found that testicle size increased markedly in species with particularly promiscuous females, and that the animals’ brains were smaller to match.

Promiscuity is known to make a difference to testicle size; when females mate with multiple males it gives males with large testicles that produce more sperm a significant reproductive advantage.

Chimpanzees, for example, are promiscuous, and their testicles are many times larger than those of gorillas, in which a single dominant male has exclusive access to a “harem” of females.

The size of human testicles lies somewhere between these two extremes.

Dr Pitnick, whose results are published today in the journal Proceedings of the Royal Society, said that the bats may be evolving according to an “expensive tissue” hypothesis — in which scarce energy is invested in either brains or testicles according to which confers the greater adaptive advantage.

“Because relatively large brains are metabolically costly to develop and maintain, changes in brain size may be accompanied by compensatory changes in other expensive tissues,” he said.

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