Bats with small brains have extra large testicles – so large that they constitute up to 8.5% of their body mass, say scientists from Syracuse University in New York. Writing in the journal Proceedings of the Royal Society B: Biological Letters, they say some bats have evolved larger genitals because in some species female bats are unusually promiscuous. High energy demands on male bats prevent them from generating both large genitals and large brains. Male bats with larger testicles but smaller brains stand a greater chance of having offspring than their smaller testicled, bigger brained rivals.

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