This just isn't fair.
It's official: too much sex saps male brains
MALE animals can produce a lot of sperm or grow big brains but cannot do both, according to a study that may confirm the suspicions of many women.
The study of 334 bat species suggests that energy-hungry brains can evolve only at the expense of other tissues.
Writing in Proceedings of the Royal Society, Biological Sciences, Scott Pitnick, of Syracuse University, New York, reported that species with promiscuous females had evolved extra large testicles but smaller brains.
"The general rule that is emerging is that sperm production can be incredibly costly," said Dr Pitnick, who first found the relationship in insects. Male fruit flies, for example, can make sperm 7.5 centimetres long.
"This led me to examine bats, as sperm competition is rife, and so testes can be ridiculously large," he said.
"Brains are metabolically expensive organs to develop and maintain, so looking for a trade-off there seemed obvious."
The most interesting implications of the study are for the co-ordinated evolution of brains, behaviour and extravagant sexually selected traits: ornaments such as the peacock's tail and armaments such as antlers.
Dr Pitnick said: "The road to sexual success can lie in being clever, in being a dull yet well-armed brute that can fight for paternity, in being spectacularly ornamented, or in providing a rich cocktail of seminal fluids and costly sperm."
Sexual selection could favour any combination of these.
Dr Pitnick carried out the study with Kate Jones, of the London Zoological Society, and Jerry Wilkinson, of the University of Maryland