1:13pm UK, Wednesday December 07, 2005

It's official, males with big testicles have smaller brains - at least in the case of bats.

Bats cannot be well-endowed in both areas, according to a discovery by scientists at Syracuse University in New York.

180-bats

A 'well-hung' bat

Their study shows a biological trade-off between brains and sperm, with the creatures 'opting' for one or the other.

It seems females are to blame for the evolution of the male species.

They are unusually promiscuous, so natural selection pressure has led to males evolving enormous testicles to tempt a mate.

The male bat is unable to sustain a large brain and high fertility levels. 

Experts at the university say the genitals of some bats make up a whopping 8.5% of their body mass. That compares to a maximum of just 0.75% for most apes.

Researchers found the link between testicle and brain size after looking at 334 different bat species.