THE REASON GEEKS DON'T GET ANY

Big brain means small testes, finds bat study

12:16 07 December 2005

NewScientist.com news service

Gaia Vince

The brainier male bats are, the smaller their testicles, according to a new study. Researchers suggest the correlation exists because both organs require a lot of energy to grow and maintain, leading individual species to find the optimum balance.
The analysis of 334 species of bat found that in species where the females were promiscuous, the males had evolved larger testes but had relatively small brains. In species, where the females were monogamous, the situation was reversed. Male fidelity appeared to have no influence over testes or brain size.
Both brain tissue and sperm cells require a lot of metabolic energy to produce and maintain. The different species appear to have evolved a preference for developing one organ more than the other, presumably determined by which will help them produce more offspring.
“An extraordinary range of testes mass was documented across bat species - from 0.12% to 8.4% of body mass. That exceeds the range of any other mammalian order,” says Scott Pitnick, from Syracuse University in New York, US, one of the research team. Primate testes vary between species from 0.02% and 0.75% of body mass.
Energy knife-edge
Efficient use of energy is crucial for bats, says Pitnick: “Bats really exist on an energy knife-edge: they are small with a large surface area, and they need to fly around, particularly during the mating season.”
Pitnick and his colleagues had predicted that, in species with promiscuous females, males would require bigger brains in order avoid being cuckolded. So they were surprised to find the opposite: “Perhaps monogamy is more neurologically demanding.”
Harry Moore, a sperm researcher at the University of Sheffield, UK, says that testis size is normally related the amount of sperm produced.
“In species with promiscuous females, the males are competing to fertilise her eggs and so need to produce a lot of sperm," he told New Scientist. “And this may be especially true in some species of bats where the females store sperm for several months.”
Journal reference: Proceedings of the Royal Society B: Biological Sciences (DOI: 10.1098/rspb.2005.3367)