Biology is all about trade-offs and it turns out that at least in bats, there is a conflict between the size of sexual organs and the brain. In the current issue of the Proceedings of the Royal Society B, a paper reports on the effects of sexual selection on neocortex and testes sizes in bats. The authors find that the testes of males are much larger in species where the females are promiscuous, than those in species where the females exhibit fidelity. The inverse relationship held for brains. The theory is that both organs are expensive metabolically (especially since bats fly). If females have many partners, the crucial competition is between the sperm and the bat with the most wins. On the other hand, if females are selective about their partners for a given breeding cycle then the quantity of sperm produced by a given male is not so important.
It’s not so clear why female faithfulness should promote a larger brain in males. One argument is that there may be a genetic constraint in that genes for both organs are co-expressed. In an earlier post, I wrote about the hypothesis that if females select for a trait in males, then any genes for that trait residing on the X chromosome would be very effectively selected for since males only carry one X. However, this genetic constraint may be a product of other selection pressures. It could simply be that selective females prefer more intelligent mates.