From Livescience.com comes this little tidbit. Smart bats have smaller testicles. Well of course smart people are sexier so that must apply to the rest of the animal world as well. But it is not so simple. It turns out that different bat species tend to have either promiscuous or relatively monogamous females. Those species with promiscuous females tend to have males with big testicles and small brains, species in which the females are faithful tend to have males with bigger brains and smaller testicles.
According to the study, the explanation for this interesting correlation is probably an evolutionary trade off. In promiscuous species, males that produce the most sperm have higher reproductive success and have to allocate more of their energy budget to bigger testicles. In fact according to the article, bats have the biggest range of test's size of any mammal- 0.12 to 8.5 percent of body mass. So bats cannot have both bigger brains and bigger testicles given energy limitations. Since those species of bat that are more or less monogamous do not have to spend as much of their energy budget on reproductive organs, evolution shifts the allocation of energy in the males to bigger brain size. No hint on what happens to female brain size.
A separate article on mate sharing between mother and daughter or grandmother and daughter horseshoe bats is of course titled "Kinky Female Bats Share Mates with Their Mothers, Avoid Incest." Of course to the bats I suppose we are the kinky ones.