Big brained bats

Yes, it's the expensive bat testicle hypothesis:

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.

This makes perfect sense to me, but the researchers were surprised:

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."

I guess it depends what they use them for. I wonder if there is some other factor correlated with both monogamy and brain size, for example -- like body size, or diet. This isn't the first time that bats have figured into the expensive tissue story, after all --- there is this paper by Jones and MacLarnon (2004) that shows that fruit-eating bats have larger brains than non-fruit-eaters, and that brain mass significantly covaries with gestation length. I suppose if monogamous species have longer gestations, that might explain that relation.