Marmosets are small primates. Think of a monkey with huge fuzzy ears. Primatologists have often wondered whether nonhuman males evaluated their potential mates, or whether their sex drive was more like an on/off switch.

We may think of animals as just responding to sexual invitations without thinking, but a study conducted in both the U.S. and Germany shows this isn't necessarily so. Male marmosets having their brains scanned were given sniffs of genital-gland secretions from ovulating females.

That's a scent that should trigger a mating response in the brain, and it did! But guess what else happened? Many other brain areas lit up as well, such as memory formation, information integration--in other words, areas associated with decision-making.

Researchers are still studying the effect, but apparently a lot more happens in a marmoset brain than a simple "have sex now" program. The specific areas of brain activity suggest that the males are evaluating potential mates before agreeing to a sexual union.

This makes special sense for marmosets, because they are monogamous and both partners raise the offspring. Under those circumstances you wouldn't want to enter into a relationship without any thought.