Sexy scent. A whiff of ovulating females activates many regions of male marmosets' brains.

Not-So-Wild Monkey Sex

Male marmosets don't act like beasts when it comes to sex, new research shows. When sniffing out a potential mate, their whole brains, including regions used in decision-making, zing with activity--just as happens with humans, researchers have found.

Male animals aren't thought to be particularly choosy when they mate, instead trying to inseminate as many females as possible. Although it sometimes may seem hard to believe, human males apparently have more than that going on in their heads. When men watch erotic films, many areas of their brains light up in brain scans, including those involved in sensory perception, motivation, and integrating and evaluating information. Some researchers believe that this full-brain response is unique to humans. But Charles Snowdon of the University of Wisconsin, Madison, and colleagues have now shown that it's not.

Using New World marmosets--which employ smell much more than sight--the researchers allowed female monkeys to rub their scents onto glass stoppers. The team pooled the chemicals from a group of ovulating females and from a group whose ovaries had been removed. The team tested one male monkey at a time by restraining his head, sliding him into the brain scanner, and letting him whiff a wooden disk dabbed with eau d'ovulator or eau d'ovariectomized.

Multiple regions of the monkey brains ramped up activity when the males sniffed ovulation scents, including areas involved in decision-making. When they smelled scents from females with no ovaries, the same areas decreased in activity from baseline, suggesting that males' sexual appetites may diminish around sterile females, the researchers report in the February issue of the Journal of Magnetic Resonance Imaging. The males appear to be making cognitive decisions about what they are smelling, says Snowdon, suggesting that they are fussy about whom they breed with and aren't just indiscriminate "sperm donors." His team now plans to put this hypothesis to the test with behavioral experiments.

The similar brain-activation patterns in man and monkey suggests that discerning sexual behavior is rooted farther back in evolution than some think, says primate biologist Fred Bercovitch of the Center for Reproduction of Endangered Species at the San Diego Zoo in California.

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Charles Snowdon
Wisconsin National Primate Research Center