Monkey Love: Male marmosets think highly of sex

Upon noticing the sexy scent of a female marmoset, a male doesn’t pursue her mindlessly, according to a new brain-imaging study. Male marmosets appear to think about—not just react to—what they’re getting into, just as people do, the authors suggest.

SCENT OF A FEMALE. A male marmoset takes a whiff of a wooden block bearing female-marmoset secretions. J. Lenon

Functional magnetic resonance imaging (fMRI) pinpoints brain areas of high oxygen consumption, indicating cellular activity. Only recently has fMRI equipment been outfitted for monkeys. Now, scientists are using the technology to see what goes on in the minds of sexually aroused common marmosets.

A team of researchers from the United States and Germany gave four male marmosets whiffs of an alcohol-water solution containing genital-gland secretions from ovulating females, the same secretions from females without ovaries, or no secretions.

Of the 21 brain areas monitored, 6 became active only when the males smelled the scent of ovulating females. These areas included not only a part of the brain associated with sexual arousal but also sections comparable to those that in people contribute to information integration, memory, basic motor control, reward and reinforcement responses, and allocating attention. The findings appear in the February Journal of Magnetic Resonance Imaging.

Coauthor Charles T. Snowdon of the University of Wisconsin–Madison says the brain activity beyond the sexual-arousal area, which is called the medial preoptic area, is a surprise. Activity in the other areas—the cingulate cortex, temporal cortex, cerebellum, and hippocampus—suggests that sexually stimulated male marmosets evaluate the quality of potential mates, Snowdon says.

If that’s the case, the findings go against common lore that nonhuman male mammals are driven in mating solely by a primal urge, Snowdon explains. But then again, he says, “the results make sense” for male marmosets, which are monogamous and contribute to parenting. The monkeys may be “making careful decisions before deciding whom to mate with,” Snowdon notes.

However, “we need to be cautious about interpreting the activations,” says James K. Rilling of Emory University in Atlanta. The brain areas that lit up unexpectedly on the fMRI scans “have been implicated in a number of functions,” he says.

Even so, says Rilling, the work paves the way for future studies. He congratulates Snowdon and his colleagues for imaging the brain’s response to “genuine social stimuli,” in this case sexual signals. “This is something that has been missing from investigations that have tried to explore the ‘social brain’ with neuroimaging,” Rilling notes.

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