Sex in the brain: How do male monkeys evaluate mates?

A pint-sized, tree-dwelling Brazilian monkey has proven to be strikingly similar to humans when it comes to sexual responses, a national research team has discovered.

Through functional magnetic resonance imaging, or fMRI, scientists from the University of Wisconsin-Madison and collaborating institutions for the first time peered into the brains of fully conscious nonhuman primates to learn what’s really on their minds when it comes to sex. The research appears in the February 2004 issue of the Journal of Magnetic Resonance Imaging.

Common marmosets, like humans, live in family groups and have to make careful choices when confronted with the scent of an attractive female, a team of marmoset experts led by Charles T. Snowdon, UW-Madison professor of psychology, discovered.

“We were surprised to observe high levels of neural activity in areas of the brain important for decision-making, as well as in purely sexual arousal areas, in response to olfactory cues,” Snowdon says. “Lighting up far more brightly than we expected were areas associated with decision-making and memory, emotional processing and reward, and cognitive control.”

The marmoset fMRI findings add strong weight to the mounting evidence that, when faced with a novel, sexually attractive and receptive female, males even in monogamous species aren’t necessarily just acting on some primal urge to procreate, without a second thought. Rather, they exhibit highly organized, complex neural processes.

“This is the first time anyone has imaged an awake nonhuman primate in response to emotionally arousing stimuli; it is also the first link between external sexual odors and the internal sexual arousal system,” Snowdon says. “This opens up a whole new field of research possibilities.”

The marmoset data corresponded surprisingly close to human fMRI studies, the scientists found. “The benefit of the nonhuman primate model is that we can control and know the developmental and social histories of our study subjects, to carry out studies not possible in humans,” Snowdon says.

Joining Snowdon in working with the marmosets were Toni Ziegler, Nancy Schultz-Darken and Pam Tannenbaum of the Wisconsin National Primate Research Center at UW-Madison. The Primate Center provided four male marmosets for the study. The researchers trained and transported them from the University of Wisconsin to the University of Minnesota, where the imaging took place.

The researchers imaged the male marmosets’ neural activity while they were presented with anogenital gland secretion samples from periovulatory females, those at or close to ovulation. Other samples, taken from ovariectomized females, gave the researchers a way to compare how the males responded to female marmoset scents containing no sexual cues, according to Ziegler. When the same males smelled the “sexier scents” from the ovulating females, the scientists could discern which neural areas showed further activation, thus identifying areas where information processing occurs.

“We were surprised to learn how great a role the neural areas related to cognitive processing play in determining how males respond to sexually receptive females,” Ziegler says.

To preempt stress to the animals, Ziegler and Schultz-Darken brought the marmosets’ cage-mates along on the road trip. “The marmosets were trained in advance, over brief periods, to get used to a mock imaging process,” Ziegler says. “By the time they underwent the real thing, they did not exhibit any signs of stress.”

“We acted as advocates for the marmosets,” adds Schultz-Darken, who is also a colony manager at the Primate Center. “It was very important to properly habituate them to the imaging equipment. We had a wonderful experience with the facility and the people at the University of Minnesota.”

Lead author Craig Ferris is a professor of psychiatry at the University of Massachusetts Medical School (UMMS) and director of the Center for Comparative Neuroimaging, a collaboration between UMMS and Worcester Polytechnic Institute. The study was funded by the National Institute of Mental Health.

The collaboration also included Reinhold Ludwig and John Sullivan of the Department of Electrical Engineering at the Worcester Polytechnic Institute. Other scientists included Jean King, UMMS associate professor of psychiatry; David Olson, Harvard Medical School; and Timothy Duong and Thomas Vaughan at the University of Minnesota, departments of Radiology, Electrical Engineering and Biomedical Engineering.

– Jordana Lenon, 608-263-7024; jlenon@primate.wisc.edu