A squirrel-sized primate with white hair dancing out of its ears, the common marmoset finally may dispel tired stereotypes about promiscuous fathers in the animal kingdom.
When psychologists exposed marmoset males to the scent of ovulating females, the researchers expected hormone levels to spike in every male as a result of heightened sexual arousal.
But in an unanticipated twist, testosterone levels in marmoset fathers barely wavered in response to the female odor, even as hormones surged in all other non-parents, reported endocrinologist and lead author Toni Ziegler in the January issue of the journal Hormones and Behavior.
"Marmosets like sex, so we expected all males to be responsive to the scent of a sexually receptive female," says Ziegler, who is a senior scientist at the National Primate Research Center, where the marmosets reside. "Instead, we were surprised to find a muted physiological response in fathers."
"This is the first time that scientists have used a primate to demonstrate an immediate testosterone response to a social situation," says senior author Charles Snowdon, chair of the psychology department. "It is a common notion that males are always interested in sex, regardless of their social status. But this study counters what has been seen in all other primates."
The common marmoset is endemic to northeastern Brazil. Living in groups of three to 12 individuals, the primate has long intrigued behavioral researchers because it lives in a cooperative breeding group, a unique social construct in which every family member chips in with infant care. Unlike many other primates, marmosets also are socially monogamous, with "committed couples" giving birth to two sets of twins every year. Snowdon says the marmoset way of life has stronger parallels with human society than with any other primate system. "Marmoset fathers are necessary to help take care of infants, so it's likely that both the female and male need to trust that their partners will always be available," says Snowdon, reflecting on the study findings. "This work demonstrates a physiological manifestation of that trust."
For the Wisconsin study, Ziegler coated wooden discs with the ovulatory scent of marmoset females and presented the discs to 15 captive males for 30 minutes. During that time, and with the help of co-author and Primate Center colony manager Nancy Schultz-Darken, Ziegler evaluated behavioral responses, tracking, for instance, the duration of marmoset erections triggered by the odor. Ziegler also drew blood samples from both study and control groups (the control group included another 15 males) to monitor hormone levels.
The marmoset work carried forward from earlier non-invasive brain tests at UW-Madison that demonstrated that specific parts of the marmoset brain - in particular, the anterior hypothalamus and the medial pre-optic area - light up when males get a whiff of female ovulatory odors. The activated brain areas in turn stimulate hormonal responses that probably help marmoset males adapt to the parenting process.
In future work, says Snowdon, the scientists plan to compare brain patterns in marmoset fathers with patterns in single males. They also hope to assess how marmosets respond to sexual cues from partners versus novel females.
An unavoidable question is whether the UW-Madison work sheds any light on the mysteries behind human parenting behavior. As human studies are substantially harder to design and control, Ziegler says the marmosets could perhaps serve as a model for future human studies.
For the time being, marmosets are at least giving guys the chance to mend reputations. "I'm tired of all men getting a bad rap for being supposedly promiscuous and irresponsible," says Snowdon. "I'm happy that we've finally found a species where, as parents, both females and males do the heavy lifting!"
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