Hormone of Monogamy: The Prairie Vole and the Biology of Mating

Article excerpt

What makes some males stay with one female while others play the field?

Researchers believe that a chemical produced in the brain may turn on monogamous behavior. But also for those who would like to package the stuff, scientists have only been able to tie this love potion to a mouse-like mammal known as the prairie vole -- not to humans. Scientists just don't know whether this chemical, or any other like it, mediates human behavior.

Field biologists have noted that the male prairie vole pairs off with a single female, probably for life. Neuroscientists have long wondered what keeps these males content with one mate while their close cousins, the montane voles, exhibit a more, shall we say, promiscuous dating style. While the stay-at-home prairie voles cuddle in their burrows, montane males mate indistriminately with one female after another.

This vast difference in lifestyle may come down to a single brain hormone, vasopressin, which in the human body is more commonly associated with regulation of water content. Research indicates that vasopressin induces the male prairie vole to stay with and protect his mate.

At the same time, vasopressin may trigger another characteristic behavior--that of the father prairie vole caring for his pups, another group of investigators finds.

What makes the monogamous praire voles so radically different from their polygamous cousins?

To answer that question, neuroscientist Thomas R. Insel of the National Institute of Mental Health (NIMH) neurophysiology lab in Poolesville, Md., turned to a discovery his team made last year (SN: 7/4/92, p.6.). The researchers found that compared to polygamous voles, prairie voles' brains had different distributions of certain receptors, proteins that sit on the surface of nerve cells. The finding led the team to the chemicals that fit with those receptors -- namely, vasopressin and the reproductive hormone oxytocin.

In their most recent research, "We asked a very simple question," says Insel "[Does] either of these hormones induce or affect pair-bonding in the monogamous animal?" Insel and his colleagues, including C. Sue Carter of the University of Maryland at College Park, provide some answers in a report published in the Oct. 7 NATURE.

Before delving into the specifics of the team's investigation, it may be helpful to define monogamy. The average person probably thinks of monogamy as a sexually exclusive relationship. Biologists, however, define the word a little differently. The monogamous animal is one that spends most of its time with one mate but is not entirely faithful, points out Insel. Most monogamous animals will, on occasion, mate with a stranger, he says. In addition, the monogamous male vole often takes a fiercely protective stance when a stranger threatens the nest. Finally, such males often help their mates with child-rearing tasks.

Insel and his colleagues began by observing mate-guarding behavior, the dramatic change that overcomes a mle prairie vole when an intruder enters his family's burrow.

Previous research had shown that, after mating, the normally timid male prairie vole will attack any strange male that happens by the nest, explains report coauthor James T. Winslow, who worked with Insel at the NIMH laboratory and is now a researcher at Hoechst-Roussel Pharmaceuticals, Inc. in Somerville, N.J.

The tream confirmed that response, showing that makes who had mated with a female would indeed show this Jekyll-to-Hyde transformation when confronted with a strange male vole. In contrast, virgin male prairie voles remained timid when a new male was placed in their cage.

Something happened during the sexual experience, the team theorized, to transform the normally shy male prairie vole into an aggressor. To test the theory, the researchers tried to prevent that behavioral change with a chemical blockade, something that would stop the action of vasopressin or oxytocin. …