What’s Neurobiology Got to Do With It?

Since Adam and Eve, monogamous relationships have been the model for religion and law. Some people seem more inclined to pair and settle down than others. Research into the mating habits of prairie voles has identified connections between their neurotransmitters and monogamous mating habits and may help understand human fidelity better.

Thomas Insel, director of the National Institute of Mental Health, said these connections may help understand why some people mate for life and others do not. Insel reported his findings in an invited address, “The Neurobiology of Social Attachment,” at the APS Annual Convention in Atlanta.

Prairie voles are highly social critters that bond in monogamous pairs after mating. When separated from their partner, prairie voles experience a high amount of distress. According to Insel, this is in contrast to montane voles, which are very similar to prairie voles genetically and in appearance, but are highly promiscuous.

Why do prairie voles form life-long pair bonds, while their cousins, the montane vole, are as promiscuous? Insel said the answer lies in the unique neurobiology of the prairie vole. Unlike other types of voles, the neurotransmitters oxytoxin (OT) and vasopression (VP) appear to enrich the concordant brain receptors in areas of reward in the prairie vole. Research has shown that mating is necessary for social attachment to develop in prairie voles. Specifically, the mating process itself appears to release OT and VP.

To further explore the important role these neurotransmitters play, inducing males with VP was shown to produce partner preference in prairie voles, ranging in form from sexual fidelity to increased aggression toward potential rivals, such as “mate guarding” behavior.

If monogamy can be induced using OT and VP in prairie voles, Insel questioned whether fidelity could be induced in the wanton montane vole using the same procedure.

According to Insel, the answer is “no.” Montane voles, he said, actually have the same amount of VP, and have the same amount of brain receptors for binding of VP, but have these receptors in different locations in the brain than the prairie vole. While the prairie vole appears to be “hard-wired” for monogamy, the montane vole seems to be predisposed to multiple mates.

Research has confirmed this finding that different receptor distributions lead to different behaviors in animals. It appears that it is not the amount of neurotransmitters present, nor the prevalence of receptor locations, but the actual location of these receptors.

Where in the brain are these receptors located? According to Insel’s research, the most likely place is the ventral pallidum, where the partner preference in prairie voles is controlled. Additionally, Insel said “the locations of receptors is species-specific in species with mating-induced pair bond formation.”

Interestingly, mating behavior and subsequent pair bond formation in male and female prairie voles appears to be induced by different neurotransmitters. In males, VP appears to be “necessary and sufficient for mating-induced behavior and pair bond formation,” whereas female copulatory and social attachment is induced primarily by OT.

Autism, a disorder of social attachment which has an approximately 91 to 93 percent heritability rate, begins to become evident in children typically before the age of three. Involving self-stimulatory behavior, as well as a lack of insight into social cues, and a failure to reciprocate socially, researchers have hypothesized, but as yet not fully discovered, a neurological basis for this disorder.

Since OT and VP have been found to be important neurotransmitters for social attachment in prairie voles, might these chemicals also be implicated in problems of social attachment, such as autism? It is quite possible that such a link exists Insel said, citing research that showed decreased levels of plasma OT in autistic children, as well as decreased cerebrospinal fluid OT levels in autistic primates, or those that displayed the marked behavioral traits indicative of autism, such as social detachment. Are humans hard-wired for social attachment like the prairie vole? Or are people more like the montane vole, neurobiologically predisposed for multiple partners? The answer appears to be the former, Insel said. Data is limited and prevents researchers from reaching definitive conclusions, but Insel said “genetically, humans may be predisposed to monogamy.”