Nature's Philanderers

By Vivienne Baillie Gerritsen

Could chemistry be at the heart of sexual wanderings? Or of sexual devotion for that matter? Well – though the idea certainly lacks romantic appeal – there are signs which point in this direction. The neuropeptide vasopressin is not a newcomer to research revolving around social behavior in animals. However, narrowing one of its roles down to what scientists coolly term ‘social cohesion’, and Christians would call ‘infidelity’, is quite a breakthrough. And deserves some thought. It is not so much the quantity of vasopressin but the tissue distribution of its receptor – in males - which seems to have a role in defining flirt or fidelity. In male voles that is.

Monogamy in the animal world is not so common. The examples are few actually. Amongst them the California mouse, the marmoset monkey and certain species of voles. In fact, only five percent of mammals stick to their mates. The ongoing theory is that most males tend to mate with more than one female, not in the pursuit of pleasure but in the pursuit of producing offspring with the same genetic buildup as themselves. Sperm –physiologically speaking – is cheap so they can afford to spread their wares in this manner.

Fig. 1 The furry philanderer (Microtus montanus)

Where do humans stand one wonders? Well, from a purely biological point of view, humans are no different from the other 95% of non-monogamous mammals. For instance, monogamous species are monomorphic, i.e. the male and female are the same size. Humans are dimorphic: men, on the whole, are larger than women. Monogamy – or polygamy – in human societies today is really set on human beliefs. In fact, monogamy probably became the name of the game in our parts of the world by way of the Roman Catholics in the 11th century. In those days, it was quite common for priests to have more than one wife, or even mistress. Only if they had a son, or more, the latter could inherit church property. So sons were declared illegitimate. The next step was to ban marriage for priests altogether. And, accordingly, celibacy became a standard of spirituality. Monogamy was all right for the spiritually inferior; infidelity was not to be heard of.

The knowledge that vasopressin has a role in male social behavior such as communication, aggression, sexual commitment and paternal care of offspring is not new. The idea had already been emitted in the 1960s following research on rats and mice. Vasopressin was administered into various tissues of the rodents and modulations in their behavior were observed. More recently, the same kind of tests were carried out on various species of voles: the monogamous prairie voles (Microtus ochrogaster) and their promiscuous relatives, the non-monogamous meadow vole (Microtus pennsylvanicus). When vasopressin antagonists were administered to male prairie voles, they turned into little furry philanderers, mating quite happily with many females. However, when vasopressin was administered to male meadow voles, it had no effect whatsoever on their mating habits.

What made the difference? The number of vasopressin receptors. Prairie voles bear far more vasopressin receptors in their ventral forebrains than their meadow vole homologues. When voles – or any mammal including humans – mate, they release quantities of vasopressin in the forebrain, which bind to their receptors. What is more, this particular region in the brain happens to be very close to what is known as the ‘reward center’. What vasopressin ultimately triggers off is the feeling that mating is quite agreeable. But – granted – this is not enough to remain faithful. The vole also has to remember its partner. Besides behavior, vasopressin has a role in social recognition and memory. The vole cue for recognition is probably olfactory. So, the ultimate feeling of something nice coupled to recognizing its partner leads the prairie vole to a form of fidelity.

How about meadow voles then? They also have vasopressin receptors in their forebrains. Yes, but not nearly so many as their prairie counterparts. To prove the fidelity theory, the vasopressin receptor gene was introduced into the forebrains of young meadow voles. And yes, you guessed, the meadow voles lost all wish to flirt. Once they had mated with one female, no other female could lead them astray.

Differences in the gene's flanking regions could echo differences in individuals' social behavior.

Why is it that prairie voles have more receptors in their forebrain than meadow voles? As a species, they are very close. Their vasopressin receptor sequences are also 99% identical. The flanking sequences on the vasopressin receptor genes differ however; and it is these flanking sequences which determine the tissue distribution of the receptors. The flanking sequences are comparable within one species but vary between species. Hence, differences in the flanking regions could echo differences in social behavior between individuals and between species.

So, does the way we behave depend solely on a bunch of hormones? How many vasopressin receptors do men have in their forebrains some of you may be asking? Can the act of adultery be waved off on the basis of chemistry? Discussing the promiscuousness of men, or women, is not the issue here. Rather, what is interesting is that the distribution of vasopressin receptors could have a role in the variability of human social behavior. For instance, it could explain in part light differences in social behavior between people but also more extreme dysfunctions such as autism where individuals have great difficulty in forming social bonds. Humans are not voles though and an individual’s behavior cannot be reduced to a single gene. Social behavior involves highly complex biological pathways which themselves interact with an individual’s particular past and present environment. And let us just hope that we have our own say in all this as well.

References
1. Lim M.M., Wang Z., Olazabal D.E., Ren X., Terwilliger E.F., Young L.J. Enhanced partner preference in a promiscuous species by manipulating the expression of a single gene Nature 429:754-757(2004) PMID: 15201909.

2. Lim M.M., Hammock E.A.D., Young L.J. The role of vasopressin in the genetic and neural regulation of monogamy J. Neuroendocrinol. 16:325-332(2004) PMID: 15089970.

3. Balaban E. Why voles stick together Nature 429:711-712(2004) PMID: 15201896.

Cross-references to group at the Swiss Institute of Bioinformatics:

Back to news