(http://images.livescience.com/images/monogamous_Prairie_Vole_03_10.jpg)
A team of scientists observed the arginine vasopressin receptor 1a gene in prairie voles (Microtus ochrogaster) who are socially monogamous animals. This is a largely unique behavior among rodents. Prairie voles form long-lasting pair bonds and both parents partake in raising their young. It is thought that two hormones, oxytocin and vasopressin, are essential in the development of monogamy. The scientists observed a gene that regulates vasopressin output in the brain to see if, when transplanted into a close promiscuous relative species, the meadow voles (Microtus pennsylvanicus), it would have the same effect in forming pair bonds. The scientists genetically modified meadow voles by transferring the viral vector V1aR gene into test subjects’ forebrain. Additionally, they isolated adult males for 24 hours with a receptive female and sought to find out whether the males preferred to return to the female with whom he had spent the previous day. After male meadow voles were given the opportunity to access new partners, would they prefer to return to the old one? They measured this empirically by timing how long affected males huddled with the female with whom they had previously been isolated. They also hoped to show that by changing a single gene they would be able to provide “a potential molecular mechanism for the rapid evolution of complex social behaviour.”
They found that males possessing the V1aR gene would go back to the females with whom they were isolated for 24-hours and huddle. Males who hadn't been genetically modified were found uniformly distributed among females after the 24-hour isolation with one female.
Monogamy is a rarely observed phenomenon in mammals. Unlike birds that lay eggs, parental investment can just as easily come from the mother as it can from the father. Female mammals, on the other hand, tend to have long gestation periods followed by long lactation periods before they can mate again. From a male perspective, if the objective is to pass your genes on to as many offspring as possible, it seems maladaptive to stay with one female when there are many other females you could potentially inseminate.
If females are the limiting factor, from an evolutionary perspective mammals should be largely polygynous. Polygyny is a mating system where one male mates with many females. From a female perspective, it may be in her best interest to be polygynous because “loser” females, or those with unattractive genetic material, would be able to mate with high quality males. Polygyny may also ensure superior habitat and resources supplied by the male.
Promiscuity is another viable option for mammals. It may be in males’ and females’ best interest to be promiscuous like the meadow voles and let sperm competition and cryptic female choice determine which males are successful in providing the gamete for the female’s egg. Given the severity of the many negative aspects of monogamy- and the advantages of other mating systems - why do certain species still engage in this behavior?
Polygyny and promiscuity favor few “winner” males, and the many “loser” males do not mate at all. This is not an argument for group selection, but if a loser male can hook a female into mating with him, it is in his best interest to invest all of his time and resources into her and not attempt to sequester many females. From a female perspective, monogamy may be the best choice because it provides ensured support for the female and her offspring. Gestation and lactation are incredibly energy-consuming and difficult if pursued solitarily.
Olivia Judson wrote about this paper in her blog, The Wild Side, and concluded with "it does raise the tantalizing prospect that, one day, it might really be possible to concoct love potions or pills that would alter brain chemistry and enhance the odds of a man forming a strong attachment to his lover." A tantalizing prospect, indeed!
Lim, M. M., Wang, Z., Olazabal, D. E., Ren, X., Terwillinger, E. F., Young, L. J. 2004. Enhanced Partner Preference in a Promiscuous Species by Manipulating the Expression of a Single Gene. Nature: 429.
