Pair bonding between humans is necessary for sustaining human life here on Earth. We encounter monogamy every day — from old married couples bickering over each other’s bad habits, to tweens smooching against the wall on subway platforms.
Indeed, monogamy can make for some very awkward moments. But what about monogamous behaviour in other mammals? As biologist Devra Kleiman reports, monogamy is relatively rare in mammals, occurring in less than 5 per cent of mammalian species.
On the flip side, studies by David Lack have shown that over 90 per cent of all bird species are monogamous. Perhaps this is not shocking to those who have heard the term “lovebirds.” In fact, the lovebird is a type of parrot known for its rigorous displays of monogamous affection.
It would be useful now to stop and define monogamy in its very basic form, in order to differentiate monogamy in humans from that seen in other mammalian species. In zoological research, monogamy is defined as “the habit of having only one mate.” In this case, monogamy does not imply sexual fidelity, frequency of sexual interaction between mates, or the degree of parental investment.
Many experimental studies have shown that monogamy generally has both a neurochemical and behavioural basis. The prairie vole, or Microtus ochrogaster (Greek for “little ear”), is a socially monogamous creature and serves as a good model for social bonding formation. If a prairie vole loses its partner, there is a 70 per cent chance that the surviving companion will not replace it with a new partner.
A monogamous prairie vole relationship — and even social bonding in prairie vole colonies for that matter — resembles that of humans. They nest together, raise their young together, groom each other, and basically have each other’s back.
The catch, however, is that prairie voles also engage in sexual infidelity. A study by Wolff and colleagues found that female prairie voles will engage in sexual infidelity with multiple males if her partner does not ward off potential competitors — a behaviour called mate-guarding.
But male prairie voles are not off the hook. A study by Reuven Dukas and John Ratcliffe suggests that male voles use the tactic of sticking around as a resident, instead of remaining a wanderer, in order to induce puberty and ovulation in females. This enhances their mating opportunities. What’s more, male prairie voles that stay as residents tend to have a larger ano-genital distance, indicating higher sperm count and larger testis.
So what happens in the brain to facilitate monogamy? Studies have clearly shown that neuropeptides such as oxytocin and vasopressin are critical for pair bond formation in both sexes. An interesting example of neuropeptide influence is found in meadow voles. The socially promiscuous meadow vole, closely related to the prairie vole, is not naturally monogamous. However, it will engage in monogamous pair bonding when a vasopressin receptor gene is transferred into its ventral forebrain.
Many are probably wondering how to get this reaction in their own mates. Perhaps love potions will work someday. Or maybe Facebook stalking will pay off in the future. But for now, let’s bask in the reflected glory of our fellow monogamous critters.
