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Prairie voles (Microtus ochrogaster) are said to be the paragons of monogamy. These brown mouse-like creatures, smaller than a palm squirrel, are among only 3 to 5 per cent of about 5,000 mammals to stick to one partner all their lives. When one of a couple dies, its mate most often lives a solitary life. But some studs give the lie to their species monogamous lifestyles. They trespass upon other voles home ranges, sneaking sex and scrapping with strangers. Female partners in stable relationships arent above having a casual fling. Their regular mates dont father even as much as a quarter of the litters. Still, a majority stick with their first sex partner, and thats a rarity in the animal world.

Philandering males of these small North American rodents hit the reproductive jackpot, producing more babies than their monogamous counterparts. But being unfaithful is not all fun and frolic. While these males are gallivanting, their lonely mates may also seek sex with others. These cuckolded males that are busy cuckolding others are more likely to raise a rivals offspring. Neither monogamy nor polygamy has an edge, and prairie voles practice both.

What makes some stick to one mate and the others wander?

Vasopressin, a hormone produced by the brains hypothalamus, influences social behaviour such as aggression, bonding with mates and care of offspring. It suffuses the brains of monogamous voles that have more vasopressin receptors (V1aR) in the ventral pallidum, the reward-seeking part of the brain. It is the same with faithful men. When receptor rich voles have sex for the first time, they imprint on their partners and thereafter stay true to them.

Meadow voles (Microtus pennsylvanicus), closely related to prairie voles, are promiscuous. By injecting vasopressin receptors into the males brains, researchers turned them monogamous.

The receptors were not the only things keeping male prairie voles in line. A lot has to do with their sense of geography, say Steven Phelps of the University of Texas at Austin, US, and his colleagues. They discovered rodents with more vasopressin receptors in the brains retrosplenial cortex, known for its role in memory and navigation, stayed within the confines of their homes. These rodents recalled sites where they had fights with rivals and avoided them.

The ones with fewer vasopressin receptors had a poorer understanding of their territories. They wandered over large areas, intruding on others territories and perhaps only dimly recollecting where their nests were located. The result: promiscuity.

A meadow vole. (Shutterstock)

Why didnt the prairie voles stick to either monogamy or polygamy? Mariam Okhovat and other students in Phelps lab used genetics in the lab combined with radio telemetry data gathered in a previous study to find out. They examined the gene avpr1a, known for predicting sexual fidelity in males and encoding the vasopressin receptors V1aR.

The specific arrangement of the four nucleotides in a DNA sequence is called encoding. It is the recipe for producing a particular protein that plays a unique role. In this case, the gene avpr1a produces a protein that acts as a receptor for vasopressin.

Not only did the number of V1aR in the ventral palladium and retrosplenial cortex anticipate male behaviour, but variations in the sequence of the gene also mattered.

The researchers discovered avpr1a comes with two alleles that they call HI and LO. The structure of the V1aR produced by the HI and LO alleles is the same, but they produce different amounts, says Okhovat. Males with the HI allele producing more vasopressin in the retrosplenial cortex were faithful and stayed within the bounds of their home range. The ones with the LO allele produced low levels of the hormone in the retrosplenial cortex and bumbled far out of their territories. Although the researchers noticed this correlation, this relationship between geography and vasopressin in the retrosplenial cortex hasnt been confirmed.

The two alleles didnt affect the amount of V1aR receptors in the ventral palladium, which regulates motivations, emotions and behaviour. The gene variations only affected receptors in parts of the brain involved in remembering places and sexual fidelity.

Phelps says his team was shocked by what they found. This brain variation isnt just there by chance. It isnt random. Natural selection had maintained the alleles in prairie voles for a long time so they had to benefit the species.

Janaki Lenin(Courtesy HarperCollins)

Since these effects are stronger in captive voles than in wild ones, the researchers say the traits cannot be explained by heredity alone. Environment and population may also influence the degree of males faithfulness.

Like many rodents, their populations boom or crash depending on food supply. Numbers explode at feast times, and home ranges shrink in size, with more than 600 rodents jammed in a hectare. Such situations are a perfect playground for promiscuous voles with little sense of territory, and a headache for possessive male voles, keen on keeping their mates away from strangers charms.

During famines, vole populations crash, leaving only one or two in a hectare. Those with better navigation and memory skills are at an advantage. They dote on their partners, defend their territories and mates from philandering males, and make successful parents.

Extending these conclusions to human behaviour is tempting, but research hasnt advanced that far. A 2014 study of 7,400 twins in Finland is the latest to suggest genes could influence faithfulness. It is possible vasopressin receptors control fidelity in women and, to a lesser degree, in men. Surprisingly, oxytocin, commonly thought to be at work when women bond with their partners, seems to have little effect. Philandering in humans may have some biological basis, but it is complicated by personality and environment.

Of his findings in voles, Phelps says, When it comes to social behaviour, maybe there isnt a normal brain. Many genes may have subtle variations so that individuals of a species can respond with flexibility to circumstances.

Wandering voles go astray in more ways than one.

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Excerpt: Every Creature Has A Story by Janaki Lenin - Hindustan Times

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