Seminar Two
Evolution of sexuality
The elixir of love
The neurobiologist Antonio Damasio has likened oxytocin to a love potion or magic elixir. Recent research in rodent behavior is highly relevant here, so this is a must-read! The prairie vole is a rarity among rodents for its monogamous matings. Unlike other vole species, prairie voles raise young as a couple, both father and mother caring for the young for many weeks. The male fiercely defends his mate and family from other males.
Meadow voles and montane voles, in contrast, are solitary and polygamous rodents, like most mammals. The females of both species mate with passing males and raise their young alone. Studies show that oxytocin and another, almost identical neurohormone—vasopressin—are present in both species of vole, but far more oxytocin and vasopressin receptors occur in the brains of prairie voles. Oxytocin is more specific for female voles and vasopressin for male voles. Surges of vasopressin occur in the brains of males after mating. Both hormones have everything to do with reproduction and bonding, provided that there are receptors in the brain which recognize the hormones.
The most amazing part of this story comes next: introducing the gene for a vasopressin receptor makes normally polygynous male meadow voles mate for life! (This is achieved by inserting the gene into a virus, which becomes a “vector” for the gene when the vole is infected with the virus.) Even transgenic male mice with a prairie vole version of this gene respond to vasopressin by bonding with females, which they never do otherwise!
The inescapable conclusion is that pairbonding in some mammals, including humans, may be enhanced by a single molecule in the female and a related molecule in the male, as long as there are receptors for the molecule which enable brain reward circuits to be activated.
A tantalizing twist on this story appeared in a 2008 study by Swedish researchers who examined the vasopressin receptor genes in over 2,000 people. Having a particular vasopressin allele was inversely linked to the strength of a person's bond to his or her mate; men with two copies of this allele were more than twice as likely to report serious marital problems as men who did not carry the allele. Women married to men with one or two copies of the allele reported less satisfactory marriages. The study supports previous findings that a single molecule can influence pairbonding, but in this study there was a surprising inverse relationship with the presence of an allele for the receptor.
As studies become more sophisticated we will surely find a network of molecular interactions, coded for by genes, which modify pair-bonding behavior in a symphony of interactions with other genes and the environment.
