Over at the Notes&Theories blog, there is a good post about the complicated role of oxytocin. Oxytocin is commonly called the ‘love hormone’, a striking simplification that should immediately set off your Overly Anthropomorphized radar. Meadow voles are the promiscuous cousins of the monogamous prairie voles:
But oxytocin and vasopressin are released in brains of all mammals, not just those that are monogamous. The differences between species have nothing to do with how much oxytocin or vasopressin is released, but rather they depend on exactly where these hormones act. Vasopressin and oxytocin act only at specific receptors – and in the brain, these receptors are only made in certain places…Then they modified a harmless virus in such a way that it carried the code for making prairie vole vasopressin receptors, and injected it into a small part of the brains of male meadow voles. This part of the brain now began to make vasopressin receptors where none had been before – and the meadow voles began to behave like prairie voles, forming strong attachments to their current sexual partners.
Like so many things in the nervous system, oxytocin and vasopressin have a multitude of possibly contradictory roles that are determined by when they are released, where their receptors are expressed, and what else is released at the same time. A burst of dopamine+oxytocin and a burst of oxytocin+testosterone will surely have different meanings in the brain! And with highly plastic gene expression, what they mean will vary between individuals.