Chemical bonding

Human behaviour may, after all, be the upshot of molecular games. Scientists working with voles -- mouse-like creatures -- found that monogamy and parenting among these animals depended critically on the ebb and flow of certain chemicals in the brain. These hormones -- oxytocin and vasopressin -- are present in all mammals.

Experiments showed oxytocin injections made ovulating rats eager to mate and male rats to have instant erections. The substances caused prairie voles to pair up quickly, but when they were prevented from reaching the brain, the animals remained single.

Says C Sue Carter, a vole researcher at the University of Maryland at College Park in the US, "It's easy to see parallels in humans." In humans, blood samples taken during sexual activity showed oxytocin levels rising several-fold.