Post Birth, Milk production stimulating-hormone brings parents closer

The University of Wisconsin-Madison study relied on hormone analyses of urine from cotton-top tamarins, a small, endangered monkey native to Colombia. They live in monogamous family groups where both parents help care for the young, which is similar to humans.

The study found a link between prolactin levels and sexual activity and cuddling among paired adults. Prolactin levels were high among pairs that frequently had sex and cuddled and low among mothers that had finished nursing, even though their infants remained nearby.

First author Charles Snowdon said that the fathers are so busy taking care of the kids, they probably had less time for cuddling and interacting with their partners, adding that when you look at mothers who had lower prolactin, they had less sex with their partners.

The discovery in other studies of high prolactin levels among males who care for infants (in humans and other primates) made prolactin seem likely to be causing the parenting behavior, but Snowdon says he and co-author Toni Ziegler believe may be prolactin is not serving as a mechanism to drive parental care, but it's a consequence, a reward for parental care.

The idea that prolactin and oxytocin may supply rewards was reinforced by a German study that found a burst of both hormones when men and women reached orgasm while making love. This suggested to Snowdon that prolactin may, among other things, function as a reward mechanism for sex.

There is other evidence that prolactin has a role in reward circuits, Snowdon says. Prolactin inhibits arousing chemicals in our nervous system, reducing our desire.

The study is published in the online journal PLoS One.