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Scientists seem to have a good handle on the neurochemical basis of maternal behavior in mammals -- including the human variety. Mothering behavior is based on a combination of prolactin, which stimulates milk letdown and nursing behavior, oxytocin, which stimulates labor, the release of milk from the breasts and, along with dopamine, the bond with the baby.

But fatherhood remains a mystery. Vasopressin, the hormone that's almost a twin to oxytocin and more important to the male brain, seems to be involved. Oxytocin also is likely involved in male bonding to mate and offspring. And prolactin, often thought of as a female hormone because of the connection to nursing, turns out to be quite important, as well.

This 2003 study of monkey fathers sheds some light on prolactin's role.

Researchers  Carsten Schradin, Deann Reeder, Sally Mendoza and Gustl Anzenberger tracked the prolactin levels in the urine of the coppery titi (Callicebus cupreus), the common marmoset (C. jacchus) and Goeldi's monkey (C. goeldii). These three species of monkeys are monogamous, and the male contributes a great deal to the care of the young, especially by keeping the baby safe and close by carrying it. In non-monogamous species, the mother and sometimes siblings and related adult females do all the carrying.

If the hormone prolactin, which stimulates maternal behavior in mammals, does the same thing for furry dads, you would expect changed in prolactin levels to correlate with the periods when fathers begin to carry babies. Moreover, you'd expect unmated males to have lower levels than fathers. Finally, if the increased prolactin was the result of tactile stimulation of that warm little body clinging to the father's back, the fathers that spent the most time carrying would have the highest levels.

These three species of monkey make for a neat comparison, because, while the males all invest in their offspring by carrying the relatively helpless babies on their backs, they don’t put in equal time.

The titi father does almost all the infant hauling. Titi monkeys live in small family groups, a monogamous pair and one to three offspring. In this species, the male is the primary caregiver. Males pick up the infants soon after birth, and carry them 70 to 90 percent of the time until weaning, sometimes continuing until the birth of the next offspring. Siblings seldom lend a hand. The titi fathers did have higher prolactin levels than females and higher than their unmated sons, but they didn’t show a significant increase after the birth of infants

The marmoset  males are the primary caregivers, but they get a bit more help from mother and siblings. Marmosets live in family groups headed by a monogamous mother/father pair. Mother, father and older offspring all help take care of the younger monkeys, carrying them around and sharing food with them, but the father is the primary caregiver.

In the marmosets in the study, mating and forming a pair bond didn’t increase prolactin levels; it began to rise shortly before the birth. Marmoset fathers had higher levels in the phase before they began carrying than their sons did.

Goeldi's monkeys also live in family groups and raise offspring cooperatively; Goeldi's typically have only one baby at a time. In this species, the mother starts out as the primary carrier. The fathers don’t start carrying the babies until about three weeks after birth. There was no difference in the urine levels of mated and unmated Goeldi's pre-pregnancy. The Goeldi's fathers, which don't begin to carry for three weeks, showed an increase in the hormone during the pre-carry period. Such an increase in prolactin immediately before the carrying stage could facilitate transition from nonparticipation to direct participation of the father in care-giving.

The experiment showed the first hypothesis, that prolactin levels are timed to paternal behavior, to be true, at least somewhat. The second, that fathers would have higher levels than non-breeders, also held true.

But the amount of carrying time didn't correlate with higher levels of prolactin. In other words, it's unlikely that the skin-on-skin contact stimulates the production of prolactin. So, the study shows a clear link between paternal behavior and prolactin, but not the mechanism.

When a son living in the family group becomes a breeder, the researchers think, the prolactin levels of marmosets and titi males goes up in preparation for their upcoming role as fathers. Maybe marmoset and titi dads don’t need priming prebirth because their levels are always higher. Sustained high levels of prolactin in these two species reflect the need to be continuously available to carry.

They don’t know exactly when prolactin levels increase in titi or jacchus, or what stimulates it, but in goeldie, one small study showed prolactin levels increased before birth and increased again after the birth.

They caution that carrying is not the whole story when it comes to parental investment in care. Goeldii fathers are immediately attracted by their infants, for example, and if a researcher hands the infant to its father, he will take it up. So other aspects of fathering may be influenced by other hormones or neurochemicals.