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Is prolactin the “breastfeeding hormone”? What is it doing in fathers?

Photo: paxye. Creative Commons.

Some think that prolactin helps bring out more “outward looking” forms of fathering, which is found in some animals, such as bird fathers.

If you are reading this and you have heard of the hormone prolactin, it is probably because it plays a central role in helping moms produce their milk for breastfeeding. That is actually where its name comes from—prolactin promotes lactation. But here’s the thing about prolactin, it is a really old signal in our bodies and in other animals. It has existed in some form for many hundreds of millions of years. Fish, amphibians, reptiles, birds, and mammals all have it.

Only in mammals do mothers nurse their babies (lactation) and from the standpoint of deep evolution, fish, amphibians, and reptiles were all around before mammals. If we step back and think about that for a second, it tells us something important about prolactin. It did not evolve simply to help promote breastfeeding. Prolactin did all sorts of important things in animals’ bodies before lactation was around, hundreds of millions of years ago.

Prolactin is commonly viewed as a “female” hormone because of breastfeeding. While its role in nursing is a rich part of the mother-baby picture in mammals, it is definitely not prolactin’s whole story, including when it comes to human fathering. In some animals prolactin goes up when fathers take care of their babies. Knowing that prolactin is an important part of the “biology of fatherhood” in other animals – including types of animals that do not have lactation – is what led a handful of scientists to study this hormone in human fathers.

We do not know nearly as much about prolactin in human fathers as we do about, for example, testosterone, but we do see some intriguing patterns. My team and I showed that fathers have higher prolactin than non-fathers, with prolactin being the highest when babies are most needy and vulnerable as infants. Interestingly, in our work, we have not found that dads with higher prolactin are more involved caregivers. This hints that prolactin might have some other role for dads. In other studies, dads with higher prolactin engaged their babies in more exploratory play, helping them gain new experiences in the world around them.

Some think that prolactin helps bring out more “outward looking” forms of fathering, which is found in some animals, such as bird fathers in which higher prolactin leads to gathering of resources and providing them to the family.

We still have much to learn about prolactin in human fathering, but all signs point to the idea that prolactin is for parenting, not just mothering!