Peter B. Gray Ph.D.

The Evolving Father

Prolactin Is For Men Too

New findings on hormones and human paternal care

Posted Jul 03, 2012

What are the hormonal correlates of paternal care? Across many animals, the two most commonly found correlates of paternal care are lower testosterone levels and higher prolactin levels. While that link with testosterone has gained considerable attention in recent years, less attention has been given to research on human prolactin and paternal care.

The name prolactin itself implies lactation (“promoting lactation”). If prolactin is elevated in fathers of young children, would that mean that men are or should be breastfeeding their children? No—though male lactation’s a fascinating topic unto itself, with some men capable of producing small amounts of milk, as has also been found in two species of fruit bat. Yet in species as diverse as emperor penguins, wolves, meerkats, ringdoves, and common marmosets, involved males exhibit prolactin increases associated with offspring care. Even that paragon of paternal investment—the pipefish, containing a pouch in which he incubates eggs before “giving birth”—develops that brood pouch under the influence of prolactin.

Two new studies add considerably to our understanding of prolactin and paternal care in humans. These findings stem from Lee Gettler, Chris Kuzawa, Thom McDade and Alan Feranil’s research based in Cebu City, Philippines. In a paper just published in American Journal of Physical Anthropology, Gettler et al. report on prolactin levels measured from morning blood spot samples collected among nearly 300 men in their early 20s beginning in 2005, with follow-up about 4 and a half years later. They found that men’s prolactin levels in 2005 did not predict who would become a father. However, they found that fathers had higher prolactin levels than non-fathers. That represents the first time in humans that fathers have been found to have higher prolactin levels than non-fathers. The Cebu City fathers of infants had marginally (p=0.054) higher prolactin levels compared with fathers of older children.

These new findings on prolactin rely on “baseline” levels, whereas a 2011 paper published in Hormones and Behavior by Gettler and colleagues investigated “reactive” prolactin levels—that is, acute changes in prolactin levels in response to infant stimuli. As with many issues in the field of hormones and behavior, there may be significance to both “baseline” and “reactive” hormones; in some cases, baseline hormone levels may be predictive of behaviors, while in other cases acute changes in hormones may be altering future behaviors. To investigate whether paternal behavior changed fathers’ prolactin levels, Gettler and colleagues collected blood spot samples before and 30 minutes after fathers engaged in a 30 minute play session. This study was focused on 42 fathers in Cebu City whose kids were about 2 and a half years of age. Here, they found that fathers’ prolactin levels decreased from baseline to post-play. Additionally, men who felt their wives viewed them positively as caregivers had more pronounced decreases in prolactin across that span.

There are now at least five published human studies that have addressed whether men’s prolactin levels change in response to short-term paternal care. The findings are mixed. Gettler and colleagues’ finding that prolactin levels declined in association with paternal interactions has support from another study of first-time Canadian fathers, where men’s prolactin levels also decreased during play with toddlers. However, it contrasts with a different Canadian study that found fathers had increased prolactin levels in response to listening to infant cries, a study of Jamaican fathers, and an Israeli study in which fathers’ prolactin levels were positively associated with coordinated play with their infants.

So where does this leave us? While the largest study yet has now found that fathers in Cebu City had higher prolactin levels than non-fathers, the findings regarding fathers’ acute changes in prolactin in response to paternal interactions are variable. Prolactin is a hormone with many functions—promoting weight gain, inhibiting reproductive function, responsive to anxiety, promoting milk production, among others. How all those functions package to serve an adaptive pattern of human paternal care thus warrants continued attention. Some of the existing studies suggest that a father’s experience, his behaviors immediately before participating in a study, and perhaps quite intricate aspects of his mental state at the time of sample collection may be relevant to sorting through the mixed findings; here, fine-grained aspects of the research design may be crucial.

All said, these new studies shine a spotlight on human prolactin and paternal care; rather than serving as a definitive word in this dynamic area of research, they’ll likely spark more studies that will draw on a variety of methods, subject pools, and more, while helping work though the conceptual and methodological challenges identified in the existing work.

Prolactin and parental care—it’s for the guys too.

References:

Ellison PT, Gray PB, eds. 2009. Endocrinology of Social Relationships. Cambridge, MA: Harvard University Press.

Fleming AS, Corter CS, Stallings J, Steiner M. 2002. Testosterone and prolactin are associated with emotional responses to infant cries in new fathers. Hormones and Behavior 42: 399-413.

Gettler LT, McDade TW, Feranil AB, Kuzawa CW. 2012. Prolactin, fatherhood,a nd reproductive behavior in human males. American Journal of Physical Anthropology 148: 362-370.

Gettler LT, McDade TW, Augustin SS, Kuzawa CW. 2011. Short-term changes in fathers’ hormones during father-child play: impacts of paternal attitudes and experience. Hormones and Behavior 60: 599-606.

Gordon I, Zagoory-Sharon O, Leckman JF, Feldman R. 2010. Prolactin, oxytocin, and the development of paternal behavior across the first six months of fatherhood. Hormones and Behavior 58: 513-518.

Gray PB, Parkin JC, Samms-Vaughan ME. 2007. Hormonal correlates of human paternal interactions: a hospital-based investigation in urban Jamaica. Hormones and Behavior 52: 499-507.

Gray PB, Anderson KG. 2010. Fatherhood: Evolution and Human Paternal Behavior. Cambridge: Harvard University Press.

Storey AE, Noseworthy DE, Delahunty KM, Halfyard SJ, McKay DW. 2011. The effects of social context on the hormonal and behavioral responsiveness of human fathers. Hormones and Behavior 60: 353-361.

Storey AE, Walsh CJ, Quinto RL, Wynne-Edwards KE. 2000. Hormonal correlates of paternal responsiveness in new and expectant fathers. Evolution and Human Behavior 21: 79-95.

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