In the animal kingdom, it’s no secret that the most masculine male usually gets the girl; big bodies, large antlers, and low frequency calls tend to drive the ladies wild. But once the guy gets the girl and the girl gets pregnant, high levels of testosterone aren’t necessarily a good thing. According to new research in PNAS, males with high testosterone are more likely to become fathers, but once they have children, their testosterone levels fall dramatically.

In animals like birds where paternal care is common, males tend to downregulate their testosterone production once babies are born. Humans, however, are one of the few mammalian species in which males help raise the offspring. Since this is a rare trait among mammals, we don’t know much about our reproductive strategies when it comes to testosterone.

This longitudinal study used data from a long-term dataset of males living in Cebu City in the Philippines. The researchers measured morning and evening salivary testosterone levels in 642 21-year old males. Then, when the guys were 26, these measurements were taken again; by that time, many were married and had children.

Not surprisingly, men with higher testosterone levels at 21 years of age were more likely to be married and have children five years later.

It's very common for men to undergo age-related decreases in testosterone, and most of the men in the study did have slightly lower testosterone levels by the time they were 26. However, men who had children by the time the second measurement was taken had much greater decreases than those that were still single and those that had gotten married but not had children.

Dads with newborns were most strongly affected; new fathers whose kids were less than a month old had the largest drops in testosterone production, when compared to their baseline. These hormone changes weren’t accounted for by their sleep quality, stress levels, or time budgeting.

Fathers that were more involved in child care and spent more time with their children had lower testosterone levels than those that didn’t spend much time caring for their kids and those that were completely uninvolved with their children. Furthermore, the baseline testosterone measurement taken at 21 years of age was completely unrelated to how invested the fathers were in their children’s care five years later. Taken together, this strongly suggests that caring for his kids can actually suppress a father’s testosterone production.

While testosterone may help attract females, it can be detrimental to a relationship’s stability. In previous research, men with high testosterone levels were more likely to have marital problems, and less likely to feel empathy when an infant cries. It’s likely that the down-regulation of testosterone production seen here is an evolutionary adaptation to increase reproductive success. A lot of testosterone may help get the girl, but a little less of it may help raise the family.

PNAS, 2011. DOI: 10.1073/pnas.1105403108 (About DOIs).