Sept. 12, 2011 -- Testosterone levels drop when men become fathers, and they are lowest for the most devoted dads, new research shows.
While earlier studies had also shown that fathers have lower testosterone levels than men of the same age without children, it has not been clear if men with low testosterone are more likely to become fathers or if fatherhood actually suppresses levels of the male sex hormone.
The new research suggests that the latter is the case.
Researchers say the findings show that unlike most other mammals, human males are biologically hardwired for parenthood.
"The classic idea is that men were the hunters and providers and the females evolved to raise the children," Northwestern University anthropologist and study co-author Christopher W. Kuzawa, PhD, tells WebMD. "I think our study shows pretty clearly that men are also wired for their role as fathers."
The researchers examined data on close to 500 young Filipino men followed for almost five years. The men were enrolled in a health and nutrition study. Their testosterone levels were measured at the beginning and end of the study.
All of the men were single and had no children when they enrolled in the study. But about a third entered into stable relationships and became first-time fathers during the follow-up.
While the single men with higher testosterone levels at the beginning of the study were more likely to find partners and become fathers, new fathers experienced a drop in levels of the sex hormone greater than drops seen in men of the same age without children over the study period.
Among the fathers in the study, those who were the most involved in the care of their children had lower testosterone than those who were not so involved.
So while testosterone may help men find mates and produce offspring, high levels of the male sex hormone may not be needed and may even be detrimental to caring for a child, the researchers say.
"Fatherhood and the demands of having a newborn baby require many emotional, psychological, and physical adjustments, and our study indicates that a man's biology can change to help meet those demands," anthropologist and lead researcher Lee T. Gettler, tells WebMD.