As soon as a man has a child his testosterone levels drop, preparing him for fatherhood, researchers from Northwestern University reported in the journal Proceedings of the National Academy of Sciences. In many species where the male helps out with caring for the young, the same steep fall in testosterone levels is observed.
Testosterone gives a male behaviors and other characteristics needed when competing for a mate, the authors explained. However, as soon as that has been achieved, hormone levels go down because mating-related behaviors in many ways are not compatible with the responsibilities of nurturing offspring.
Co-author, Christopher W. Kuzawa, said:
"Humans are unusual among mammals in that our offspring are dependent upon older individuals for feeding and protection for more than a decade. Raising human offspring is such an effort that it is cooperative by necessity, and our study shows that human fathers are biologically wired to help with the job."
Kuzawa is associate professor of anthropology in the Weinberg College of Arts and Sciences, and a faculty fellow at the Institute for Policy Research at Northwestern.
Researchers had not been sure whether fatherhood lowered testosterone levels, or whether males with established low testosterone levels were automatically better fathers. Previous studies on fatherhood and testosterone had been mainly small and non-conclusive ones, the researchers wrote.
In this new study, 624 males in the Philippines who were not fathers, were followed until and after they became fathers. At the start of the study they were aged between 21.5 and 26 years. The scientists wanted to determine whether their testosterone levels altered during that period.
Co-author, Lee Gettler, said:
"It's not the case that men with lower testosterone are simply more likely to become fathers. On the contrary, the men who started with high testosterone were more likely to become fathers, but once they did, their testosterone went down substantially. Our findings suggest that this is especially true for fathers who become the most involved with child care."
It appears that their testosterone-level-drop is particularly steep when the newborn baby first arrives home. The scientists added that this sharp fall is temporary.
"Fatherhood and the demands of having a newborn baby require many emotional, psychological and physical adjustments. Our study indicates that a man's biology can change substantially to help meet those demands."
Kazawa believes that the drop in hormone levels experienced by men with offspring may also protect their long-term health. Several studies have shown that long-term single men have worse health than married men and fathers.
"If fathers have lower testosterone levels, this might protect them against certain chronic diseases as they age."
Written by Christian Nordqvist