Testosterone levels plummet when guys become dads: Study

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(CBS) How come tough guys sometimes turn into big softies once they have kids? A new study suggests there might be a biological reason behind why new dad's become more sensitive.

Pictures: 11 Warning Signs of Low Testosterone

The study found men's testosterone levels drop following the birth of a newborn. And those hands-on diaper-changing dads see the biggest testosterone drops.

Looks like women aren't the only ones biologically engineered to raise children.

"Classic models of human evolution emphasized the role of men as hunters and providers, with the assumption being that mothers were responsible for childcare," study co-author Dr. Christopher Kuzawa, associate professor of biological anthropology at Northwestern University in Evantson, Ill., said in a written statement. "Our findings challenge this model by showing that human fathers also are wired to care for their children."

For the study - published in the September 12 issue of Proceedings of the National Academy of Sciences - researchers tested saliva from 600 childless Filipino men. Researchers tracked the men for five years, and those who became fathers saw their testosterone drop 34 percent. Men without kids saw a 14 percent testosterone drop, a factor of normal aging.

Testosterone tanked most in men with a child less than one month old. These guys' testosterone levels dropped a whopping 50 percent - and their levels remained low until their kid was a toddler.

"Fathers who are the most involved with physically taking care of their children have the lowest testosterone," study author Lee Gettler, an anthropology doctoral candidate at Northwester, said in a written statement. "This is an important distinction, as we did not find that fathers with the lowest testosterone were simply inclined to become caregivers."

While higher levels of testosterone may help men be more successful at finding mates, high testosterone levels may hinder the nurturing required for childcare, so the drop is like a trade-off, the researchers said.

Some experts hope men take these findings to heart, and stop associating low testosterone with being a "wimp," and lose the notion that men are supposed to be hunter-gatherers while women care for kids.

"My hope would be that this kind of research has an impact on the American male," Dr. Peter Ellison, professor of human evolutionary biology at Harvard, who was not involved in the study, told the New York Times. "It would make them realize that we're meant to be active fathers and participate in the care of our offspring."

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