National Geographic News
Children pose for a portrait.
Filipino children pose for a portrait (file photo).

Photograph by J. Baylor Roberts, National Geographic

Christine Dell'Amore

National Geographic News

Published September 13, 2010

Males who gain weight quickly as babies continue their lives in the fast lane—growing up to have more sex partners, according to new research.

Previously researchers had known that all male newborns get a surge of adult-strength testosterone for a brief period. But the study team was curious whether nutrition has a hand in how the hormone affects a man's sexual development.

(Read why men are sloppy kissers.)

Beginning in 1983, the team examined nearly 800 Filipino men from birth to between the ages of 20 and 22.

A "remarkably consistent" result emerged: The men who were well fed—and thus gained weight faster—as infants reached puberty earlier, started having sex earlier, and had more sex partners over their lifetimes.

Men who'd grown fast as babies had an average of 3.2 sexual partners by the time of the 2005 survey, while those who'd grown slowly had an average of 1.7 sexual partners.

The faster-growing men also had more testosterone, were taller and more muscular, and had a stronger grip, among other factors.

The approximately 700 women tested in the study, who do not have an early spike in testosterone, did not show a link between fast infant growth and an earlier onset of puberty.

"Our study provides additional evidence that our fates are not hardwired at birth," said study leader Christopher Kuzawa, an anthropologist at Northwestern University in Illinois.

"We can't rule out genetic contributions, [but] I think we have good evidence that it's not simply a genetic story."

Men's Beefier Bods Come at a Cost

Evolution may favor faster maturity, because being taller and stronger offers certain reproductive advantages among men, Kuzawa said.

For instance, there is some scientific evidence that, in humans, men who are tall and strong are more attractive to women. Evolutionary biologists suspect that such traits—associated with high testosterone—may be cues that the man would produce stronger children.

(Related: "Watching 'Sexy' Males Leads to Better Chicks, Study Says.")

But such benefits come at a cost: Maintaining a more robust body requires more calories, something that may not be a given—such as in some poverty-stricken regions of the Philippines.

That's why Kuzawa hypothesizes that the body can "sense" nutritional status early in life and adjust accordingly. When food is abundant, a male can afford to build a beefier body. But in an environment where food is scarce, it "pays to be smaller," Kuzawa said.

(Watch a video of babies in the womb.)

Kuzawa cautioned that this study is the first time a link has been found between nutrition and early puberty, which means it's too early to extend the findings to babies in other countries.

Still, the research does reinforce the notion that undernutrition can have long-lasting effects on development, Kuzawa noted.

"This should be a priority—making sure that kids are well nourished."

The fast-growing babies study was published Monday in the journal Proceedings of the National Academy of Sciences.

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