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Fast-Growing Boys Have More Sex as Men

13 September 2010 3:00 pm
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Elizabeth Quinn

Fast track. When these Filipino men were babies, they adapted their growth rates to grow up big and strong fast.

The faster a baby boy gains weight in the first 6 months of life, the more sex he'll probably have as a young adult. That's one of the startling new findings of a long-term study of more than 700 men from birth to early adulthood in the Philippines. The fastest-growing baby boys reached puberty earlier than babies who gained weight more slowly. By their early 20s, they were stronger, taller, and had more testosterone (and sex partners). "Your characteristics as an adult male—body size, lean mass, and sexual maturation—are programmed by your early experiences," says lead author Christopher Kuzawa, a biological anthropologist at Northwestern University in Chicago, Illinois.

Researchers have long known that babies adapt to their world even before they're born—and that those adaptations affect their physiology and metabolism as adults, including their risk for chronic disease. Infants born to malnourished mothers, for example, are more likely to be born small and have efficient metabolisms that require less fuel to survive. Conversely, infants whose mothers were well-fed are often born larger and are more likely to grow bigger more rapidly, requiring more calories to maintain their larger bodies that can make them more attractive mates. Problems arise, however, when low-birth-weight babies are fed diets rich in sugars or fat, which stress their smaller organs, such as pancreases, for example, leading to diabetes—or when fast-growing children face famine.

Scientists have debated whether these responses to good or bad times early in development are adaptive adjustments to their environment—reflecting that infants have a window of time early in life when they can fine-tune their developmental trajectories—or whether early deficiencies in nutrition and stress simply predispose these infants to more disease as adults.

Kuzawa and his colleagues believe they have found evidence for the adaptive hypothesis in data on 770 men in the Philippines who are part of a long-term study tracking their development since birth. They found that baby boys who were breast-fed and had less diarrhea, probably because they lived in wealthier households with better hygiene, had faster rates of growth in the first 6 months of life than did bottle-fed infants or breast-fed infants from poorer mothers. They note that this is a time when testosterone surges briefly to adult levels in males, presumably influencing the development of resource-costly male traits such as height and muscle. Indeed, these boys reached puberty earlier, were taller and more muscular, and had higher levels of testosterone in their blood as adults than did males who gained weight more slowly as infants. Babies on the fast track also grew up to have sex earlier and were more likely to report having sex in the past month, resulting in more lifetime sex partners, the researchers report online today in the Proceedings of the National Academy of Sciences.

Kuzawa speculates that the boys made adjustments to their environments based on early nutritional cues. For instance, males fed breast milk from well-nourished mothers could "learn" from hormone levels and nutrients in breast milk that they would live in a food-rich world, and thus spend precious resources boosting their rate of growth so they reached puberty and began reproducing earlier. Males whose mothers could give them fewer nutrients or were stressed, meanwhile, grew slower to conserve all of their resources to survive a lean world.

"The authors have shown that the early-life environment has enduring impact on traits important to human males," says evolutionary biologist Stephen Stearns of Yale University. But human evolutionary biologist Peter Ellison of Harvard University says he doesn't think the researchers have actually proved that these boys' rapid maturation is an adaptive response to the early environment. The differences in growth rates could reflect genetic traits inherited from parents that influence how sensitive different boys are to testosterone, for example, which would make them grow faster and stronger. Regardless, Ellison says this is a very "rich" result that suggests many angles of new research, such as testing whether levels of testosterone in newborns, really influence rates of weight gain.