Study: Teen Growth Spurt Helps Moms

Jan. 5, 2006 — -- Parents of small children may not feel like they have it easy, but compared to chimpanzee parents, they do.

Research has shown that humans have evolved so that their rate of growth makes it easier on the parents, particularly the mother. And this accommodating growth rate, anthropologists say, allows human mothers to have more children -- a trait that might have been key in humans' early evolutionary success.

"Kids can't meet their own needs until their teens, sometimes even late teens. And, as we know, a teenager can eat you out of house and home," said Michael Gurven, an anthropologist at the University of California, Santa Barbara. "The irony is it would be even more difficult if our children grew like primates."

Chimpanzees, which are believed to be humans' closest ancestral relative, grow at a steady rate. Human babies, meanwhile, stay fairly small until adolescence when they quickly shoot up in size. In a recent report in Proceedings of the Royal Society B, Gurven examined two hunter-gatherer societies in Paraguay and Botswana to show how this comparatively slower rate of growth lessens the load for parents since smaller children means smaller bellies to feed.

Gurven, who worked with Robert Walker of the University of New Mexico in Albuquerque, used past studies to calculate how much the children of the two foraging groups of people would eat if they grew as chimps did. Their model showed that the Ache girls and boys of Paraguay would eat 9 percent and 25 percent more calories respectively if they grew as chimps did. The children of Botswana's Dobe Ju/'hoansi people, meanwhile, grow more slowly. Their girls would eat 38 percent more and the boys -- 54 percent more.

"The human strategy is to stay small as long as possible and then shoot up and get big just before you're about to be useful," Gurven said. "It's good economics."

He points out that chimpanzees get bigger sooner because their foraging skills aren't as advanced so a 4-year-old chimp can learn to be mostly self-sufficient. Humans, however, have a long training period in order to, in Gurven's words, "get more bang for the buck." In primitive societies, children can't even begin to forage at all until at least age 7 and most aren't self-sufficient until their teens.

Barry Bogin, of the University of Michigan, points out that the human-growth rate creates something no other animal species has -- a childhood. Instead of going rapidly from breast-feeding to self-sufficiency, human children are weaned long before they're ready to fend for themselves. Bogin's studies based on hunter-gatherer societies show that mothers tend to wean their babies around 30 months after birth -- or when the child is about 3. Chimps continue nursing until their young are 4 to 4.5 years of age, at which point they begin foraging for themselves.

"This means that chimps can reproduce only every five to seven years, while people can reproduce every two to four years," Bogin said. "Chimps are endangered; we're not."

Of course, having more children who can't fend for themselves means mothers can be quickly overwhelmed -- and that's where humans' unique family structures kick in.

"We lean on our relatives -- grandmothers, fathers," Bogin said. "No other animal does this to same degree. If a mother chimp dies before her young is age 4, the baby dies."

Bogin explains humans also rely on their ingenuity to make sure their babies get the nutrition they need even after weaning. He argues this began with homo habilus who apparently figured out about 2.5 million years ago how to get at bone marrow through the use of tools.

"Marrow is the perfect weaning food," he said. "It's high in fat and protein."

These days most babies eat cereal, not marrow, once they're weaned, and most 7-year-olds aren't out foraging for themselves. Gurven also suspects that babies and children of Western societies may be getting bigger in height and weight as food resources are in abundance. One obvious example is the 14-pound, 3-ounce baby born to an Oklahoma woman last month.

That said, one thing appears to have remained consistent within humans -- teenagers still grow fast.

"Kids in Western cultures still hit that growth spurt in their teens," Gurven said. "It happens just before they're ready to strike out on their own -- if you're lucky."