Metabolism Key To Length Of Human Pregnancy
August 28, 2012

Long-held Theory On Human Gestation Called Into Question

April Flowers for - Your Universe Online

It's not all in the hips, baby.

The length of human pregnancy is limited primarily by a mother's metabolism, not the size of the birth canal, suggests a new study from the University of Rhode Island.

This new research, published in the Proceedings of the National Academy of Sciences, challenges the long-held notion of an evolutionary trade-off between childbirth and a pelvis adapted for walking upright.

Big brains and the ability to walk upright are traits that set humans apart from other primates. These traits are seemingly at odds when it comes to childbirth. Big brains and big heads that encase them are difficult to push through the human birth canal, but a wider pelvis would compromise walking.

It's long been the scientific theory that nature's solution to this problem, which is known as the "obstetric dilemma," was to shorten gestation so that babies are born before their heads get too big, leaving human babies relatively helpless and seemingly underdeveloped in terms of motor and cognitive ability when compared to other primates.

"All these fascinating phenomena in human evolution–bipedalism, difficult childbirth, wide female hips, big brains, relatively helpless babies–have traditionally been tied together with the obstetric dilemma," said Holly Dunsworth, an anthropologist at the University of Rhode Island and lead author of the research. "It's been taught in anthropology courses for decades, but when I looked for hard evidence that it's actually true, I struck out."

The first problem that Dunsworth and her team encountered with the obstetric dilemma theory is that there is no evidence that hips wide enough to deliver a more developed baby would be a detriment to walking. Anna Warrener, a post-doctoral researcher at Harvard University and one of the paper's co-authors, has studied how hip breadth affects locomotion with women on treadmills. She found that there is no correlation between wider hips and a diminished locomotor economy.

"That throws doubt on the assumption that the size of the birth canal is limited by bipedalism," Dunsworth said. "Wide hips don't mean you can't walk efficiently."

The team then looked for evidence that human pregnancy is shortened compared to other primates and mammals. What they found was well-established research to the contrary.

"Controlling for mother's body size, human gestation is a bit longer than expected compared to other primates, not shorter," Dunsworth said. "And babies are a bit larger than expected, not smaller. Although babies behave like it, they're not born early."

For humans and mammals in general, gestation length and offspring size are determined by the mother's body size. Because body size is a good proxy for an animal's metabolic rate and function, Dunsworth started to wonder if metabolic rate might offer a better explanation for the timing of human birth than pelvis width.

Peter Ellison of Harvard University and Herman Pontzer of Hunter College, two experts in human physiology and energetics, joined the team to find out. Building on Ellison's previous work on human pregnancy and childbirth, the team developed a new hypothesis for the timing of human birth called EGG (energetics, gestation, and growth).

"Under the EGG, babies are born when they're born because mother cannot put any more energy into gestation and fetal growth," Dunsworth explains. "Mom's energy is the primary evolutionary constraint, not the hips."

The study shows that women give birth just as they are about to cross into a metabolic danger zone using metabolic data on pregnant women.

"There is a limit to the number of calories our bodies can burn each day," says Pontzer. "During pregnancy, women approach that energetic ceiling and give birth right before they reach it. That suggests there is an energetic limit to human gestation length and fetal growth."

These constraints explain why human babies are so helpless compared to our primate cousins, like chimpanzees. Human babies don't begin to crawl until they are seven months, for example, whereas chimp babies are crawling by one month. For a human mother to give birth to a child as developmentally advanced as a chimp, however, would take a 16-month gestation. This would put the mother well past their energetic limits. In fact, even one extra month of gestation would cross into the metabolic danger zone.

"It would be physiologically impossible, regardless of pelvic bone anatomy, to birth a more developed baby," Dunsworth said. "Our helplessness at birth is just a sign of how much more brain growth we have to achieve once we start living outside our mother."

The energetics, gestation and growth hypothesis would downplay an implication of the obstetric dilemma that Dunsworth finds odd.

"We've been doing anthropology with this warped view of the male pelvis as the ideal form, while the female pelvis is seen as less than ideal because of childbirth," she said. "The female births the babies. So if there's an ideal, it's female and it's no more compromised than anything else out there. Selection maintains its adequacy for locomotion and for childbirth."

"If it didn't, we'd have gone extinct," Dunsworth said.