Timing of Childbirth Evolved to Match Women's Energy Limits

Timing of Childbirth Evolved to Match Women's Energy Limits
Posted By: Erin Wayman
Smithsonaian Magazine
August 29, 2012
https://blogs.smithsonianmag.com/hom...energy-limits/

New research suggests the timing of human gestation is
not a compromise between the size of a woman's hips and
the size of a baby's head. Instead, it's determined by a
woman's energy limits.

Have you ever wondered why women stay pregnant for nine
months? For decades, anthropologists have explained the
timing of human gestation and birth as a balance between
two constraints: the size of a women's hips and the size
of a newborn's brain. But new research says that's not
the case. Instead, the timing of childbirth occurs when
women's bodies can no longer keep up with the energy
demands of pregnancy. That happens at around nine
months, Holly Dunsworth of the University of Rhode
Island and colleagues report online August 27 in the
Proceedings of the National Academy of Sciences.

The traditional explanation of gestation length is known
as the obstetric dilemma. The hypothesis suggests that
the width of the pelvis, and thus the width of the birth
canal, is limited by the demands of efficient upright
walking. But as brain size expanded over hominid
evolution, heads got bigger. To make sure a baby's head
could fit through the birth canal, gestation decreased
and babies were born at an earlier stage of development;
today, newborns enter the world with the least developed
brain of all primates at less than 30 percent adult
size.

Dunsworth and her colleagues wanted to see if they could
find any actual evidence to support the obstetric
dilemma. First, they considered gestation length.
Traditionally, human gestation has been considered short
when looking at how much additional growth the brain
needs to reach adult size. But such a measure is unfair
when compared to other primates since humans have
abnormally large brains, the researchers say. Instead,
Dunsworth's team compared gestation length to maternal
body size and found humans actually have relatively long
pregnancies-37 days longer than would be expected for a
typical primate our size. Our gestation is also
relatively extended compared with chimpanzees or
gorillas, suggesting pregnancies got longer, not
shorter, in hominids.

The team also looked for evidence that widening the
pelvis to accommodate bigger brained babies would make
walking less efficient. Researchers have assumed that
broadening the hips would increase the force needed by
hip muscles to walk and run, thus making locomotion less
energy efficient. But one recent study shows the
dimensions of the hips don't actually affect the
muscle's required force, calling into question the long-
held belief that wider hips would interfere with women's
walking. Furthermore, the team calculated how much wider
the hips would have to be if humans were born with the
same brain development as chimps (40 percent adult
size). All that would be needed is a three-centimeter
increase. Women's hips already vary by three or more
centimeters, the researchers say, suggesting that hip
size really doesn't limit gestation.

Instead, gestation is determined by energy. Studies of
mammals show that during pregnancy females reach their
species' "metabolic ceiling," the upper limit of the
amount of energy they can expend. In humans, the
metabolic ceiling is 2 to 2.5 times the baseline amount
of energy needed during rest. Dunsworth and her
colleagues say women reach that limit by their sixth
month of pregnancy. Then at nine months, the energy
demands of a fetus go beyond this metabolic threshold.
"Extending gestation even by a month would likely
require metabolic investment beyond the mother's
capacity," the team writes.

But even though hip size doesn't appear to limit the
size of a baby's head, women around the world often have
trouble delivering babies because of the tight fit of
the head going through the birth canal. One possible
explanation is that childbirth has only become
problematic recently in human evolution. Changes in diet
that have led to increased energy consumption may be
allowing women to produce bigger babies, and natural
selection hasn't had enough time to broaden the hips.
Figuring out why modern childbirth is so difficult, and
dangerous, is an area that needs further research.