Reading 3 Flashcards
Is there evidence for the human gestation, hip width, and brain size belief?
Researchers find no evidence for the long-held view that the length of human gestation is a compromise between hip width and brain size
What does new research suggest the timing of human gestation is not a compromise between? What is it actually?
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.
How have anthropologists explained the timinf of human gestation and birth as a balance of for decades? What does new research say about this?
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
What is the traditional explanation of gestational length known as?
the obstetric dilemma
What is 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.
Why is it unfair to say that human gestation is abnormally short compared to other primates? What has research found?
But such a measure is unfair when compared to other primates since humans have abnormally large brains
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.
Why have people always thought that wide hips make walking less efficient? What does the research show about this?
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.
What have we found that 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,”
Why might smaller birth canals be making birth difficult?
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.
