Other Animals Share Human Mothers’ Pain
Even in the era of advanced maternal health care, childbirth remains a difficult activity, with long delivery times and many potential complications. Evolutionary biologists have blamed our laborious childbirth on the “obstetric dilemma,” or the paradox between walking upright and having a big brain. The hypothesis is that when humans became bipedal, the pelvis narrowed to allow for more efficient walking. At the same time, our heads tended to enlarge to fit our brains, causing a mismatch between the size of the baby’s head and the dimensions of the pelvis.
A recent article in Biological examinationshowever, challenges the idea that human births are particularly difficult among mammals. Evolutionary biologist Nicole Grunstra of the University of Vienna reviewed the research literature on cases of birth difficulties (“dystocia”) in humans and other mammals that give birth to living things. She included studies of captive and wild animals, noting in each case the underlying causes of dystocia.
Read more: “Laying or live birth: how evolution chooses”
From 170 studies, Grunstra determined that birth complications are widespread among mammals, writing that “parturition [birth] causes maternal death in non-human mammals at significant frequencies, even in domestic and farmed mammals where life-saving intervention is also available. Precocial mammals such as primates or elephants, which give birth to one or more large, well-developed babies, have particularly difficult births.
The same is true even in wild mammals, where one would expect natural selection to eliminate genes for traits that result in risky births. For example, calves sometimes become stuck during birth and perish. And wild ungulates – hoofed animals like deer and antelope – have particularly high rates of birth complications, rivaling maternal mortality in humans. Their heads aren’t big, but all those long limbs have to extend out of the birth canal, too.
These results raise the question of why natural selection has not led to changes aimed at reducing risky births. Grunstra attributes the persistence of risk to “a life-course trade-off between the improved survival of larger children and the occasional costs of birth complications.” There is ample evidence that larger babies survive better, creating an ongoing evolutionary tension between offspring body size and the mechanical limitations of the birth canal imposed by the pelvis.
In what Grunstra calls a “cliff edge selection model,” the survival advantages of larger children will continue to increase their size at birth until it reaches a dangerous maximum where they cannot exit the birth canal.
So bigger is better, until it becomes too dangerous.
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Main image: Théodore / Adobe Stock
