Difficult birth also in chimpanzees

Difficult birth also in chimpanzees

A chimpanzee mother with her baby. © Neurobite/iStock

It’s not just us humans who have a difficult time giving birth: chimpanzees also have a narrower birth canal than previously thought. This is shown by a study using three-dimensional models of the pelvic anatomy in chimpanzees and humans. Until now, it was assumed that the difficult birth in humans was primarily due to our upright walk and the baby’s relatively large brain. However, the new results suggest that the pelvis gradually narrowed over the course of evolution – even before the development of upright walking.

Most births in the animal kingdom are quick and uncomplicated. We humans, on the other hand, typically struggle for several hours until the baby is born and often have to struggle with birth complications. There are two main reasons for this: the upright walk, which has led to a reduction in the size of the human pelvis, and the large brain, which ensures that the fetus has a large head. In order to at least partially compensate for these problems, sexual dimorphism has developed over the course of human evolution, so that women have significantly wider pelvises than men. In addition, human babies are born comparatively immature, which at least somewhat limits the size of their heads at birth.

Comparison of humans and chimpanzees

Great apes, on the other hand, usually move on all fours and the head of the baby monkeys is relatively smaller than that of human babies. “In accordance with the birth dilemma theory, it was assumed for a long time that great apes had no discernible differences between male and female pelvic shapes,” explains a team led by Nicole Webb from the University of Zurich. “However, new studies show that sexual dimorphism in pelvic shape also occurs in chimpanzees, although to a lesser extent than in modern humans.”

To get to the bottom of this phenomenon, Webb and her team created detailed three-dimensional virtual simulations of the birth process in humans and chimpanzees. “In this way, we found a similarly narrowed central pelvis in chimpanzees as in humans, with even narrower outlet dimensions,” report the researchers. “At the same time, our analyzes show that even in chimpanzees, females have a more spacious pelvis than males, even though they are smaller overall.”

The researchers also found clear parallels between chimpanzees and humans when it came to the head size of the newborns: “Chimpanzee babies have a significantly smaller brain relative to that of fully grown individuals, although the difference is not quite as great as in humans,” says Webb. “Accordingly, young macaques begin to walk and climb independently at around three weeks, young great apes at five to six months and human children at one year.”

Constriction even before walking upright

Based on these results, the researchers hypothesize that the birth dilemma did not only develop as a result of upright walking. “We propose a new hypothesis that the birth dilemma has developed gradually over the course of evolution and has become increasingly severe,” explains Webb’s colleague Martin Häusler. The upright walk then led to the already difficult passage through the narrow pelvic canal becoming even more complicated: “The pelvic blades shortened and led to a twisted birth canal, which caused a complicated rotation, flexion and extension movement of the fetal head and, at different times, the rest body requires,” says Häusler.

From the researchers’ point of view, the twisted pelvic canal is the actual reason for difficult birth in humans, not the tight space. “The space conditions are similar in chimpanzees, but the birth canal is straight and the fetus can simply slip through,” says Häusler. Even if birth is more difficult for chimpanzees than for many other monkeys, it is at least easier than for humans. “We humans are just at the end of one extreme – but we are not unique among primates,” says Webb. “In fact, there are even isolated observations of ‘obstetrics’ among captive orangutans. Births of great apes in the wild are extremely rarely observed – we urgently need more data on their behavior during birth.”

Source: Nicole Webb (University of Zurich) et al., Nature Ecology & Evolution, doi: 10.1038/s41559-024-02558-7

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