Birth: It’s a Tight Squeeze for Chimpanzees, Too

The birth process in chimpanzees and other great apes is generally considered to be easy. This is usually attributed to their relatively large pelvis and the small head of their newborns. In contrast, human childbirth is both more complex and riskier when compared to other mammals. According to the original “obstetrical dilemma” hypothesis, our birth difficulty stems from a conflict that arose during human evolution between adaptations in the pelvis for upright walking and an increase in our infants’ brain size. On the one hand, the pelvis shortened to improve balance while moving bipedally, while the baby’s larger head still had to fit through the birth canal. As a solution to this dilemma, the shape of the pelvic bones differs between the sexes (with females having larger dimensions despite smaller body sizes), and human babies are born more neurologically immature than other primates so that brain growth is delayed to the postnatal period.

An international team of researchers led by Nicole M. Webb of the Senckenberg Research Institute and Martin Haeusler of the Institute of Evolutionary Medicine, University of Zurich, simulated birth in chimpanzees and humans and quantified the space between the bony birth canal and the fetal head. The study shows that narrow birth canals in relation to the infant head size are not unique to humans. Accordingly, the “obstetrical dilemma” hypothesis, which had previously been explained solely by the development of bipedalism and the size of the human brain, did not suddenly appear during the development of modern humans, but rather developed gradually over the course of primate evolution – and then intensified in humans thus explaining the high rates of birth complications observed today.

To test the “obstetrical dilemma” hypothesis, the research team first compared the available space in the birth canal of chimpanzees and humans, using the average distance between the fetal head and the pelvic bones while accounting for soft tissue contributions. “Using a three-dimensional virtual simulation of the birth process, we were able to show that the space in the chimpanzee pelvis is actually just as tight as it is in humans,” explains paleoanthropologist Nicole M. Webb. Interestingly, after a detailed shape analysis they also found that female chimpanzees have a more spacious pelvis than males, especially the smaller females, providing evidence of adaptations to deal with these space limitations. The researchers also show that the great apes appear to trend towards humans in how neurologically immature, or how secondarily altricial their infants are compared to monkeys — again surprisingly similar to humans, although to a lesser magnitude.

“Based on these intriguing parallels, we propose a new hypothesis that the obstetrical dilemma developed gradually and became increasingly exacerbated over the course of evolution. This contradicts the previous theory that our long and difficult births emerged abruptly with the enlargement of the brain in Homo erectus,” explains Martin Haeusler. The increase in body size in the ancestors of the great apes made their pelvis stiffer, which limited the ability of their ligaments to stretch during birth. In early hominins, the upright gait also led to a twisted bony birth canal, which required complex movements of the fetal head. This mechanism, rather than the narrowness of the birth canal, is likely the main cause of the difficult birth process in humans, the researchers argue.

The study shows that the remarkably complex human birth process is the result of gradual compromises during hominoid evolution. “The difficult birth and the neurological immaturity of our newborns, with the long learning phase that follows, are a prerequisite for the evolution of our intelligence. At the same time, we humans are only at one extreme – we are not unique among primates,” states Haeusler. “There have even been isolated observations of birth assistance among captive orangutans. However, births of great apes in the wild are only observed extremely rarely – we urgently need more behavioral data,” insisted Webb.

Literature

Webb N., Fornai C., Krenn V., Watson L., Herbst E. and Haeusler M. Gradual exacerbation of obstetric constraints during hominoid evolution implied by re-evaluation of cephalopelvic fit in chimpanzees. Nature Ecology & Evolution. 23 October 2024. DOI: 10.1038/s41559-024-02558-7