The tight fit of a baby's head through a mother's birth canal, previously believed to cause uniquely difficult childbirth in humans, also occurs in some small-bodied primates, according to research published on Jun. 29 by University College London researchers.
The study found that certain small-bodied primate babies have heads almost twice as large as their mothers' pelvic space. The findings challenge the longstanding theory of an exclusively human 'obstetrical dilemma,' which posits that large heads and narrow pelvises adapted for upright walking make childbirth particularly difficult for humans.
Researchers at University College London revisited previous evidence and reported that while constricted birth is not experienced by other apes, it is common among many small-bodied primates, especially American monkeys such as bushbabies and squirrel monkeys. For example, the heads of newborn squirrel monkeys can be nearly double the size of their mother's pelvic space.
Co-corresponding author Dr. Nicole Torres Tamayo said, "Much of the data that informed earlier studies was flawed. It had been collected in a human-centric way that failed to consider the anatomy of other species. As well as greatly expanding the number of species considered, we collected measurements that took into account the specific anatomy of different species. This data then informed our 3D modeling. In the past, the measurement for the newborns' heads were from the forehead to the back of the skull. This assumed that all babies are born crown-first, as most humans are. But species like the gelada monkey, with their pronounced snouts, are often birthed face-first. We took this positioning into account."
Using advanced 3D modelling techniques and by expanding their study from eight to 29 species, researchers found tight fits at birth were especially common among proportionally smaller primates.
The study received funding from a Sasakawa Foundation Butterfield Award, a Leverhulme Trust Project grant and two Kyoto University Cooperative Research Programme Awards; data collection was supported by National Science Foundation grants.
