Why humans struggle with childbirth, but snow monkeys don't

Yahoo

25-03-2025

While bringing new life into the world is often viewed as one of life's most joyful moments, childbirth continues to pose serious risks for women - with more than 287,000 maternal deaths recorded during or shortly after pregnancy in 2020 alone.
Even though maternal mortality has significantly declined in recent decades, and most high-income countries report fewer than 10 deaths per 100,000 births, childbirth can still lead to severe complications, especially in regions with limited access to quality healthcare.
Interestingly, recent research led by the University of Vienna in Austria has shown that, unlike humans, Japanese macaques (Macaca fuscata), commonly known as snow monkeys, experience no maternal deaths linked to childbirth.
The study, which was published in the journal Proceedings of the National Academy of Sciences of the United States of America (PNAS), reveals that despite having a similar pelvis-to-head ratio as humans, these Old World monkeys—a diverse group of primates within the Cercopithecidae family, notable for their non-prehensile tails and wide geographic distribution—rarely encounter complications during delivery.
One of the main causes of maternal mortality in humans is the evolution of large brains, resulting in large fetal heads that struggle to pass through a narrow birth canal. This issue contributes to mortality rates of up to 1.5 percent in countries lacking sufficient medical care.
To investigate whether macaques face similar birth complications and maternal mortality as humans, scientists and midwives from the University of Vienna, the Medical University of Vienna, the Konrad Lorenz Institute for Evolution and Cognition Research (Klosterneuburg), and Kyoto University (Japan) analyzed long-term demographic data from semi-free-living Japanese macaques at Affenberg in Landskron, Austria.
Barbara Fischer, PhD, an evolutionary biologist at the University of Vienna and one of the study's authors, explains that among 281 infants born to 112 female macaques during this period, not a single maternal death occurred.
In an interview with Interesting Engineering (IE), Fischer says it was fascinating to study macaque births to understand what makes them similar and different from the human experience. "Why did childbirth evolve to be so difficult in humans," Fischer told IE. "Macaques are closely related to humans, they are primates, and they also give birth to really large fetuses, compared to the size of the birth canal of their mothers - just like humans."
Fischer emphasizes that the key finding is especially striking because Japanese macaques give birth to fetuses that are proportionally as large as those in humans. Yet, they show no signs of birth-related maternal mortality.
On why Japanese macaques face no mortality risks, the evolutionary biologist explains that this remains unknown. However, she suggests one possible reason is their different pelvic anatomy, as their birth canal does not require the fetus to rotate during delivery, unlike humans.
"This makes birth presumably simpler and less error-prone in macaques," Fischer highlights. "They also possibly have a pelvic girdle and pelvic floor that becomes more flexible around birth, which they can afford in contrast to humans because they do not walk upright."
"We think that the pelvic girdle and pelvic floor muscles in macaques have greater flexibility during labor compared to humans and that the birth dynamics are less restricted due to differences in pelvic morphology," she points out.
Moreover, Fischer says that loosening the pelvis would pose greater problems for an upright-walking species like humans than for quadrupedal animals such as macaques. "Macaque fetuses might also have skulls that are just as malleable as human fetuses' heads, which aids during birth."
Meanwhile, Katharina Pink, PhD, an evolutionary anthropologist and midwife at the Medical University of Vienna, and lead author of the study, believes there may be valuable lessons to learn from macaque births that could help improve approaches to human childbirth.
"We were able to show for this population that not a single female has died in connection with the birth of her young within the last 27 years," Pink says, emphasizing that macaques move freely during birth and they often squat and change their birthing positions.
"Movement, in combination with the anatomical condition, is key for a successful birth," Fischer continues. "This should be actively enabled for women who give birth."
Scientific reports on births among free-living, non-human primates are rare, as most births occur at night or in the early morning hours, making them difficult to observe and document. However, observed birth events suggest that non-human primates instinctively adopt standing or squatting labor positions, likely making optimal use of their pelvic flexibility to ease delivery.
According to Fischer, the freedom to move and adopt intuitive birthing positions plays a crucial role in how macaques easily navigate labor. "Birth is a highly dynamic process," she adds. "Since there is not much space in the birth canal, using it efficiently by moving around and changing position is key."
Fischer highlights that this study offers the first reliable long-term data for estimating maternal mortality in non-human primates. "Maternal mortality in connection with birth is very difficult to measure, even in humans," she says. "There is no reliable data available for other primate species - single observations of birth events do exist, but this does not give a good overview of how risky birth is in other primate species, compared to humans, under natural conditions."
The team spent years gathering the data, which Fischer describes as extensive and demanding work. "This research was only possible thanks to the effort the researchers at Affenberg have put into collecting these data over tens of years," she adds. "Our work also shows how valuable long-term data is for answering diverse research questions, not just about primate behavior, but also about evolution."
The researchers now aim to expand their comparative studies to other primates, hoping to understand the evolution of childbirth better and identify precisely why macaques avoid birth complications.
Pink believes studying natural primate birthing behaviors could inspire new, less invasive approaches to maternal healthcare. "These observations could inspire future studies to understand better how freedom of movement during physiological birth can lead to more individualized and less invasive care for mothers," Pink concludes.