Human childbirth has long been considered exceptionally difficult. A common explanation is the so-called obstetrical dilemma: Humans walk upright and have large brains, leading to a tight fit between the baby and the mother's pelvis. This is thought to make birth especially risky in our species. But this idea has rarely been tested against data on birth outcomes from other mammals.

Looking beyond humans

In the new study, Nicole Grunstra from the Department of Evolutionary Biology at the University of Vienna reviewed a wide range of scientific literature and compiled data on birth complications across mammals. The analysis included domestic animals such as cows and sheep, as well as wild species living under natural conditions such as seals and deer.

The goal was to find out whether humans are truly exceptional, or whether difficult births are widespread in the animal kingdom.

Birth complications across mammals

The results show that birth difficulties are not unique to humans. They occur across many placental mammal species, including those in the wild where natural selection might be expected to eliminate such risks. Even whales and dolphins can have calves get stuck during birth, despite lacking a bony pelvis.

In some species, such as deer and antelope, rates of birth complications and female mortality are similar to those seen in human populations, including hunter-gatherer populations without modern medical care. The types of birth complications and underlying causes are also comparable. A tight fit between the fetus and the birth canal, for example, is common in species that give birth to large, well-developed offspring, such as monkeys, ungulates, and elephants. And overnutrition can cause the fetus to grow bigger in humans, other primates, and rodents.