This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: A new comparative review, published in The Anatomical Record, examines factors that may lead to birth complications among humans and other mammals. The study suggests that fetal sex may play an important role in risky births in humans and several other species.
It's often thought that human childbirth is unusually difficult among mammals. The idea is that an evolutionary "obstetrical dilemma" arose because of our bipedal pelvic anatomy and increasing brain size over time. Larger babies passing through a narrow birth canal can increase the chance of obstructed labor, affecting mortality for both the baby and mother.
Yet difficult births, including cases in which a fetus is too large for the birth canal, occur across many placental mammals. Some research has suggested that the obstetrical dilemma was solved by relatively earlier births, but other studies found no evidence that humans have shorter gestations than other apes, even after controlling for body size. Furthermore, researchers involved in the new study say there is a maximum fetus size compatible with safe delivery, and when it is exceeded, obstructed birth results.
In other words, birth size and weight are known to be associated with difficult delivery. The new study suggests, however, that because male babies also tend to be larger, male births tend to be riskier. The team says this pattern is not restricted to humans.
"Most taxa, including humans, displayed a bias toward larger male fetuses. Fetal sex has nonetheless not been incorporated in evolutionary perspectives of childbirth, perhaps because it is often assumed that the sex of each baby is determined at random, since most mothers are likely to produce both sons and daughters," the study authors write. "Every mother has the potential to gestate both offspring through the reproductive career.
Hence, it might be assumed that selection has acted on offspring size in general to resolve the obstetrical dilemma. As we show, however, the risks of birth complications are not equal for sons and daughters." The team compiled sex-specific birth-weight data for 140 placental mammal species across 11 mammal orders and compared sex differences among newborns with adult body-size differences and developmental style. They then searched for reports of difficult birth by offspring sex.
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