Amazing stuff! This about the obstetrical dilemma. Was it not known for thousands of years that male babies are on average larger than female babies and thus impose more risks during childbirth?
Apparently, the only new discovery here is that these risks are found across mammal species.
I am not sure why the AAAS (American Association for the Advancement of Science) picked this article to promote it!
Again the AAAS gets caught using the ideological term "people" instead of women! DEI demagoguery is still alive!
The costly son hypothesis is a very unfortunate term. I hope it will be quickly end up in the dustbin of history and gain no traction!
"Every day, about 700 [women] people die giving birth around the world. A leading cause is obstructed labor, often due to the baby’s head exceeding the size of the birth canal. ...
Reviewing birth data for 140 species, the authors argue that sons are more difficult to birth than daughters for many mammals, including humans and our ancient ancestors.
The authors found this pattern in five domesticated animals: cattle, water buffalo, sheep, goats, and a breed of American racehorse.
That’s also the case in captive antelope and Asian elephants.
The pattern holds in cattle and sheep, even when controlling for birth weight, suggesting in addition to sheer size, factors such as sex differences in fetal shape and behavior could contribute to males causing more trouble during birth. ...
This added risk of this phenomenon, which the researchers dubbed the “costly son” hypothesis ..."
From the abstract:
"Human childbirth has often been considered uniquely difficult among mammals, yet recent evidence suggests that obstructed labor and fetopelvic disproportion are widespread across placental mammals.
Here, we propose the “costly son” hypothesis: because male offspring are typically larger at birth, they impose greater obstetric risk on mothers and themselves by increasing the likelihood of fetopelvic disproportion and dystocia. We compiled sex-specific birth weight data for 140 placental mammal species across 11 orders and reviewed published evidence on sex differences in dystocia risk.
Male neonates were more commonly larger at birth than females, and male-biased birth size differences were strongly associated with male-biased adult sexual size dimorphism. Species with male-biased adult size dimorphism had sixfold greater odds of exhibiting male-biased birth weight differences than species with female-biased dimorphism.
Male-biased birth weight differences existed irrespective of developmental mode (precocial or altricial), indicating that sex differences in fetal growth arise early in gestation.
Evidence from humans, livestock, and at least one non-human primate suggests that male fetal sex is associated with elevated dystocia risk, that is, precocial taxa where birth size approaches the mechanical limits of the maternal birth canal.
We further hypothesize and discuss how parity and sex ratio adjustment may modulate these risks and associated trade-offs in a Trivers–Willard context.
Our findings identify fetal sex as an underappreciated dimension of mammalian obstetrical dilemmas and suggest that the evolutionary benefits of producing larger sons are balanced by increased risks of birth complications and mortality."
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