Sons may be harder to birth than daughters in many mammal species

News from Science reports that a study reviewing birth data for 140 species argues sons are harder to birth than daughters in many mammals, including humans and their ancient ancestors. That adds a wrinkle to an explanation that links difficult human births to babies’ large heads and a pelvis shaped for upright walking. The study suggests the added risk may be offset because bigger male babies grow into bigger male adults who tend to have more grandbabies.