Sex differences in the pelvis are not uniquely human

Updated: Apr 21

Wider hips in human females is a feature that has long been thought to have evolved to birth babies whose brains ballooned during the Pleistocene (about 2 million years ago). This phase in our evolutionary past, characterised by rapidly increasing brain-size is called encephalization.

Now, a study by Barbara Fischer et al. has found that sex differences in pelvis did not evolve during encephalization, but probably preceded it. The researchers compared the pelvic sex differences in humans with those in chimpanzees (whose lineage diverged from ours 4 million years ago). 44 anatomical 3D landmarks and 65 curve landmarks on each pelvis were used for the comparison. The sample comprised 34 adult chimpanzee pelves (20 female, 14 male) and 99 adult human pelves (53 female, 46 male).

The researchers found that the two species share the same pattern of sex differences in pelvis shape even though the magnitude of sex differences was two times larger in humans than in chimpanzees (explained by the fact that humans are much bigger than chimpanzees). The researchers also compared the sex differences in pelvic inlet size, which determines the dimensions of the birth canal. The inlet size was found to be 11% larger in human females and 10% larger in chimpanzee females (compared with males), yielding, once again, identical patterns of sex-difference.

So, why should sex differences in two species that supposedly faced different selection pressures be identical? The researchers think that the most parsimonious explanation is that the sex difference in anatomy existed in the common ancestor of chimpanzees and humans. Developmentally, pelvic sex differences come about due to hormone-induced remodelling of bones during puberty. The researchers go on to propose that relevant aspects of the endocrine system (and underlying genetic machinery) remain conserved since the common ancestor of all primates. What did evolve separately in species, they say, is the duration and amount of hormones released during puberty, producing species-specific magnitudes of sex difference in pelvic size.

Read more about the study here.