Small-bodied primate babies can have heads nearly twice as large as their mothers' pelvic space, suggesting that the tight fit of a baby's head through the birth canal is not unique to humans, as previously thought, a new study has found.

Findings published in the journal Nature Ecology and Evolution challenge the theory of an exclusively human ''obstetrical dilemma'the idea that our large heads and narrow pelvises (adapted for upright walking), have made childbirth uniquely difficult for our species, researchers said.