Using biomechanical studies in tandem with the metabolism of pregnancy Dunsworth et al. have ruled out birth canal size as the limiting factor where gestation length is concerned. Instead they posit an energetic equation whereby the pregnant female is constrained to give birth at around nine months by her body's inability to produce the energy necessary to maintain her life and that of her foetus. Brilliant concept! It's a good day to be a palaeoanthropologist.
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