Evolutionary trade-offs in the human and mammalian pelvis

Human childbirth appears to be more difficult than birth in many other mammals. In part, this is due to the large size of the baby relative to the mothers birth canal, i.e. the lower part of the pelvis (two hipbones and the sacrum). Sometimes this fit is so tight that the baby gets stuck (obstructed labor) and mothers and babies sustain injuries or even die, especially when adequate medical care is not available. It is puzzling why natural selection has not reduced these adverse risks by making the birth canal more spacious. The answer lies in the fact that the human pelvis has to perform multiple different functions and the optimal size and shape for each of those functions differs. A more spacious birth canal is advantageous for birth, whereas smaller pelvic dimensions may be favored by pelvic floor support or bipedal locomotion, among others, and hence these functions impose a trade-off. In such cases, natural selection tends to favor the optimal compromise between function-specific morphologies. However, the underlying functions that have been important in shaping the human bony pelvis are not yet clear. I will analyze the shape of the human pelvis in comparison to that of other mammals. The human and non-human sample varies in adult body size (which has implications for the required size and shape of bones for adequate body support), in the relative size of their neonates (ranging from minute in marsupials to nearly half of maternal body mass as in some bats), mode of locomotion (e.g., quadrupedal running in the cheetah, slow, suspensory climbing in the sloth, and jumping in kangaroos and some rodents), and body posture (upside down as in bats, horizontal/quadrupedal as in ungulates, and upright as in humans). I will quantify how pelvic shape varies in relation to these different functions, thus identifying what pelvic shape features are associated with individual functions and different combinations of functions. By analyzing humans in this comparative context I will also quantify the relative importance of e.g. mode and speed of locomotion, or body posture in explaining human pelvic shape, shedding light on the longstanding question of which functions have been particularly important during human pelvic evolution. Methods include a combination of 3D landmark placement to capture complex pelvic geometry and multivariate statistics. If the pelvic joints connecting the different pelvic bones are flexible, they can mitigate the health risks imposed by a tight fit between the baby and maternal pelvis. Pelvic joint flexibility is highly variable among mammals, so I will also study under what conditions pelvic joints are more or less flexible. Lastly, in species that have relatively large neonates, the female pelvic canal tends to be markedly larger than the males. I will test that this adaptation first evolved in an ancient egg-laying ancestor, by studying male-female pelvic differences in relation to egg size in birds and reptiles.