Humans as inverted bats: A comparative approach to the obstetric conundrum

Abstract

Objectives The narrow human birth canal evolved in response to multiple opposing selective forces on the pelvis. These factors cannot be sufficiently disentangled in humans because of the limited range of relevant variation. Here, we outline a comparative strategy to study the evolution of human childbirth and to test existing hypotheses in primates and other mammals. Methods We combined a literature review with comparative analyses of neonatal and female body and brain mass, using three existing datasets. We also present images of bony pelves of a diverse sample of taxa. Results Bats, certain non‐human primates, seals, and most ungulates, including whales, have much larger relative neonatal masses than humans, and they all differ in their anatomical adaptations for childbirth. Bats, as a group, are particularly interesting in this context as they give birth to the relatively largest neonates, and their pelvis is highly dimorphic: Whereas males have a fused symphysis, a ligament bridges a large pubic gap in females. The resulting strong demands on the widened and vulnerable pelvic floor likely are relaxed by roosting head‐down. Conclusions Parturition has constituted a strong selective force in many non‐human placentals. We illustrated how the demands on pelvic morphology resulting from locomotion, pelvic floor stability, childbirth, and perhaps also erectile function in males have been traded off differently in mammals, depending on their locomotion and environment. Exploiting the power of a comparative approach, we present new hypotheses and research directions for resolving the obstetric conundrum in humans.

Evolution of the human pelvis and obstructed labor: new explanations of an old obstetrical dilemma

“…However, in several other mammals with relatively large neonates, flexibility of the pubic symphysis is much larger than in humans. 47 In guinea pigs, for instance, the mean diameter of the fetal head is 20 mm, whereas the pelvic canal in early pregnancy is just 11 mm wide. To accommodate the fetal head, the pubic bones separate up to 23 mm during late pregnancy in response to estrogen and relaxin, and a ligament appears in the middle of the joint.…”

“…53,54 In other mammals, by contrast (eg, in many odd-and even-toed ungulates and in big cats), the pubic symphysis is completely fused by ossification. 47,49 Why is the flexibility of the pubic symphysis so variable across mammals, and why is it so inflexible in humans, thus contributive to difficulty in childbirth? A widened pubic symphysis increases the birth canal and thus the vulnerability of the pelvic floor.…”

“…An ossified symphysis, by contrast, allows for a higher net force applied by the muscles to the rest of the body and thus may facilitate more energetically efficient locomotion. 47 Apart from many fastrunning species, pubic flexibility is also strongly reduced in other large-bodied bipedal species, such as kangaroos.…”

Variation in mouse pelvic morphology maps to locations enriched in Sox9 Class II and Pitx1 regulatory features

“…The pelvic girdle forms the anatomical interface between the hind limb and the axial skeleton and serves important roles in bearing the weight of and propelling an organism through its environment. In mammals, the pelvis can show strong sexual dimorphism including in aspects of the morphology of the birth canal, which plays important roles in parturition (Grunstra et al, 2019; McPherson & Chenoweth, 2012). Quantitative genetic investigations into pelvic morphology across several species (Carrier, Chase, & Lark, 2005; Chase et al, 2002; Chase, Carrier, Adler, Ostrander, & Lark, 2005; Kenney‐Hunt et al, 2008; Kohn & Atchley, 1988) have provided insight into the genetic architecture of variation in this complex skeletal element.…”

“…The pelvic girdle forms the anatomical interface between the hind limb and the axial skeleton and serves important roles in bearing the weight of and propelling an organism through its environment. In mammals, the pelvis can show strong sexual dimorphism including in aspects of the morphology of the birth canal, which plays important roles in parturition (Grunstra et al, 2019;McPherson & Chenoweth, 2012).…”