Variation in Pubic Symphysis Fusion Across Primates: Implications for Obstetric Adaptation.

Area of Science:

• Primate Anatomy
• Evolutionary Biology
• Paleoanthropology

Background:

• The unfused human pubic symphysis is theorized as an obstetric adaptation for large-brained infants and bipedal pelves.
• This study investigates pubic symphysis fusion across primate species to assess its role as an obstetric adaptation.

Purpose of the Study:

• To evaluate the degree of pubic symphysis fusion in adult primates.
• To determine if an open symphysis is an obstetric adaptation in humans and other primates.

Main Methods:

• Assessed symphyseal fusion in 718 individuals from 67 nonhuman primate species.
• Examined fusion variation by age and sex in four species (Galago moholi, Macaca mulatta, Microcebus murinus, Pan troglodytes).
• Conducted logistic regressions for detailed pubic changes in macaques.

Main Results:

• Pubic fusion is common across most primate species.
• Fusion timing varied by sex and species; earlier in males than females in macaques and chimpanzees, only in males in mouse lemurs, and absent in galagos.
• Delayed or absent fusion was more common in females, but not consistently linked to tight cephalopelvic fits.

Conclusions:

• While delayed pubic fusion suggests obstetric adaptation, the link is not definitive across all species.
• Other birthing mechanisms or non-obstetric pressures may influence symphysis fusion.
• The persistent unfused pubic symphysis in humans and some primates is best explained as convergent evolution driven by obstetric selection.