Age dependent changes in pelvic shape during adulthood.

The human pelvis has been reported to change in shape throughout adult lifetime, and also the expression of parturition scars, or "pelvic features", increases with age. However, little is known about the causes and timing of these changes. Here we investigate changes in pelvic shape and the expression of pelvic features by applying a comprehensive geometric morphometric approach to a modern sample of 167 adult individuals with known age, parity, and cause of death. Our results confirm that the pelvis changes in shape during adult life, but to a larger magnitude in females compared to males. Moreover, females showed three different phases of pelvic shape change, coinciding with the main period of reproduction (from 17 to about 37-40 years), the period after that but before menopause, and the postmenopausal period (after 50 years of age). Males exhibited two phases with relatively similar shape changes. The expression of parturition scars increased in females until about 40 years of age and stayed relatively constant thereafter. Only a very weak increase of feature expression was found in males. We hypothesize that changes of adult pelvic shape and feature expression result from a combination of hormone-mediated and mechanically induced bone remodeling. Estrogen-induced and pregnancy-related bone remodeling dominates in premenopausal women, whereas bone remodeling in response to mechanical factors may underlie pelvic shape changes in men and postmenopausal women. The continual widening of the birth canal during the reproductive period eases childbirth in a population, but it is unlikely that this remodeling pattern specifically evolved as an obstetric adaptation in the human lineage. The highly conserved endocrine system and estrogen-induced pelvic bone remodeling were already in place when the neonatal brain increased in the human lineage. But the regularity control of this conserved pathway may have been "fine-tuned" by selective forces in the human lineage.