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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.

The aim of this study was to compare mandibular form (i.e., size and shape) between patients with agenesis of the lower second premolar (P2) and a control group with no agenesis. Three hypotheses were tested: (H1) agenesis causes a change in mandibular morphology because of inadequate alveolar ridge development in the area of the missing tooth (mandibular plasticity); (H2) agenesis is caused by spatial limitations within the mandible (dental plasticity); and (H3) common genetic/epigenetic factors cause agenesis and affect mandibular form (pleiotropy). A geometric morphometric analysis was applied to cross-sectional images of computed tomography (CT) scans of three matched groups (n=50 each): (1) regularly erupted P2; (2) agenesis of P2 and the primary second molar in situ; and (3) agenesis of P2 and the primary second molar missing for >3 months. Cross-sections of the three areas of interest (first premolar, P2, first molar) were digitized with 23 landmarks and superimposed by a generalized Procrustes analysis. On average, the mandibular cross-sections were narrower and shorter in patients with P2 agenesis compared with that in the control group. Both agenesis groups featured a pronounced submandibular fossa. These differences extended at least one tooth beyond the agenesis-affected region. Taken together with the large interindividual variation that resulted in massively overlapping group distributions, these findings support genetic and/or epigenetic pleiotropy (H3) as the most likely origin of the observed covariation between mandibular form and odontogenesis. Clinically, reduced dimensions and greater variability of mandibular form, as well as a pronounced submandibular fossa, should be expected during the treatment planning of patients with P2 agenesis.

The cervical vertebral column bears or balances the weight of the head supported by the nuchal muscles that partly originate from the cervical vertebrae. The position of the head relative to the vertebral column, and consequently locomotion and posture behavior, could thus be associated with the form of the cervical vertebrae. In spite of this assumption and some empirical indications along these lines, primate vertebral morphologies have been reported to be very similar and not clearly related to locomotion. We therefore study the relationship between the morphology of the first cervical vertebra, the atlas, and the locomotion pattern within primates using a geometric morphometric approach. Our analysis is based on a total of 116 vertebrae of adult Homo sapiens, Gorilla gorilla, Pan troglodytes, Pongo pygmaeus, Hylobates lar, Macaca mulatta, Papio hamadryas, Ateles geoffroyi, and Alouatta palliata. On each atlas, 56 landmarks were digitized and superimposed by Procrustes registration. The resulting shape variables were analyzed by principal component analysis, multivariate regression, and partial least‐squares analysis. We found that the nine primate species differ clearly in their atlas morphology and that allometric shape change is distinct between the nonhuman primates and Homo sapiens. We could further identify morphological features that relate to the species' locomotion pattern. Human atlas shape, however, cannot be predicted by an extrapolation of the nonhuman primate model. This implies that either the primate atlas is generalized enough to allow bipedal locomotion or else the human atlas morphology is a unique adaptation different from that in the more orthograde nonhuman primates. Anat Rec (Part B: New Anat) 289B:184–194, 2006. © 2006 Wiley‐Liss, Inc.