Morphological integration and changes in pelvic dimensions with age in adult female humans

Abstract

This study examines morphological integration (MI) in the human pelvis with respect to sex and age. Anatomists and biological anthropologists have often cited the flair of the iliac blades, sub‐pubic angle, and breadth between the ischia among the morphologies that distinguish females and males, though the magnitude of difference between the sexes varies among groups. Despite these differences, it is assumed that patterns of MI within the human pelvis are the same between the sexes; the few studies of evolvability and integration in the pelvis to date remove effects of sexual dimorphism on the variance‐covariance matrix. In addition, the age of adult individuals is never considered a factor in quantitative genetic studies of the evolution of the pelvis, as the pelvis is thought to reach its definitive morphology with the cessation of primary growth. However, studies by Tague (1994) and Auerbach et al. (in review) show that older adult females have mediolaterally wider pelvic outlets (MLPO) and anteroposteriorly deeper pelvic inlets (APPI), while no age‐related differences in pelvic dimensions are observed among males. Though covariation within the pelvis should be similar between the sexes and among age groups within the sexes, no assessment of overall integration within the pelvis has been compared in light of these factors.In this study, 18 linear dimensions were measured from the rearticulated pelves of 327 adult human skeletons (188 females, 139 males) recovered from archaeological sites in North America dating from the last millennium. Fusion of the iliac crest was the criterion for inclusion. We aged the skeletons based on diagnostic changes in pelvic articular morphology, and placed the skeletons into two groups: “Young” (<~25 years) and “Not Young” (>~25 years). MI was measured by comparing relative eigenvalue variances between sex and age‐and‐sex groups, following Pavličev et al. (2009. Evol Biol 36:157–170), as well as by comparing mean‐scaled covariance matrices of the traits within each group. Relative eigenvalue variances have a scale of zero to one and higher values indicate more integration.Results indicate that MI differs notably between age groups within sex, but not between the sexes. Both females and males have similar relative eigenvalues (females = 0.162; males = 0.172). The slightly lower value for females may be driven by the influence of the Young female group, which has a relative eigenvalue variance of 0.139. In contrast, the Not Young female group has relative eigenvalue variance of 0.202. This is a stark difference, and indicates that younger females have less integration than older females. Examination of the covariance matrices indicates lower covariances among most traits, but especially between MLPO and APPI. This indicates that covariation of traits within the pelvis continue to change in adults, and older females have more integrated pelves. Further developmental and evolutionary implications are considered.Support or Funding InformationNational Science Foundation, BCS Grant #0962752