Morphological Integration of the Human Pelvis with Respect to Age and Sex

Abstract

Considerable research has shown that modern human pelvic dimensions, especially of the birth canal, are sexually dimorphic. Studies also suggest that females with younger ages-at-death have narrower canal dimensions than those who die at older ages, possibly due to continued independent growth of the pubis. A recent examination of this pattern argued that it is unlikely that these differences relate to mortality, but the source of the difference in pelvic dimensions with age remains unresolved. We use pelvic dimensions to assess differences in magnitudes of morphological integration between adult females and males across agesat-death. We first ascertain whether the sexes have different strengths of integration, and then assess if differences in magnitudes of integration are associated with age-at-death. Pelvic dimensions of all groups were moderately integrated. Females and males have similar magnitudes of integration, and there is no change in the strength of integration with age. Examining individual regions of the pelvis indicates that the ilium, pubis, and pelvic inlet and outlet have stronger integration than the overall pelvis. This was particularly true of the pelvic outlet, which demonstrated the strongest integration. Our findings suggest that regions of the pelvis are more strongly integrated internally, and less integrated with each other, which would allow for proportional growth among regions of the pelvis with age that do not affect its overall integration. No single region of the pelvis appears to be motivating the difference in pelvic dimensions between age groups. We further consider the implications of these findings on evolutionary constraints. Anat Rec, 300:666-674, 2017. V C 2017 Wiley Periodicals, Inc.

Variation in obstetric dimensions of the human bony pelvis in relation to age‐at‐death and latitude

“…In contrast, Betti et al () and Betti () showed that most os coxae shape variation reflects neutral evolutionary processes, but they did not rearticulate pelves in their sample and so could not examine obstetrical dimensions. These two sets of evolutionary model results are not necessarily in conflict, as most true pelvis dimensions are independent from BIB (Grabowski et al, ; Mallard et al, ). Larger BIBs do not guarantee that we would find most dimensions of the true pelvis larger in high latitude groups.…”

“…Studies of integration in the human pelvis (Grabowski, ; Grabowski & Roseman, ; Mallard et al, ) have shown that the dimensions of the three bones that comprise the os coxa are less independent than the os coxa as a whole; that is, the ischium, pubis, and ilium are more integrated individually than they are with each other. We may therefore conclude that the changes that occur between younger and older females in our sample are likely changes to the entire pelvis, and not necessarily driven by one region of the pelvis (cf.…”

“…This may be reflected in our data, which show that all pelvic dimensions (except sacral length and IBL in females) are larger in the high latitude groups than mid‐latitude groups. Natural selection may have driven BIB to become narrower in the mid‐latitude groups (as these groups likely descended from wider‐bodied ancestral populations), but the independence of bi‐iliac breath and true pelvis dimensions (indicated in Mallard et al, ) would have allowed for the latitudinal differences in true pelvis dimensions to arise in response to a mixture of selection and neutral evolutionary forces. Natural selection is not likely driving morphological differences occurring with age in females (age‐related metric distinctions that do not occur in males).…”

“…Sexual dimorphism of the human bony pelvis has long been the focus of intense investigation, reflecting the functional importance of this complex skeletal structure (Berge, Orban‐Segebarth, & Schmid, ; Betti, ; Brown, ; Caldwell & Moloy, ; Krogman, ; Kurki, ; Laitman & Albertine, ; Lewton, ; Lovejoy, ; Mallard, Savell, & Auerbach, ; Rosenberg, ; Ruff, ; Schaffer & Thane, ; Tague, ; Tague, ; Tague & Lovejoy, ; Thoms & Greulich, ; Washburn, ). Central to these studies have been differences in the obstetric canal (the true pelvis), which exist because of demands placed on the female pelvis for childbirth.…”

“…This relationship may relate to the cylindrical model (Ruff, ) for surface area‐to‐mass ratios, and thus thermoregulation. Analyses of morphological integration in the pelvis (Grabowski, ; Mallard et al, ) indicate independence of the dimensions of the iliac blades and the obstetric canal, and higher constraints in some of the dimensions of the latter (such as ischial ramus length). Nonetheless, variation in overall body size, especially related to BIB, may drive differences in the size and shape of the entire pelvis.…”