This research examines the association of pelvic dimensions and natural selection as it relates to both age-at-death in females and climate among past indigenous North American groups. The investigators predict that narrow pelvic inlets in females, a measure of obstetric efficiency (the ability to give birth without complications) will have caused higher death rates in young aged individuals. In addition, previous studies have linked pelvic shape, especially hip breadth, to body heat regulation. Individuals with wider bodies have less surface area relative to body mass, and therefore are more efficient at retaining body heat. Indigenous North Americans have been shown to possess wider bodies than would be expected for the latitudes in which they lived. Thus, their body shape may be related to common ancestral occupation of colder climates, a trait then shared by descendent populations. Although climate may have selected for narrower bodies in warmer climates, obstetric efficiency may have attenuated this pressure. Although multiple factors are related to the age at which individuals died, a correlation between young aged mean mortality and narrow dimensions in the pelvic outlet would support pelvic shape and obstetrical complications as a selective factor among females. Furthermore, if the shape of the pelvic outlet covaries with overall hip breadth, this would indicate that pelvic shape is indeed a balance between at least two selective pressures: climate and obstetrics.
Broadly, this project contributes to the understanding of biological changes encountered by migrating humans and factors influencing those changes. It contributes further to an understanding of body shape variation among humans. The study will incorporate pre-doctoral students in data collection at major skeletal repositories and encourages students toward the development of further studies. Furthermore, new methods for capturing and comparing shape data, developed in collaboration with a postdoctoral researcher at the Max Planck Institute in Leipzig, will be created and shared with other researchers.
This project examines the shape of the pelvis (hip bones)—especially of the pelvic outlet (the bony enclosure for the birth canal)—among peoples who lived in North America prior to European colonization. Pelvic shape has been thought to reflect a balance between potential environmental selection pressures. The shape of the pelvis relates to obstetric efficiency (the ability to give birth without complications). In addition, previous studies have linked pelvic shape to heat regulation in the body: wider hips, reflecting overall wider bodies, are associated with human populations living in colder climates. This is because, for the same height, wider individuals will have less surface area relative to body mass, and therefore are more efficient at retaining body heat. The project assesses the effects these potential factors have on variation in human pelvic shape. Indigenous humans from various latitudes across North America are specifically targeted for study in this study. Previous studies have shown them to collectively possess wider bodies than would be expected for the latitudes in which they lived, especially when compared with populations living in similar climates in Africa and Europe. This body shape may be related to common ancestral occupation of colder climates, a trait shared by descendent populations. Although climate may have selected for narrower bodies in warmer climates, obstetric efficiency may have attenuated this pressure. Thus, this study examined two hypotheses. First, the wider bi-iliac (maximum pelvic breadth) dimensions observed in peoples from North America are associated with overall larger birth canals, and thus reflect selection against a decrease in the overall size of the pelvis after humans colonized the Americas. Second, this selection pressure will be observed in the differential age-at-death for females; females with smaller obstetric dimensions will die at younger ages, and will also have narrower body breadths. We therefore investigated the association of pelvic dimensions with the age-at-death among past groups. Although multiple factors are related to the age at which individuals die, a correlation between young mean mortality and narrow dimensions in the pelvic outlet would support pelvic shape as a selection factor. Furthermore, if the shape of the pelvic outlet covaries with overall hip breadth, this would indicate that pelvic shape is indeed a balance between at least two environmental factors: climate and obstetrics. The results of this research has indicated that females dying at younger ages (<25 years) have significantly different pelvic morphologies from females dying at older ages; males do not have significant differences between older and younger individuals. Specifically, younger females have narrower pelvic inlets in the mediolateral plane, and narrower pelvic outlets in the anteroposterior plane. These two dimensions are crucial to provide obstetric sufficiency in childbirth. In addition, these dimensions have been shown to be the subject of evolutionary changes in pelvic shape that occurred between Australopithecines and the Homo lineage. This pattern occurs regardless of climate; other changes in pelvic shape correlated with climatic factors do not correspond with these two dimensions. Ultimate causes, and whether the differences between older and younger females are due to continued growth in the pelvis, cannot be determined at this time. Yet these findings suggest that the obstetric dimensions of the pelvis are independent of the dimensions associated with body breadth and, thus, the regulation of body heat. In addition to these scientific findings, the grant was successful in meeting the broader impact goals. The researchers successfully sampled indigenous North American collections from three institutions, representing a dozen archaeological sites dating from throughout the last millennium. At least three of these sites will be repatriated and returned to living native communities in the next two to three years, and so this study helped preserve additional data from those sites while research is still possible. In addition, two graduate students successfully trained in data collection and made significant contributions to the project with measurements they obtained. Their experience has directly led to their successful completion of doctoral qualifying exams and the development of their doctoral dissertation projects, which will involve extensive data collection from indigenous North American skeletons. Finally, the project was successful in developing a new method for obtaining metric data from skeletons, and the research team is already developing new applications for these methods to other aspects of the skeleton.
