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It seems puzzling why humans have evolved such a small and rigid birth canal that entails a relatively complex process of labor compared with the birth canal of our closest relatives, the great apes. This study reviewed insights into the evolution of the human birth canal from recent theoretical and empirical studies and discussed connections to obstetrics, gynecology, and orthopedics. Originating from the evolution of bipedality and the large human brain million years ago, the evolution of the human birth canal has been characterized by complex trade-off dynamics among multiple biological, environmental, and sociocultural factors. The long-held notion that a wider pelvis has not evolved because it would be disadvantageous for bipedal locomotion has not yet been empirically verified. However, recent clinical and biomechanical studies suggest that a larger birth canal would compromise pelvic floor stability and increase the risk of incontinence and pelvic organ prolapse. Several mammals have neonates that are equally large or even larger than human neonates compared to the size of the maternal birth canal. In these species, the pubic symphysis opens widely to allow successful delivery. Biomechanical and developmental constraints imposed by bipedality have hindered this evolutionary solution in humans and led to the comparatively rigid pelvic girdle in pregnant women. Mathematical models have shown why the evolutionary compromise to these antagonistic selective factors inevitably involves a certain rate of fetopelvic disproportion. In addition, these models predict that cesarean deliveries have disrupted the evolutionary equilibrium and led to new and ongoing evolutionary changes. Different forms of assisted birth have existed since the stone age and have become an integral part of human reproduction. Paradoxically, by buffering selection, they may also have hindered the evolution of a larger birth canal. Many of the biological, environmental, and sociocultural factors that have influenced the evolution of the human birth canal vary globally and are subject to ongoing transitions. These differences may have contributed to the global variation in the form of the birth canal and the difficulty of labor, and they likely continue to change human reproductive anatomy. [ABSTRACT FROM AUTHOR]

Based on a longitudinal study of radiographs of the Denver Growth Study, we investigated the morphological development of individual and gender differences in the anterior neurocranium, face, and basicranium. In total, 500 X-rays of 14 males and 14 females, each with 18 landmarks and semilandmarks, were digitized and analyzed using geometric morphometric methods. Sexual dimorphism in shape and form is already present at the earliest age stage included in the analysis. However, the nature of dimorphism changes with age. Four factors apper to contribute to cranial sexual dimorphism in human postnatal development: 1) initial, possibly prenatal, differences in shape; 2) differences in the association of size and shape; 3) male hypermorphosis; and 4) some degree of difference in the direction of male and female growth trajectories. Studying changes in individuals, we find a low correlation between newborn and adult morphology, while 3-year-olds already show a high correlation with their adult form. We conclude that the adult pattern of interindividual difference in facial form in a single human population is established within the first few years of life. KEY WORDS facial growth; longitudinal; geometric morphometrics; frontal bone; sexual dimorphism; Denver Growth Study

Without cesarean delivery, obstructed labor can result in maternal and fetal injuries or even death given a disproportion in size between the fetus and the maternal birth canal. The precise frequency of obstructed labor is difficult to estimate because of the widespread use of cesarean delivery for indications other than proven cephalopelvic disproportion, but it has been estimated that at least 1 million mothers per year are affected by this disorder worldwide. Why is the fit between the fetus and the maternal pelvis so tight? Why did evolution not lead to a greater safety margin, as in other primates? Here we review current research and suggest new hypotheses on the evolution of human childbirth and pelvic morphology. In 1960, Washburn suggested that this obstetrical dilemma arose because the human pelvis is an evolutionary compromise between two functions, bipedal gait and childbirth. However, recent biomechanical and kinematic studies indicate that pelvic width does not considerably affect the efficiency of bipedal gait and thus is unlikely to have constrained the evolution of a wider birth canal. Instead, bipedalism may have primarily constrained the flexibility of the pubic symphysis during pregnancy, which opens much wider in most mammals with large fetuses than in humans. We argue that the birth canal is mainly constrained by the trade-off between 2 pregnancy-related functions: while a narrow pelvis is disadvantageous for childbirth, it offers better support for the weight exerted by the viscera and the large human fetus during the long gestation period. We discuss the implications of this hypothesis for understanding pelvic floor dysfunction. Furthermore, we propose that selection for a narrow pelvis has also acted in males because of the role of pelvic floor musculature in erectile function. Finally, we review the cliff-edge model of obstetric selection to explain why evolution cannot completely eliminate cephalopelvic disproportion. This model also predicts that the regular application of life-saving cesarean delivery has evolutionarily increased rates of cephalopelvic disproportion already. We address how evolutionary models contribute to understanding and decision making in obstetrics and gynecology as well as in devising health care policies. [ABSTRACT FROM AUTHOR]

Skull Reference Models (SRM's), which incorporate empirical sample distributions of shape in a low-dimensional morphospace, can be created from landmark data for modern taxa, fossil taxa, or the combination. They contain information both about the morphological average (the 'virtual morphotype') and about withinand between taxon covariance structures. Developmental and evolutionary constraints such as ontogenetic allometry, phylogenetic allometry, heterochrony, or sexual dimorphism can be seen in these spaces as general factors shaping these covariance structures, while such considerations as locomotion, diet, behavior, or relative brain size may be treated as correlated causes or effects of skull form. In this statistical context, SRM's derived from landmark configurations are useful for ecophenotypic analysis of skull shape variation, classification of new specimens, completion of incomplete forms, and relating variation in extinct taxa to variation in extant ones. In this paper, a preliminary application of SRM's to real fossil data, we show that the estimated ontogenetic shape trajectory for the data set combining Sts 5, Sts 71, and the Taung child lies roughly parallel to those of the other Hominoidea, particularly Pan. By exploiting this observation we can present three-dimensional visualizations of further intermediate stages in this reconstructed ontogeny of A. africanus as well as the morphological average derived from these significant specimens.

Facial and body attractiveness in humans are proposed to be evidence of developmental and hormonal health. Attractiveness ratings by males of female faces and their nude bodies correlate positively, which underpins the notion that face and body comprise one single ornament and refute objections that facial attractiveness in women is instead a deceptive signal such as youth. Two questions arise: (1) which physical features drive these judgements, and (2) can these features be associated with aspects of the women's actual developmental and hormonal status? As image data we use photographs of 100 young American women in three standardized views: faces only, nude fronts (faces covered) and nude backs. Each image was rated for attractiveness by 60 males. Our morphometric data consist of 50 cranial and 50 postcranial soft tissue landmarks, digitized as 2D-coordinates. We analyze the three image sets separately, using relative warps analysis, shape regression (upon rated attractiveness) and Procrustes symmetry analysis. We then apply singular warps analysis to consider explicitly the interrelations among the three different parts of the same body. In all three views, we find the attractiveness ratings significantly associated with both (1) specific localized shape differences in regions of known estrogen sensitivity, and (2) the amount of fluctuating asymmetry. Interestingly, the grids of the first singular warp (all three sets) seem to concur with the shape changes that predict attractiveness. These results support the notion that the psychology by which women's physical attractiveness is rated may exploit indicators of their hormonal and developmental status.

We collected a total of 96 traditional landmarks and semilandmarks on the face and cranial base of 268 adult and sub-adult crania for a geometric morphometric analysis of five different hominoid species--Homo sapiens, Pan paniscus, Pan troglodytes, Gorilla gorilla and Pongo pygmaeus. The standard relative warp (RW) morphospace was sheared so that pooled within-group size allometry, the axis assumed to express the 'common' ontogenetic shape change, lies horizontally. In the resulting analysis there are differences in size-adjusted mean forms--these appear in sheared RW's 2,3 ...--and there are also differences in the within-taxon ontogenetic trajectories (which appear as within-group correlations of RW's that must, per definition, be uncorrelated in the pool). When visualized, both correspond to regionalized shape differences. The shape differences associated with the second sheared RW, which separates Homo from Pongo and the African apes, are already manifest in the youngest forms we have. These ape ontogenies seem parallel, but the vector for H. sapiens is different (P=0.002). Shape features of the third component, which distinguishes among the African apes, develop during postnatal ontogeny, and thus express actual divergence of ontogenies. Assuming that Hominoidea is monophyletic, the average growth trajectory is a reasonable estimate of the ancestral ontogeny. This trajectory is closest to those observed in our two samples of chimps, particularly to that for bonobo. In the second and third dimensions, furthermore, the bonobos lie nearly at the grand mean. We suggest therefore that bonobo is a particularly good model species for speculations about hominoid ancestral ontogeny. Research supported by the Austrian Science Foundation Project P14738 and a Ph.D. grant of the University of Vienna.

We present statistical and geometrical techniques to reconstruct incomplete crania and exemplify our approach by two fossil specimens, Bodo and Kabwe. Our techniques formalize the biologist's prior understanding of the considerations that govern form: considerations like continuity, symmetry or integration. The modern morphometrics of landmarks and curves makes it possible to blend statistical and biological reasoning in this domain. Factors such as size allometry or sex and also directional asymmetry whether zero or nonzero can be explicitly incorporated into the data estimation by way of the corresponding covariance structures. For tasks of estimation based on very small samples we show two variants based on the continuity assumption of the thin-plate spline: one for landmarks, one for semilandmarks. When complete specimens are adequate in number our estimation can be regression-driven instead. All the missing points can be estimated at once by maximizing the likelihood of the resulting configuration in a reduced-rank model of a multivariate Gaussian distribution. Whatever integration the form possesses is automatically exploited in the course of these regressions. We demonstrate the accuracy of these approaches using a dataset of 388 anatomical landmarks and semilandmarks on 52 complete H. sapiens crania. After deliberately deleting regions of landmarks we estimate the missing data and compare the estimated specimen to the original. As our results indicate that the accuracy of estimation is sufficiently close to the precision of measurement, we apply our algorithm to complete the incomplete Middle-Pleistocene Homo specimens. Research supported by the Austrian Science Foundation Project P14738.

Hominoid social groups vary in size, in composition (females versus males) and in mating and residence patterns. Comparative analyses demonstrate that the extent of sexual dimorphism in primates corresponds somewhat to the structure of these groups. To explore this relationship in more detail, we investigate cranial sexual dimorphism in five hominoid species' Homo sapiens, Pan paniscus, Pan troglodytes, Gorilla gorilla and Pongo pygmaeus. 3D-coordinates of 35 traditional landmarks and 61 semilandmarks are measured for each of the 268 adult and sub-adult specimens and analyzed using geometric morphometric methods. There is an association between the amount of size dimorphism and one aspect of the mating system, namely, the ratio of males to females per group. Furthermore, we find an association between a quite different measure of intrasexual competition among males (sperm competition) and another measure of dimorphism specific to shape: the divergence angle between within-sex growth allometry and between-sex mean shape difference. We interpret these findings by considering various male mating strategies and inter-specific differences in the actual pattern of growth allometry. This research is supported by a PhD grant from the University of Vienna; by the Austrian Council for Science and Technology 200.049/3--VI/I/2001; and by the Austrian Science Foundation P14738.

Allopatric speciation often yields ecologically equivalent sister species, so that their secondary admixis enforces competition. The shores of Lake Tanganyika harbor about 120 distinct populations of the cichlid genus Tropheus, but only some are sympatric. When alone, Tropheus occupies a relatively broad depth zone, but in sympatry, fish segregate by depth. To assess the effects of competition, we studied the partial co-occurrence of Tropheus moorii 'Kaiser' and 'Kirschfleck' with Tropheus polli. A previous study demonstrated via standardized breeding experiments that some observed differences between Tropheus 'Kaiser' living alone and in sympatry with T. polli have a genetic basis despite large-scale phenotypic plasticity. Using geometric morphometrics and neutral genetic markers, we now investigated whether sympatric populations differ consistently in body shape from populations living alone and if the differences are adaptive. We found significant differences in mean shape between non-sympatric and sympatric populations, whereas all sympatric populations of both color morphs clustered together in shape space. Sympatric populations had a relatively smaller head, smaller eyes and a more anterior insertion of the pectoral fin than non-sympatric populations. Genetically, however, non-sympatric and sympatric 'Kaiser' populations clustered together to the exclusion of 'Kirschfleck'. Genetic distances, but not morphological distances, were correlated with geographic distances. Within- and between-population covariance matrices for T. moorii populations deviated from proportionality. It is thus likely that natural selection acts on both phenotypic plasticity and heritable traits and that both factors contribute to the observed shape differences. The consistency of the pattern in five populations suggests ecological character displacement. [ABSTRACT FROM AUTHOR]

Copyright of Collegium Antropologicum is the property of Croatian Anthropological Society and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Summary: Craniofacial sexual dimorphism in primates varies in both magnitude and pattern among species. In the past two decades, there has been an increasing emphasis in exploring the correlations of these patterns with taxonomy and the variation in patterns within and among the craniofacial regions. Scrutinising these relationships for hominids, we decompose the craniofacial morphology in five taxa: Homo sapiens, Pan paniscus, Pan troglodytes, Gorilla gorilla and Pongo pygmaeus. 3D coordinates of 35 traditional landmarks and 61 semilandmarks, covering five ridge curves, are measured for each of 268 adult and sub-adult specimens and analysed using geometric morphometric methods. A multivariate analysis in size-shape space shows that ontogenetic scaling contributes to the development of sexual dimorphism in all five taxa, but to a varying extent. In absolute as well as in relative terms P. pygmaeus shows the greatest allometric component, followed by G. gorilla. Homo is intermediate, while in Pan the non-allometric constituent part contributes a large fraction to the actual sexual dimorphism, most markedly in the pygmy chimpanzee. An eigendecomposition of the five vectors of sexual dimorphism reveals two dimensions independent of allometry. One separates orang-utan sexual dimorphism from the African apes and Homo, and the other differentiates between the great apes and Homo with Pan mediating. We discuss these patterns and speculate on their use as characters for taxonomic analysis in the fossil record. [Copyright &y& Elsevier]

Copyright of Collegium Antropologicum is the property of Croatian Anthropological Society and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)