Sexual dimorphism in the baboon facial skeleton

Age- and sex-related differences in baboon (Papio anubis) gray matter covariation

Age-related changes in cognition, brain morphology, and behavior are exhibited in several primate species. Baboons, like humans, naturally develop Alzheimer's disease-like pathology and cognitive declines with age and are an underutilized model for studies of aging. To determine age-related differences in gray matter covariation of 89 olive baboons (Papio anubis), we used source-based morphometry (SBM) to analyze data from magnetic resonance images. We hypothesized that we would find significant age effects in one or more SBM components, particularly those which include regions influenced by age in humans and other nonhuman primates (NHPs). A multivariate analysis of variance revealed that individual weighted gray matter covariation scores differed across the age classes. Elderly baboons contributed significantly less to gray matter covariation components including the brainstem, superior parietal cortex, thalamus, and pallidum compared to juveniles, and middle and superior frontal cortex compared to juveniles and young adults (p < 0.05). Future studies should examine the relationship between the changes in gray matter covariation reported here and age-related cognitive decline.

Developmental instability in wild Nigerian olive baboons (Papio anubis)

Background

Developmental instability in archaeological samples can be detected through analysis of skeletal and dental remains. During life, disruptions to biological internal homeostasis that occur during growth and development redirect bodily resources to returning to homeostasis and away from normal processes such as symmetrical development. Because dental enamel does not remodel in life, any deviations from normal development are left behind. Even subtle disturbances to developmental trajectory may be detected in asymmetrical development of traits, specifically a random variation in sides termed fluctuating asymmetry. Human dental fluctuating asymmetry studies are common, but here we investigate the permanent dentition of a non-human primate Papio anubis, for potential fluctuating asymmetry relative to sex, weaning, and reproductive maturity. The sample stems from an outlier population that lives in the wettest and most humid habitat of any studied baboon group.

Methods

The skulls of adult baboons were collected after their natural death in Gashaka Gumti National Park, Nigeria. The permanent dentition of antimeric teeth (paired) were measured for maximum length and breadth using standard methods. The metrics were analyzed to assess the presence of fluctuating asymmetry in adult permanent mandibular and maxillary dentition. Measurement error and other forms of asymmetry (antisymmetry, directional asymmetry) were considered and dental measures expressing true fluctuating asymmetry were used to address three research questions.

Results

Males exhibit greater fluctuating asymmetry than females, suggesting that males experience greater overall instability during the developmental period. While weaning is not more stressful than other life history stages for males and females (using the first molar fluctuating asymmetry index as a proxy compared to other teeth), it is more stressful for females than males. The onset of reproduction is also not more stressful than other life history stages for males and females (using the third molar fluctuating asymmetry index as a proxy compared to other teeth), but it is more stressful for males than females. We explore possible explanations for these findings in the discussion.

How did modern morphology evolve in the human mandible? The relationship between static adult allometry and mandibular variability in Homo sapiens

Key to understanding human origins are early Homo sapiens fossils from Jebel Irhoud, as well as from the early Late Pleistocene sites Tabun, Border Cave, Klasies River Mouth, Skhul, and Qafzeh. While their upper facial shape falls within the recent human range of variation, their mandibles display a mosaic morphology. Here we quantify how mandibular shape covaries with mandible size and how static allometry differs between Neanderthals, early H. sapiens, and modern humans from the Upper Paleolithic/Later Stone Age and Holocene (= later H. sapiens). We use 3D (semi)landmark geometric morphometric methods to visualize allometric trends and to explore how gracilization affects the expression of diagnostic shape features. Early H. sapiens were highly variable in mandible size, exhibiting a unique allometric trajectory that explains aspects of their 'archaic' appearance. At the same time, early H. sapiens share a suite of diagnostic features with later H. sapiens that are not related to mandibular sizes, such as an incipient chin and an anteroposteriorly decreasing corpus height. The mandibular morphology, often referred to as 'modern', can partly be explained by gracilization owing to size reduction. Despite distinct static allometric shape changes in each group studied, bicondylar and bigonial breadth represent important structural constraints for the expression of shape features in most Middle to Late Pleistocene hominin mandibles.

Allometry and advancing age significantly structure craniofacial variation in adult female baboons

Primate craniofacial growth is traditionally assumed to cease upon maturation or at least be negligible, whereas bony remodeling is typically associated with advanced adult age and, in particular, tooth loss. Therefore, size and shape of the craniofacial skeleton of young and middle-aged adults should be stable. However, research on both modern and historic human samples suggests that portions of the CFS exhibit age-related changes in mature individuals, both related to and independent of tooth loss. These results demonstrate that the age-category 'adult' is heterogeneous, containing individuals demonstrating post-maturational age-related variation, but the topic remains understudied outside of humans and in the cranial vault and base. Our research quantifies variation in a sample of captive adult female baboons (n = 97) in an effort to understand how advancing age alters the mature CFS. Craniometric landmarks and sliding semilandmarks were collected from computed tomography (CT) scans of adult baboons aged 7-32 years old. To determine whether craniofacial morphology is sensitive to aging mechanisms and whether any such effects are differentially distributed throughout the cranium, geometric morphometric techniques were employed to compare the shapes of various cranial regions among individuals of increasing age. Unexpectedly, the biggest form differences were observed between young and middle-aged adults, rather than between adults with full dentitions and those with some degree of tooth loss. Shape variation was greatest in masticatory and nuchal musculature attachment areas. Our results indicate that the craniofacial skeleton changes form during adulthood in baboons, raising interesting questions about the molecular and biological mechanisms governing these changes.

A missing piece of the Papio puzzle: Gorongosa baboon phenostructure and intrageneric relationships

Most authors recognize six baboon species: hamadryas (Papio hamadryas), Guinea (Papio papio), olive (Papio anubis), yellow (Papio cynocephalus), chacma (Papio ursinus), and Kinda (Papio kindae). However, there is still debate regarding the taxonomic status, phylogenetic relationships, and the amount of gene flow occurring between species. Here, we present ongoing research on baboon morphological diversity in Gorongosa National Park (GNP), located in central Mozambique, south of the Zambezi River, at the southern end of the East African Rift System. The park exhibits outstanding ecological diversity and hosts more than 200 baboon troops. Gorongosa National Park baboons have previously been classified as chacma baboons (P. ursinus). In accordance with this, two mtDNA samples from the park have been placed in the same mtDNA clade as the northern chacma baboons. However, GNP baboons exhibit morphological features common in yellow baboons (e.g., yellow fur color), suggesting that parapatric gene flow between chacma and yellow baboons might have occurred in the past or could be ongoing. We investigated the phenostructure of the Gorongosa baboons using two approaches: 1) description of external phenotypic features, such as coloration and body size, and 2) 3D geometric morphometric analysis of 43 craniofacial landmarks on 11 specimens from Gorongosa compared to a pan-African sample of 352 baboons. The results show that Gorongosa baboons exhibit a mosaic of features shared with southern P. cynocephalus and P. ursinus griseipes. The GNP baboon phenotype fits within a geographic clinal pattern of replacing allotaxa. We put forward the hypothesis of either past and/or ongoing hybridization between the gray-footed chacma and southern yellow baboons in Gorongosa or an isolation-by-distance scenario in which the GNP baboons are geographically and morphologically intermediate. These two scenarios are not mutually exclusive. We highlight the potential of baboons as a useful model to understand speciation and hybridization in early human evolution.

Additive genetic variation in the craniofacial skeleton of baboons (genus Papio) and its relationship to body and cranial size

OBJECTIVES

Determining the genetic architecture of quantitative traits and genetic correlations among them is important for understanding morphological evolution patterns. We address two questions regarding papionin evolution: (1) what effect do body and cranial size, age, and sex have on phenotypic (V_P ) and additive genetic (V_A ) variation in baboon crania, and (2) how might additive genetic correlations between craniofacial traits and body mass affect morphological evolution?

MATERIALS AND METHODS

We use a large captive pedigreed baboon sample to estimate quantitative genetic parameters for craniofacial dimensions (EIDs). Our models include nested combinations of the covariates listed above. We also simulate the correlated response of a given EID due to selection on body mass alone.

RESULTS

Covariates account for 1.2-91% of craniofacial V_P . EID V_A decreases across models as more covariates are included. The median genetic correlation estimate between each EID and body mass is 0.33. Analysis of the multivariate response to selection reveals that observed patterns of craniofacial variation in extant baboons cannot be attributed solely to correlated response to selection on body mass, particularly in males.

DISCUSSION

Because a relatively large proportion of EID V_A is shared with body mass variation, different methods of correcting for allometry by statistically controlling for size can alter residual V_P patterns. This may conflate direct selection effects on craniofacial variation with those resulting from a correlated response to body mass selection. This shared genetic variation may partially explain how selection for increased body mass in two different papionin lineages produced remarkably similar craniofacial phenotypes.

Cranial sexual dimorphism in the Kinda baboon (Papio hamadryas kindae)

OBJECTIVES

The smallest extant member of genus Papio, the Kinda baboon exhibits low sexual dimorphism and a distinctive cranial shape. Ontogenetic scaling accounts for most cranial-shape differences within Papio, but studies have shown that the Kinda follows a separate ontogenetic trajectory. If so, its cranial-dimorphism pattern should differ from other subspecies. To evaluate this hypothesis, morphometric analysis was used to investigate cranial dimorphism in Papio.

MATERIALS AND METHODS

Three-dimensional landmarks were digitized on 434 adult crania representing six Papio subspecies. Size- and shape-dimorphism magnitudes were quantified using centroid size and Procrustes distances. Patterns of sex- and size-related variation were explored using MAN(C)OVA, multivariate regression, and form-space PCA. Canine dimorphism was investigated using dental metrics.

RESULTS

Kinda size and shape dimorphism are significantly lower than in other Papio subspecies. The relative magnitude of Kinda shape dimorphism is similar to other southern baboons; Kinda canine dimorphism is unremarkable. MAN(C)OVA results support subspecies differences in cranial dimorphism and scaling. Allometric and dimorphism vectors differ significantly in some subspecies, and their vector-angle matrices are strongly correlated. The Kinda's allometric vector angles are divergent. Form-space PC3, summarizing size-independent dimorphism, separates the Kinda from other subspecies.

DISCUSSION

The Kinda baboon exhibits significantly lower size and shape dimorphism than other baboons, but its relative dimorphism levels are unexceptional. The Kinda differs from other subspecies in patterns of allometry, size-related shape dimorphism, and residual shape dimorphism. Kinda facial shape is "masculinized" relative to size, especially in females, suggesting female sexual selection contributed to the evolution of Kinda dimorphism.

Convergent evolution of sexual dimorphism in skull shape using distinct developmental strategies

Studies integrating evolutionary and developmental analyses of morphological variation are of growing interest to biologists as they promise to shed fresh light on the mechanisms of morphological diversification. Sexually dimorphic traits tend to be incredibly divergent across taxa. Such diversification must arise through evolutionary modifications to sex differences during development. Nevertheless, few studies of dimorphism have attempted to synthesize evolutionary and developmental perspectives. Using geometric morphometric analysis of head shape for 50 Anolis species, we show that two clades have converged on extreme levels of sexual dimorphism through similar, male-specific changes in facial morphology. In both clades, males have evolved highly elongate faces whereas females retain faces of more moderate proportion. This convergence is accomplished using distinct developmental mechanisms; one clade evolved extreme dimorphism through the exaggeration of a widely shared, potentially ancestral, developmental strategy whereas the other clade evolved a novel developmental strategy not observed elsewhere in the genus. Together, our analyses indicate that both shared and derived features of development contribute to macroevolutionary patterns of morphological diversity among Anolis lizards.

Middle Pleistocene human facial morphology in an evolutionary and developmental context

Neanderthals and modern humans exhibit distinct facial architectures. The patterning of facial morphology of their predecessors, the Middle Pleistocene humans, is more mosaic showing a mix of archaic and modern morphologies. Significant changes in facial size and robusticity occurred throughout Pleistocene human evolution, resulting in temporal trends in both facial reduction and enlargement. However, the allometric patterning in facial morphology in archaic humans is not well understood. This study explores temporal trends in facial morphology in order to gain a clearer understanding of the polarity of features, and describes the allometric patterning of facial shape. The modern human sample comprises cross-sectional growth series of four morphologically distinct human populations. The fossil sample covers specimens from the Middle Pleistocene to the Upper Paleolithic. We digitized landmarks and semilandmarks on surface and computed tomography scans and analyzed the Procrustes shape coordinates. Principal component analyses were performed, and Procrustes distances were used to identify phenetic similarities between fossil hominins. In order to explore the influence of size on facial features, allometric trajectories were calculated for fossil and modern human groups, and developmental simulations were performed. We show that facial features can be used to separate Pleistocene humans into temporal clusters. The distinctly modern human pattern of facial morphology is already present around 170 ka. Species- and population-specific facial features develop before two years of age, and several of the large-scale facial differences between Neanderthals and Middle Pleistocene humans are due to scaling along a shared allometric trajectory. These features include aspects of the frontal bone, browridge morphology, nasal aperture size and facial prognathism. Infraorbital surface topography and orientation of the midface in the European Middle Pleistocene hominins is intermediate between the African Middle Pleistocene and Neanderthal condition. This could suggest that the European Middle Pleistocene hominins display incipient Neanderthal features.

Regional, ontogenetic, and sex-related variations in elastic properties of cortical bone in baboon mandibles

Understanding the mechanical features of cortical bone and their changes with growth and adaptation to function plays an important role in our ability to interpret the morphology and evolution of craniofacial skeletons. We assessed the elastic properties of cortical bone of juvenile and adult baboon mandibles using ultrasonic techniques. Results showed that, overall, cortical bone from baboon mandibles could be modeled as an orthotropic elastic solid. There were significant differences in the directions of maximum stiffness, thickness, density, and elastic stiffness among different functional areas, indicating regional adaptations. After maturity, the cortical bone becomes thicker, denser, and stiffer, but less anisotropic. There were differences in elastic properties of the corpus and ramus between male and female mandibles which are not observed in human mandibles. There were correlations between cortical thicknesses and densities, between bone elastic properties and microstructural configuration, and between the directions of maximum stiffness and bone anatomical axes in some areas. The relationships between bone extrinsic and intrinsic properties bring us insights into the integration of form and function in craniofacial skeletons and suggest that we need to consider both macroscopic form, microstructural variation, and the material properties of bone matrix when studying the functional properties and adaptive nature of the craniofacial skeleton in primates. The differences between baboon and human mandibles is at variance to the pattern of differences in crania, suggesting differences in bone adaption to varying skeletal geometries and loading regimes at both phylogenetic and ontogenetic levels.

Genetic and environmental contributions to variation in baboon cranial morphology

The development, function, and integration of morphological characteristics are all hypothesized to influence the utility of traits for phylogenetic reconstruction by affecting the way in which morphological characteristics evolve. We use a baboon model to test the hypotheses about phenotypic and quantitative genetic variation of traits in the cranium that bear on a phenotype's propensity to evolve. We test the hypotheses that: 1) individual traits in different functionally and developmentally defined regions of the cranium are differentially environmentally, genetically, and phenotypically variable; 2) genetic covariance with other traits constrains traits in one region of the cranium more than those in others; 3) and regions of the cranium subject to different levels of mechanical strain differ in the magnitude of variation in individual traits. We find that the levels of environmental and genetic variation in individual traits are randomly distributed across regions of the cranium rather than being structured by developmental origin or degree of exposure to strain. Individual traits in the cranial vault tend to be more constrained by covariance with other traits than those in other regions. Traits in regions subject to high degrees of strain during mastication are not any more variable at any level than other traits. If these results are generalizable to other populations, they indicate that there is no reason to suppose that individual traits from any one part of the cranium are intrinsically less useful for reconstructing patterns of evolution than those from any other part.

Genetic variation in baboon craniofacial sexual dimorphism

Sexual dimorphism is a widespread phenomenon and contributes greatly to intraspecies variation. Despite a long history of active research, the genetic basis of dimorphism for complex traits remains unknown. Understanding the sex-specific differences in genetic architecture for cranial traits in a highly dimorphic species could identify possible mechanisms through which selection acts to produce dimorphism. Using distances calculated from three-dimensional landmark data from CT scans of 402 baboon skulls from a known genealogy, we estimated genetic variance parameters in both sexes to determine the presence of gene-by-sex (G x S) interactions and X-linked heritability. We hypothesize that traits exhibiting the greatest degree of sexual dimorphism (facial traits in baboons) will demonstrate either stronger G x S interactions or X-linked effects. We found G x S interactions and X-linked effects for a few measures that span the areas connecting the face to the neurocranium but for no traits restricted to the face. This finding suggests that facial traits will have a limited response to selection for further evolution of dimorphism in this population. We discuss the implications of our results with respect to the origins of cranial sexual dimorphism in this baboon sample, and how the genetic architecture of these traits affects their potential for future evolution.

Ontogeny and diachronic changes in sexual dimorphism in the craniofacial skeleton of rhesus macaques from Cayo Santiago, Puerto Rico

Insight into the ontogeny of sexual dimorphism is important to our understanding of life history, ecology, and evolution in primates. This study applied a three-dimensional method, Euclidean Distance Matrix Analysis, to investigate sexual dimorphism and its diachronic changes in rhesus macaque (Macaca mulatta) skulls. Twenty-one landmarks in four functional areas of the craniofacial skeleton were digitized from macaques of known age and sex from the Cayo Santiago collections. Then, a series of mean form matrices, form difference matrices, and growth matrices were computed to demonstrate growth curves, rates and duration of growth, and sexual dimorphism within the neurocranium, basicranium, palate, and face. The inclusion of fully adult animals revealed a full profile of sexual dimorphism. Additionally, we demonstrate for the first time diachronic change in adult sexual dimorphism caused by extended growth in adult females. A quicker growth rate in males from ages 2 to 8 was offset by a longer duration of growth in adult females that resulted in diminished dimorphism between the ages of 8 and 15. Four functional areas showed different sex-specific growth patterns, and the rate and duration of growth in the anterior facial skeleton contributed most to the changing profiles of sexual dimorphism. The late maturation in size of the female facial skeleton corresponds to later and less complete fusion of facial sutures. The prolongation of growth in females is hypothesized to be an evolutionary response to high levels of intrasexual competition, as is found in other primate species such as common chimpanzees with similar colony structure and reproductive behavior. Further investigation is required to determine (1) if this phenomenon observed in craniofacial skeletons is linked to sexual dimorphism in body size, and (2) whether this diachronic change in sexual dimorphism is species specific. The changing profile of sexual dimorphism in adult rhesus macaques suggests caution in studying sexual dimorphism in fossil primate and human forms.

The use of cranial variables for the estimation of body mass in fossil hominins

Estimating body mass/size/weight remains a crucial precursor to the evaluation of relative brain size and to achieving an understanding of brain evolution in fossil species. Despite the obvious close association between the metrics of postcranial elements and body mass a number of factors combine to reduce their utility. This study examines the feasibility of cranial variables for predicting body mass. The use of traditional regression procedures, independent contrasts analysis, and variance partitioning all support the hypothesis that cranial variables are correlated with body mass even when taking phylogeny into account, with r values typically ranging between 0.52 and 0.98. Body mass estimates derived for fossil hominins using cranial variables are similar to those obtained from previous studies using either cranial or postcranial elements. In particular, upper facial breadth and orbital height display strong predictive capability. Average body masses derived from Least Squares Regression (LSR) equations were used to calculate estimates of body mass for three hominin species. This resulted in estimates of between 30 kg and 47 kg for Australopithecus africanus, 48 kg and 52 kg for Paranthropus robustus, and 75 kg for Homo neanderthalensis. It is proposed that regression equations derived for the order primates are used to estimate body mass for archaic hominins, while hominoid based equations are most suited for Homo.

Cranial ontogeny ofPapio baboons (Papio hamadryas)

Cranial form in subspecies of Papio baboons (Papio hamadryas) varies in relation to size, geography, and sex. However, knowledge about this variation is based mainly on adults, precluding direct assessments of the evolutionary factors that are ultimately responsible for adult shape variation. Consequently, this study tests hypotheses about the development of size and shape differences among subspecies of Papio baboons, anticipating limited evolutionary divergences in the ontogenetic pathways leading to adult endpoints. Geometric morphometric and bivariate allometric analyses are used to explore developmental size and shape variation. Allometric scaling in adult Papio baboons occurs because both sexes and all subspecies follow similar developmental pathways to a variety of adult forms. However, complex allometry contributes to form differences, producing potentially important shape differences that emerge during development. Modest shape differences that are statistically independent of size distinguish chacma baboons (P. h. ursinus) from other forms. A small-headed subspecies, the Kinda baboon (P. h. kindae), also presents a distinctive ontogeny, and may provide insights into the evolution of size change in this species. Variation among subspecies that is statistically independent of size involves the rostrum, zygomatic breadths, and cranial flexion. These features may be related to diet, but the precise biomechanical correlates of baboon form variation remain unclear.

Ontogeny of craniofacial sexual dimorphism in the orangutan (Pongo pygmaeus). I: Face and palate

The orangutan is widely recognized as a highly dimorphic species. An ontogenetic approach to the study of sexual dimorphism can assist researchers in understanding both where and when these differences develop. In this study, 357 orangutans from Borneo were divided into five developmental stages representing infancy to mature adulthood. Three-dimensional (3D) coordinate data from 16 landmarks representing the face and palate were analyzed by means of a Euclidean distance matrix analysis (EDMA), a quantitative method for the comparison of forms. Three separate analyses (an age-specific static comparison of forms, a sex-specific analysis of growth trajectories, and an intersex comparison of patterns of relative growth) were carried out with the intent to describe the rate, timing, magnitude, and pattern of growth in the orangutan face and palate. The results indicate that generally males and females share a similar, but not identical, pattern of growth or local form change, but differ in growth rate, timing, and magnitude of difference. Dimorphism in the face and palate can be localized in infancy and traced throughout all age intervals. Orangutan females grow slightly faster than males from infancy to adolescence, at which time male growth exceeds female growth. Female growth ceases with the advent of adulthood, while male growth continues (i.e., both the number and magnitude of the dimorphic dimensions increase). Males and females are similar in facial dimensions and growth related to the orbits, upper face, and palate width. They maintain these similarities throughout development. However, they differ in facial and nasal height, palate length, snout projection, depth of the nasopharynx, and hafting of the face onto the skull. The face broadens and the zygomatic bone flares dramatically in adult males, corresponding to the development of cheek pads. While growth patterns are similar between the two sexes, they differ in the lateral orbit, snout projection, and hafting of the face onto the cranium. Adult dimorphism is the result of growth patterns experienced throughout life, and it is not equally expressed across the cranium. An understanding of patterns of dimorphism, along with the magnitude of difference, may be helpful for interpreting dimorphism in the fossil record.

Cranial allometry, phylogeography, and systematics of large-bodied papionins (primates: Cercopithecinae) inferred from geometric morphometric analysis of landmark data

The cranial morphology of the African Old World monkeys Mandrillus, Papio, and Theropithecus (i.e., baboons) has been the subject of a number of studies investigating their systematic relationships, patterns of scaling, and growth. In this study, we use landmark-based geometric morphometrics and multivariate analysis to assess the effects of size, sex, taxonomy, and geographic location on cranial shape. Forty-five landmarks were digitized in three dimensions on 452 baboon crania and subjected to generalized Procrustes analysis (GPA), which standardizes geometric size but leaves scaling-based shape differences in the data. The resulting shape coordinates were submitted to regression analysis, principal components analysis (PCA), partial least-squares (PLS) analysis, and various clustering techniques. Scaling (shape differences correlated with size) was the largest single factor explaining cranial shape variation. For instance, most (but not all) of the shape differences between the sexes were explained by size dimorphism. However, central tendencies of shape clearly varied by taxon (both specific and subspecific) even after variations in size and sex were adjusted out. Within Papio, about 60% of the size- and sex-adjusted shape variations were explained by the geographic coordinates of the specimen's provenance, revealing a stepped cline in cranial morphology, with the greatest separation between northern and southern populations. Based on evidence from genetic studies, and the presence of at least two major hybrid/interbreeding zones, we interpret the phylogeographic pattern of cranial variation as indicating that these populations are best ranked as subspecies of a single species, rather than as two or more distinct biological species. This objective approach can be applied to other vertebrate species or species groups to help determine the taxonomic rank of problematic taxa.

Ontogeny and phylogeny in papionin primates

This study investigates the developmental bases of size and shape variation in papionin primates (Macaca, Cercocebus, Mandrillus, Lophocebus, and Papio). The analysis tests hypotheses predicting that heterochronic changes in ontogeny, particularly in the degree of overall size growth, can account for cranial diversity and "allometric scaling" in this clade. Large developmental samples of extant papionin crania are examined to test heterochronic hypotheses using bivariate allometric methods. Analyses indicate that the crania of larger papionins (Mandrillus and Papio) are generally peramorphic, surpassing size and shape ranges of smaller, and probably less-derived, macaques and mangabeys. At least two heterochronic processes, including acceleration and hypermorphosis, can account for this pattern. Ontogenetic changes include decoupling of growth and development among cranial regions, along with simple shifts in size. Allometric scaling has complex developmental bases. Size change itself is not sufficient to explain all developmental differences among papionins, but these changes are extremely important in comparisons within cranial regions such as the face. Results imply that Papio exhibits strongly derived patterns of brain growth that impact postnatal patterns of size and shape transformation. Consideration of these results in the context of recent socioecological analyses suggests that derived patterns of cranial growth in Papio may be a response to selection during the early periods of ontogeny, resulting in a distinctive life history pattern.

Growth-related shape changes in the fetal craniofacial complex of humans (Homo sapiens) and pigtailed macaques (Macaca nemestrina): a 3D-CT comparative analysis

This study investigates whether macaques and humans possess a common pattern of relative growth during the fetal period. The fetal samples consist of 16 male pigtailed macaques (mean age, 20.5 gestational weeks) and 17 humans (9 males and 8 females; mean age, 29.5 gestational weeks). For each individual, three-dimensional coordinates of 18 landmarks on the skull were collected from three-dimensional computed tomographic (CT) reconstructed images and two-dimensional CT axial slices. Early and late groups were created from the human (early mean age, 24 weeks, N = 8; late mean age, 34 weeks, N = 9) and macaque samples (early mean age, 17.7 weeks, N = 7; late mean age, 23 weeks, N = 9). Inter- and intraspecific comparisons were made between the early and late groups. To determine if macaques and humans share a common fetal pattern of relative growth, human change in shape estimated from a comparison of early and late groups was compared to the pattern estimated between early and late macaque groups. Euclidean distance matrix analysis was used in all comparisons. Intraspecific comparisons indicate that the growing fetal skull displays the greatest amount of change along mediolateral dimensions. Changes during human growth are primarily localized to the basicranium and palate, while macaques experience localized change in the midface. Interspecific comparisons indicate that the two primate species do not share a common pattern of relative growth, and the macaque pattern is characterized by increased midfacial growth relative to humans. Our results suggest that morphological differences in the craniofacial skeleton of these species are in part established by differences in fetal growth patterns.

Scaling relationships between craniofacial sexual dimorphism and body mass dimorphism in primates: implications for the fossil record

Craniofacial remains (the most abundant identifiable remains in the fossil record) potentially offer important information about body size dimorphism in extinct species. This study evaluates the scaling relationships between body mass dimorphism and different measures of craniofacial dimorphism, evaluating taxonomic differences in the magnitude and scaling of craniofacial dimorphism across higher taxonomic groups. Data on 40 dimensions from 129 primate species and subspecies demonstrate that few dimensions change proportionally with body mass dimorphism. Primates show general patterns of greater facial vs. neurocranial and orbital dimorphism, and greater dimorphism in lengths as opposed to breadths. Within any species, though, different craniofacial dimensions can yield very different reconstructions of size dimorphism. There are significant taxonomic differences in the relationships between size and craniofacial dimorphism among primate groups that can have a significant impact on reconstructions of body mass dimorphism. Hominoids tend to show lower degrees of facial dimorphism proportional to size dimorphism than other primates. This in turn implies that strong craniofacial dimorphism in Australopithecus africanus could imply very strong body size dimorphism, conflicting with the relatively modest size dimorphism inferred from postcrania. Different methods of estimating the magnitude of size dimorphism from craniofacial measurements yield similar results, and yield comparatively low percent prediction errors for a number of dimensions. However, confidence intervals for most estimates are so large as to render most estimates highly tentative.

Size and shape changes during late fetal growth (137-157 gestational days) in the pigtailed macaque (Macaca nemestrina) craniofacial complex: an application using three-dimensional coordinate data and finite element scaling analysis

Form changes within the fetal pigtailed macaque (Macaca nemestrina) craniofacial complex was documented using finite element scaling analysis (FESA) and three-dimensional (3D) coordinate data for 35 craniofacial landmarks. Coordinate data were digitized from 3D reconstructions of computed tomography (CT) images and 2D axial slices. Twenty-two fetal pigtailed macaques ranging in age from 137 to 157 gestational days were included (in this species, birth is estimated at 170 gestational days). The null hypothesis that the craniofacial complex grows with isometry during late fetal growth of the craniofacial complex was tested (P < 0.05), and the prediction that morphological change along an anteroposterior axis dominates late fetal growth was also investigated. The null hypothesis was rejected, indicating that allometric growth is present during late fetal growth. Growth along an anteroposterior axis is localized in the palate and mandible. The neurocranium grows along a superoinferior axis, while the neurofacial junction displays growth along both the anteroposterior and superoinferior axes. Mediolateral changes are localized between asterions, the external auditory meati, and maxillary and mandibular alveolar points. Finally, a 3D model of craniofacial growth for this species was created, localizing size and shape changes that occur during late fetal growth for each of the 35 craniofacial landmarks defined in this study.

Patterns of cranial shape variation in the Papionini (Primates: Cercopithecinae)

Traditional classifications of the Old World monkey tribe Papionini (Primates: Cercopithecinae) recognized the mangabey genera Cercocebus and Lophocebus as sister taxa. However, molecular studies have consistently found the mangabeys to be diphyletic, with Cercocebus and Mandrillus forming a clade to the exclusion of all other papionins. Recent studies have identified cranial and postcranial features which distinguish the Cercocebus-Mandrillus clade, however the detailed similarities in cranial shape between the mangabey genera are more difficult to reconcile with the molecular evidence. Given the large size differential between members of the papionin molecular clades, it has frequently been suggested that allometric effects account for homoplasy in papionin cranial form. A combination of geometric morphometric, bivariate, and multivariate methods was used to evaluate the hypothesis that allometric scaling contributes to craniofacial similarities between like-sized papionin taxa. Patterns of allometric and size-independent cranial shape variation were subsequently described and related to known papionin phylogenetic relationships and patterns of development. Results confirm that allometric scaling of craniofacial shape characterized by positive facial allometry and negative neurocranial allometry is present across adult papionins. Pairwise comparisons of regression lines among genera revealed considerable homogeneity of scaling within the Papionini, however statistically significant differences in regression lines also were noted. In particular, Cercocebus and Lophocebus exhibit a shared slope and significant vertical displacement of their allometric lines relative to other papionins. These findings give no support to narrowly construed hypotheses of uniquely shared patterns of allometric scaling, either between sister taxa or across all papionins. However, more general allometric trends do appear to account for a substantial proportion of papionin cranial shape variation, most notably in those features which have influenced traditional morphological phylogenies. Examination of size-uncorrelated shape variation gives no clear support to molecular phylogenies, but underscores the absence of morphometric similarities between the mangabey genera when size effects are controlled. Patterns of allometric and size-uncorrelated shape variation indicate conservatism of cranial form in non- Theropithecus papionins, and suggest that Papio represents the primitive morphometric pattern for the African papionins. Lophocebus exhibits a divergent morphometric pattern, clearly distinguishable from other papionins, most notably Cercocebus. These results clarify patterns of cranial shape variation among the extant Papionini and lay the groundwork for studies of related fossil taxa.

Taxonomic variation in the patterns of craniofacial dimorphism in primates

Understanding sexual dimorphism in living primates is important for interpreting the biological and taxonomic significance of variation in the primate fossil record. In the past two decades, there has been an increasing emphasis on the fact that sexual dimorphism varies in both magnitude and pattern among species. Several studies have suggested that distinct patterns of dimorphism may assist in species recognition and perhaps phylogenetic analysis. This study evaluates patterns of craniofacial dimorphism in samples of 82 anthropoid primates. Dimensions of the viscerocranium tend to be more dimorphic than those of the neurocranium and orbits. Principal components analysis of phylogenetically controlled data demonstrates a basic pattern of dimorphism in overall skull proportions, and a distinction between length and breadth measurements. For any given species there can be substantial variation in the magnitude of dimorphism among dimensions, and different species can show substantially different patterns of dimorphism within and between regions of the skull and jaws. Patterns of dimorphism are clearly associated with phylogeny. Pattern similarity is not dependent on the overall magnitude of craniofacial dimorphism, or body mass dimorphism. Among all anthropoids, there are few combinations of characters that consistently show greater or lesser degrees of dimorphism. Such "stability" of patterns increases within genera. Patterns of dimorphism are likely to be useful for interpreting the taxonomic significance of variation in the fossil record. However, phylogenetic propinquity alone is not reason to use an extant species as a model for variation in an extinct species. Rather, care must be taken to identify stable patterns of dimorphism within a group of closely related extant species.

Thin-plate spline analysis of allometry and sexual dimorphism in the human craniofacial complex

The relationship between allometry and sexual dimorphism in the human craniofacial complex was analyzed using geometric morphometric methods. Thin-plate splines (TPS) analysis has been applied to investigate the lateral profile of complete adult skulls of known sex. Twenty-nine three-dimensional (3D) craniofacial and mandibular landmark coordinates were recorded from a sample of 52 adult females and 52 adult males of known age and sex. No difference in the influence of size on shape was detected between sexes. Both size and sex had significant influences on shape. As expected, the influence of centroid size on shape (allometry) revealed a shift in the proportions of the neurocranium and the viscerocranium, with a marked allometric variation of the lower face. Adjusted for centroid size, males presented a relatively larger size of the nasopharyngeal space than females. A mean-male TPS transformation revealed a larger piriform aperture, achieved by an increase of the angulation of the nasal bones and a downward rotation of the anterior nasal floor. Male pharynx expansion was also reflected by larger choanae and a more posteriorly inclined basilar part of the occipital clivus. Male muscle attachment sites appeared more pronounced. In contrast, the mean-female TPS transformation was characterized by a relatively small nasal aperture. The occipital clivus inclined anteriorly, and muscle insertion areas became smoothed. Besides these variations, both maxillary and mandibular alveolar regions became prognathic. The sex-specific TPS deformation patterns are hypothesized to be associated with sexual differences in body composition and energetic requirements.

Facial growth in Cercocebus torquatus: an application of three-dimensional geometric morphometric techniques to the study of morphological variation

This paper presents a study of 3-dimensional growth in the facial skeleton of the mangabey, Cercocebus torquatus. The pattern of facial cortical remodelling in this species is already well mapped from an earlier study. In this paper we consider the extent to which these remodelling maps relate to ontogenetic changes in size and shape of the face. This study is based on 31 facial landmarks taken from 49 adult and subadult faces. Our analysis draws on some of the tools of geometric morphometrics and we take this opportunity to describe our implementation of these tools for 3D data. The geometric analysis permits the known remodelling maps to be interpreted in the context of the general pattern of facial growth in this species. We are also able to examine sexual dimorphism in the face of this species and consider the extent to which males and females share similar ontogenetic allometries. Our findings indicate that the general pattern of size-related shape variation during facial growth is more or less identical for males and females up to eruption of the third permanent maxillary molar (M3). After this, ontogenetic allometries appear to diverge. The finding of a common growth allometry that is well approximated for younger specimens by a simple linear model is consistent with the earlier findings of a consistent pattern of facial remodelling up to M3 eruption. We consider the implications of these findings in terms of the potential for these approaches in the study of comparative growth in related species.

Sexual dimorphism and interspecific cranial form in two capuchin species: Cebus albifrons and C. apella

Ontogenetic patterns of sexual dimorphism and cranial form in two capuchin monkeys, Cebus albifrons and C. apella, are investigated by means of univariate, bivariate, and multivariate statistics. The analyses are based on 23 linear variables. Univariate analyses indicate that similar ontogenetic patterns of cranial sexual dimorphism are present; however, interspecific differences exist in timing. Ontogenetic scaling is present in both species' crania; however, it is more prevalent in C. albifrons. Several departures are present in cranial regions associated with orbital shape, the dental arcade, and the muscles of mastication. The latter two indicate that sexual differences in diet and/or foraging strategies may exist. Sexual selection is suggested as being the primary selective regime underlying the observed patterns of cranial sexual dimorphism in each species. Interspecific comparisons confirm that C. apella possesses a more dimorphic cranium than C. albifrons and that sexual dimorphism in C. apella begins earlier in development. Although interspecific ontogenetic scaling is present in some cranial variables, C. apella is not just a scaled-up version of C. albifrons. These sympatric congeners seem to be differentiated by variables related to the orbital region and the masticatory apparatus, as indicated by both departures from ontogenetic scaling and results of the discriminant function analysis. Ecological selection, rather than varying degrees of sexual selection, is likely to be responsible for this finding given that C. apella is known to consume hard-object foods. This is consistent with the predicted outcome of the competitive exclusion principle.

Sex differences in absolute rates of bone resorption in young rats: appendicular versus axial bones

This study compares absolute rates of bone resorption and formation at the organ level in adolescent Sprague-Dawley rats as a function of sex and type of bone. Bone resorption and formation were quantified in rapidly growing male and female rats (4-7 weeks of age) who were multiply prelabeled with [3H]tetracycline. Ten different whole bones were compared: four cranial or appendicular bones and six axial bones. Absolure rate of bone resorption was measured isotopically by the loss of 3H-tetracycline from each whole bone. Bone growth was quantified in terms of relative and absolute increase in bone calcium mass. When the rates of bone resorption (loss of [3H]-tetracycline as percent of whole bone per 3 weeks) were compared between sexes, the six axial bones showed significantly higher rates (P < 0.05-0.001) in males (64-73) than in females (37-66). No significant sex differences were observed in rate for the two cranial and two appendicular bones. During 4-7 weeks of age, a comparison of bone masses showed that only one bone (calvaria) gained more mass in the male and two bones (mandible and humerus) gained more mass in the female. In contrast, five of six axial bones gained more mass in the female. Thus, 7 out of 10 bones were larger in the female. In growing male and female rats, an inverse relationship appears between rate of bone resorption and mass for most of the axial bones; this relationship was not apparent for cranial or appendicular bones. Sexual dimorphism was consistently seen by greater axial bone mass in females. However, greater rates of bone resorption were seen in male axial bones but not in cranial or appendicular bones. It is apparent that the different types of bones are heterogeneous in their rates of resorption and formation during this period of growth.

Cranial growth and growth dimorphism in Ateles geoffroyi

With the exception of the work of Schultz (1960), cranial growth in Ateles is not well documented. This paper describes the results of a detailed quantitative study of cranial ontogeny in male and female Ateles geoffroyi. Using Euclidean Distance Matrix Analysis (EDMA), local areas of form change due to growth within spider monkey crania are identified. We found substantial change local to the zygomatic region in the face, identified mediolaterally directed changes in the palate, detected relatively larger amounts of change local to the anterior neurocranium compared to the posterior neurocranium, and demonstrate a greater amount of basicranial growth along a mediolateral axis than previously reported. Cranial sexual dimorphism is also examined. A. geoffroyi is noted for being monomorphic, and we found a general similarity between male and female cranial forms at all developmental ages. However, differences in overall cranial size between the sexes were found in the oldest subadult age group but not between male and female adults. This difference suggests that A. geoffroyi females attain their adult cranial form slightly before males and implies a pattern of earlier onset of female maturity relative to males.