An ontogenetic perspective on the study of sexual dimorphism, phylogenetic variability, and allometry of the skull of European ground squirrel, Spermophilus citellus (Linnaeus, 1766)

Most studies of morphological variability in or among species are performed on adult specimens. However, it has been proven that knowledge of the patterns of size and shape changes and their covariation during ontogeny is of great value for the understanding of the processes that produce morphological variation. In this study, we investigated the patterns of sexual dimorphism, phylogenetic variability, and ontogenetic allometry in the Spermophilus citellus with geometric morphometrics applied to cross-sectional ontogenetic data of 189 skulls from three populations (originating from Burgenland, Banat, and Dojran) belonging to two phylogenetic lineages (the Northern and Southern). Our results indicate that sexual dimorphism in the ventral cranium of S. citellus is expressed only in skull size and becomes apparent just before or after the first hibernation because of accelerated growth in juvenile males. Sexes had the same pattern of ontogenetic allometry. Populations from Banat and Dojran, belonging to different phylogroups, were the most different in size but had the most similar adult skull shape. Phylogenetic relations among populations, therefore, did not reflect skull morphology, which is probably under a significant influence of ecological factors. Populations had parallel allometric trajectories, indicating that alterations in development probably occur prenatally. The species’ allometric relations during cranial growth showed characteristic nonlinear trajectories in the two northern populations, with accelerated shape changes in juveniles and continued but almost isometric growth in adults. The adult cranial shape was reached before sexual maturity of both sexes and adult size after sexual maturity. The majority of shape changes during growth are probably correlated with the shift from a liquid to a solid diet and to a lesser degree due to allometric scaling, which explained only 20 % of total shape variation. As expected, viscerocranial components grew with positive and neurocranial with negative allometry.     

Postmetamorphic ontogenetic allometry and the evolution of skull shape in Nest‐building frogs Leptodactylus (Anura: Leptodactylidae)

Allometry constitutes an important source of morphological variation. However, its influence in head development in anurans has been poorly explored. By using geometric morphometrics followed by statistical and comparative methods we analyzed patterns of allometric change during cranial postmetamorphic ontogeny in species of Nest‐building frogs Leptodactylus (Leptodactylidae). We found that the anuran skull is not a static structure, and allometry plays an important role in defining its shape in this group. Similar to other groups with biphasic life‐cycle, and following a general trend in vertebrates, ontogenetic changes mostly involve rearrangement in rostral, otoccipital, and suspensorium regions. Ontogenetic transformations are paralleled by shape changes associated with evolutionary change in size, such that the skulls of species of different intrageneric groups are scaled to each other, and small and large species show patterns of paedomorphic/peramorphic features, respectively. Allometric trajectories producing those phenotypes are highly evolvable though, with shape change direction and magnitude varying widely among clades, and irrespective of changes in absolute body size. These results reinforce the importance of large‐scale comparisons of growth patterns to understand the plasticity, evolution, and polarity of morphological changes in different clades.     

Phylogenetic and size constrains on cranial ontogenetic allometry of spiny rats (Echimyidae,Rodentia)

Analysis of ontogenetic development is crucial for understanding the emergence of phenotypic discrepancies between animal taxa. The study of allometric trajectories within a phylogenetic context is a feasible approach to assess the morphological change across different evolutionary lineages. Here, we report the disparity of multivariate ontogenetic allometry in the Echimyidae, a taxonomically diverse rodent family, as well as the effects of size on the evolution of skull ontogeny. The ontogenetic trajectories of 15 echimyid operational taxonomic unities (12 genera plus one genus with three species) belonging to all subfamilies and major clades, when plotted in allometric space, revealed strong and significant phylogenetic signals. Allometric trajectories were found to be constrained by phylogenetic ancestry, with changes approximately adjusting to a Brownian motion model of evolution. Moreover, the occupation of allometric space by echimyid taxa was significantly correlated with adult size rather than with shape, suggesting that the variation in adult size might result in critically intrinsic and structural constraints on allometric coefficients. These findings disagreed with the hypothesis that allometric disparities might be mainly adaptive with undetectable phylogenetic signals.     

草兔胚后发育过程中头骨大小与形状变化的几何形态学分析(兔形目:兔科)

在本文中,我们使用几何形态学的研究方法对58 个处于不同年龄阶段的草兔头骨进行了分析。分析中总共使用来自头骨背面、腹面和侧面的180 个标点和半标点。研究结果表明,草兔在胚后发育早期即迅速建立起与成年个体近似的形态结构,组成头骨的不同形态单元存在显著的异速生长现象,主要的形变发生在幼年至年龄1 阶段,即出生后的6 个月以内。从大小的变化来看,鼻骨在胚后发育过程中呈现正的异速生长,额骨和眼眶区则与头骨整体大小变化基本等速,而顶骨、听泡和枕骨大孔则呈现显著的负异速生长。我们亦用几何形态学的方法绘制出头骨在生长发育早期和晚期不同部位的形态变化轨迹,这一结果显示较显著的形状变化发生在鼻骨、前颌骨、眶上突和头骨的纵轴方向。头骨整体形态在胚后发育过程中伸长并变窄。这些变化将有利于幼兔较早实现完善的头部系统的建立,尤其有利于提高呼吸系统的通风能力,提高在高强度的捕食压力下保持警觉,在多样的运动过程中保持身体平稳的能力,也可能进一步提高了对固态食物的处理能力。这种异速生长模式可能是善于奔跑的植食性哺乳动物在功能需求上的一种适应性进化特征。     

Developmental dynamics of ecomorphological convergence in a transcontinental lizard radiation

Phenotypic convergence has confounded evolutionary biologists for centuries, explained as adaptations to shared selective pressures, or alternatively, the result of limited developmental pathways. We tested the relative roles of adaptation and constraint in generating convergent cranial morphologies across a large lizard radiation, the Lacertidae, whose members inhabit diverse environments throughout the Old World and display high amounts of homoplasy associated with ecological niche. Using 3D X‐ray computed tomography, we quantified cranial shape variation associated with ontogeny, allometry, and ecology, covering all lacertid genera and one‐third of species diversity. Landmark‐based geometric morphometrics showed that cranial shape varied significantly among biomes, with substantial convergence among arid‐dwelling lineages. Comparisons of species cranial growth trajectories between biomes revealed that allometric postdisplacement, as evidenced by decreased elevation of a constant ontogenetic slope, drives the convergent paedomorphic appearance of independent arid‐dwelling forms. We hypothesize that observed heterochronic changes reflect temporal compression of ancestral life history in response to extreme environments, with associated phenotypes occurring as by‐products of adaptive shifts in reproductive investment. Although allometry has long been considered a developmental constraint, our results demonstrate that allometric flexibility during early ontogeny produces convergent ecomorphologies over vast temporal and spatial scales, thus dramatically obscuring underlying phylogenetic signals.     

Ontogeny, phylogeny, and morphology in anuran larvae: morphometric analysis of cranial development and evolution in Rana tadpoles (Anura: Ranidae)

Comparative studies of chondrocranial morphology in larval anurans are typically qualitative in nature, focusing primarily on discrete variation or gross differences in the size or shape of individual structures. Detailed data on chondrocranial allometry are currently limited to only two species, Rana sylvatica and Bufo americanus. This study uses geometric morphometric and multivariate statistical analyses to examine interspecific variation in both larval chondrocranial shape and patterns of ontogenetic allometry among six species of Rana. Variation is interpreted within the context of hypothesized phylogenetic relationships among these species. Canonical variates analyses of geometric morphometric datasets indicate that species can be clearly discriminated based on chondrocranial shape, even when whole ontogenies are included in the analysis. Ordinations and cluster analyses based on chondrocranial shape data indicate the presence of three primary groupings (R. sylvatica; R. catesbeiana + R. clamitans; and R. palustris + R. pipiens + R. sphenocephala), and patterns of similarity closely reflect phylogenetic relationships. Analysis of chondrocranial allometry reveals that some patterns are conserved across all species (e.g., most measurements scale with negative allometry, those associated with the posterior palatoquadrate tend to scale with isometry or positive allometry). Ontogenetic scaling along similar allometric trajectories, lateral transpositions of individual trajectories, and variable allometric relationships all contribute to shape differences among species. Overall patterns of similarity among ontogenetic trajectories also strongly reflect phylogenetic relationships. Thus, this study demonstrates a tight link between ontogeny, phylogeny, and morphology, and highlights the importance of including both ontogenetic and phylogenetic data in studies of chondrocranial evolution in larval anurans.     

Stochastic ontogenetic allometry: the statistical dynamics of relative growth

Background

In the absence of stochasticity, allometric growth throughout ontogeny is axiomatically described by the logarithm-transformed power-law model, , where and are the logarithmic sizes of two traits at any given time t. Realistically, however, stochasticity is an inherent property of ontogenetic allometry. Due to the inherent stochasticity in both and , the ontogenetic allometry coefficients, and k, can vary with t and have intricate temporal distributions that are governed by the central and mixed moments of the random ontogenetic growth functions, and . Unfortunately, there is no probabilistic model for analyzing these informative ontogenetic statistical moments.

Methodology/Principal Findings

This study treats and as correlated stochastic processes to formulate the exact probabilistic version of each of the ontogenetic allometry coefficients. In particular, the statistical dynamics of relative growth is addressed by analyzing the allometric growth factors that affect the temporal distribution of the probabilistic version of the relative growth rate, , where is the expected value of the ratio of stochastic to stochastic , and and are the numerator and the denominator of , respectively. These allometric growth factors, which provide important insight into ontogenetic allometry but appear only when stochasticity is introduced, describe the central and mixed moments of and as differentiable real-valued functions of t.

Conclusions/Significance

Failure to account for the inherent stochasticity in both and leads not only to the miscalculation of k, but also to the omission of all of the informative ontogenetic statistical moments that affect the size of traits and the timing and rate of development of traits. Furthermore, even though the stochastic process and the stochastic process are linearly related, k can vary with t.     

Ontogenetic allometry, heterochrony, and interspecific differences in the skull of African apes, using tridimensional Procrustes analysis

Ontogenetic studies of African ape skulls lead to an analysis of morphological differences in terms of allometry, heterochrony, and sexual dimorphism. The use of geometric morphometrics allows us 1) to define size and shape variations as independent factors (an essential but seldom respected condition for heterochrony), and 2) to calculate in percentage of shape changes and to graphically represent the parts of shape variation which are related to various biological phenomena: common allometry, intraspecific allometry, and allometric and nonallometric shape discrimination. Three tridimensional Procrustes analyses and the calculation of multivariate allometries, discriminant functions, and statistical tests are used to compare the skulls of 50 Pan troglodytes, and 50 Gorilla gorilla of different dental stages. The results both complement and modify classical results obtained from similar material but with different methods. Size and Scaling in Primate Morphology, New York: Plenum, p. 175-205). As previously described by Shea, the common growth allometric pattern is very important (64% of total shape variation). It corresponds to a larger increase of facial volume than of neurocranial volume, a more obliquely oriented foramen magnum, and a noticeable reshaping of the nuchal region (higher inion). However, the heterochronic interpretation based on common allometry is rather different from Shea. Gorillas differ from chimpanzees not only with a larger magnitude of allometric change (rate peramorphosis), as is classically said, but also grow more in size than in shape (size acceleration). In other words, for a similar stage of growth, gorillas have the size and shape corresponding to older chimpanzees, and for a similar shape, gorillas have a larger size than chimpanzees. In contrast, sexual dimorphism actually corresponds to allometric changes only, as classically demonstrated (time hypermorphosis). Sexual dimorphism is here significant in adult gorillas alone, and solely in terms of allometry (size-related shape and size, given that sagittal and nuchal crests are not taken into account). The study also permits us to differentiate two different shape variations that are classically confused in ontogenetic studies: a very small part of allometric shape change which is specific to each species (1% of the total shape variation), and nonallometric species-specific traits independent of growth (8% of total shape change). When calculated in terms of intraspecific allometries (including common allometry and noncommon allometry), shape changes are more extensive in gorillas (36% of total shape change) than in chimpanzees (29% of total shape change). The allometric differences mainly concern the inion, which becomes higher; the position of the foramen magnum, more dorsally oriented; and the palate, more tilted in adult gorillas than in adult chimpanzees. In contrast, nonallometric species-specific traits in gorillas are the long and flat vault characterized by a prominent occipital region, the higher and displaced backward glabella, and the protrusive nose. Biomechanical schemes built from shape partition suggest that the increased out-of-plumb position of the head during growth is partially compensated in gorillas by a powerful nuchal musculature due to the peculiar shape of the occipital region.     

Phenetic relationships among four Apodemus species (Rodentia, Muridae) inferred from skull variation

Phenetic relationships among four Apodemus species (A. agrarius, A. epimelas, A. flavicollis and A. sylvaticus) inferred from skull (mandible and cranium) variation were explored using landmark-based geometric morphometrics. Analysis of size variation revealed that mandibles and crania of A. epimelas were the largest, followed by those of A. flavicollis, while A. agrarius and A. sylvaticus had the smallest ones. Phenetic relationships inferred from mandible shape variation better reflected phylogenetic relationships among the analyzed Apodemus species than those inferred from cranial differences. Concerning cranial shape variation, the most differentiated species was A. epimelas, whose ecology clearly differs from the other three species. Thus, differentiation of the mandible provided a pattern fully concordant with the phylogeny, while the cranium differentiation was in agreement with ecology expectations. The most evident shape changes of mandible and cranium involved the angular process and facial region, respectively. We also found that allometry had a significant influence on shape variation and that size-dependent shape variation differed among the analyzed species. Moreover, mandible and cranium are differently influenced by allometric changes. Different phenetic relationships inferred from mandible and cranium shape variation imply that phylogeny, ecology, together with factors related to size differences are all involved in the observed morphological divergence among the analyzed Apodemus species.     

Influence of Evolutionary Allometry on Rates of Morphological Evolution and Disparity in strictly Subterranean Moles (Talpinae,Talpidae, Lipotyphla,Mammalia)

The adaptation to a particular function could directly influence the morphological evolution of an anatomical structure as well as its rates. The humeral morphology of moles (subfamily Talpinae) is highly modified in response to intense burrowing and fully fossorial lifestyle. However, little is known of the evolutionary pathways that marked its diversification in the two highly fossorial moles tribes Talpini and Scalopini. We used two-dimensional landmark-based geometric morphometrics and comparative methods to understand which factors influenced the rates and patterns of the morphological evolution of the humerus in 53 extant and extinct species of the Talpini (22 extant plus 12 extinct) and Scalopini (six extant plus 13 extinct) tribes, for a total of 623 humeri. We first built a synthetic phylogeny of extinct and extant taxa of the subfamily Talpinae based on all the available information from known phylogenies, molecular data, and age ranges of fossil records. We tested for evolutionary allometry by means of multivariate regression of shape on size variables. Evolutionary allometric trajectories exhibited convergence of humeral shape between the two tribes, even when controlling for phylogeny, though a significant differences in the evolutionary rates was found between the two tribes. Talpini, unlike Scalopini, seem to have reached a robust fossorial morphology early during their evolution, and their shape disparity did not change, if it did not decrease, through time. Furthermore, the basal Geotrypus spp. clearly set apart from the other highly fossorial moles, exhibiting a significant acceleration of evolutionary shifts toward higher degree of fossorial adaptation. Our observations support the hypothesis that the evolution of allometry may reflect a biological demand (in this case functional) that constrains the rates of evolution of anatomical structures.     

Ontogenetic shape changes in Pomacentridae (Teleostei,Perciformes) and their relationships with feeding strategies: a geometric morphometric approach

The present study explores the shape changes of cranial structures directly involved in food capturing during growth after reef settlement in two species of Pomacentridae (Dascyllus aruanus and Pomacentrus pavo). Landmark‐based geometric morphometrics were used to study allometric patterns and related shape changes in four skeletal units: neurocranium, suspensorium and opercle, mandible and premaxilla. At settlement, the larvae of both species have a relatively similar morphology, especially with respect to the mandible. Their shapes suggest a feeding mode defined as ram/suction‐feeding. Ontogenetic shape changes show a shift to a suction feeding mode of prey capture. The main transformations involved are an increase in height of the suspensorium and the opercle, an elevation of the supraoccipital crest, a relative shortening of the mandible, and a lengthening of the ascending process of the premaxilla. Shape changes of the mandible in the two studied species also reflect an increase of biting capacities. The high disparity between adult shape results from differences in the rate and in the length of ontogenetic trajectories, from divergence of the ontogenetic trajectories (neurocranium, mandible, and premaxilla) and parallel shifts of the trajectories in the size‐shape space (suspensorium and opercle). In an evolutionary context, allometric heterochronies during ontogeny of different skeletal unit of the head may be considered as a basis for the explanation of the diversity of damselfishes. © 2008 The Linnean Society of London, Biological Journal of the Linnean Society, 2008, 95 , 92–105.     

Growth trajectories of prenatal embryos of the deep-sea shark Chlamydoselachus anguineus (Chondrichthyes)

Chlamydoselachus anguineus, Garman 1884, commonly called the frilled shark, is a deep-sea shark species occurring up to depths of 1300 m. It is assumed to represent an ancient morphotype of sharks (e.g., terminal mouth opening, more than five gill slits) and thus is often considered to represent plesiomorphic traits for sharks. Therefore, its early ontogenetic developmental traits are important for understanding the evolution of its particular phenotype. Here, we established six stages for prenatal embryos and used linear measurements and geometric morphometrics to analyse changes in shape and size as well as their timing during different embryonic stages. Our results show a change in head shape and a relocation of the mouth opening at a late stage of development. We also detected a negative allometric growth of the head and especially the eye compared to the rest of the body and a sexual dimorphism in total body length, which differs from the known data for adults. A multivariate analysis of covariance shows a significant interaction of shape related to the logarithm of centroid size and developmental stage. Geometric morphometrics results indicate that the head shape changes as a covariate of body size while not accounting for differences between sexes. The growth pattern of stages 32 and 33 indicates a shift in head shape, thus highlighting the moment in development when the jaws start to elongate anteriorly to finally achieve the adult condition of terminal mouth opening rather than retaining the early embryonic subterminal position as is typical for sharks. Thus, the antero-terminal mouth opening of the frilled shark has to be considered a derived feature.     

Shape Variation in the Dermatocranium of the Greater Short-Horned Lizard <Emphasis Type="Italic">Phrynosoma hernandesi</Emphasis> (Reptilia: Squamata: Phrynosomatidae)

Dermatocranial shape and horn morphology display great disparity among the species of Phrynosoma. Ontogenetic change in dermatocranial shape in a series of 79 specimens of the short-horned Phrynosoma hernandesi (54F: 25M) was examined using geometric morphometric techniques. A multivariate ANCOVA of Procrustes residuals with sex as a factor and ln(centroid size) as the covariate indicated sexual shape dimorphism. Separate multivariate regressions of Procrustes residuals on ln(centroid size) for each sex indicated that allometry accounts for ~52–54% of the total sample shape variance. Comparisons of ontogenetic shape change between sexes indicate that sexual shape dimorphism is minimal and of uncertain biological significance. Groupings of multivariate regression coefficients by magnitude and sign suggest that allometric integration of the dermatocranium is not uniform over the dermatocranium. Principal component analysis of the landmark configurations corrected for sex and allometry yields a first principal component which describes shape variance concentrated in the posterolateral and posterior regions of the dermatocranium, and again is indicative of non-uniform shape variation over the dermatocranium. Our findings for P. hernandesi indicate that the adult shape of the dermatocranium may contribute to a passive defence against predation. We hypothesize that the complexity in dermatocranial shape demonstrated here for P. hernandesi indicates parcellation of shape variance, which may contribute to explanations of the pronounced dermatocranial disparity exhibited by the species of Phrynosoma.     

Mode of miniaturisation influences body shape evolution in New World anchovies (Engraulidae)

We explored the macroevolutionary dynamics of miniaturisation in New World anchovies by integrating a time-calibrated phylogeny, geometric morphometrics and phylogenetic comparative methods. We found that the paedomorphic species Amazonsprattus scintilla occupies a novel region of shape space, while the dwarf species Anchoviella manamensis has an overall shape consistent with other anchovies. We found that miniaturisation did not increase overall clade disparity in size or shape beyond the expectations of Brownian motion, nor were there differences in rates of size or shape evolution among clades. Overall, our study shows that while the mode of miniaturisation influences shape evolution, the phenotypic novelty produced by the evolution of miniaturisation did not seem to alter macroevolutionary dynamics.     

Postnatal ontogenetic size and shape changes in the craniums of plateau pika and woolly hare(Mammalia: Lagomorpha)

In the present study, postnatal ontogenetic size and shape changes in the cranium of two lagomorph species, the plateau pika(Ochotona curzoniae) and woolly hare(Lepus oiostolus), were investigated by geometric morphometrics. The ontogenetic size and shape changes of their cranium exhibited different growth patterns in response to similar environmental pressures on the Qinghai-Tibetan Plateau. The overall size change in the cranium of the plateau pika was slower than that of the woolly hare. The percentage of ontogenetic shape variance explained by size in the woolly hare was greater than that in the plateau pika. The overall shape of the cranium was narrowed in both species, and morphological components in relation to neural maturity showed negative allometry, while those responsible for muscular development showed isometric or positive allometry. The most remarkable shape variations in the plateau pika were associated with food acquisition(temporalis development), though other remarkable shape variations in the incisive and palatal foramen in the ventral view were also observed. The most important shape change in the woolly hare was demonstrated by the elongation of the nasal bones, expansion of the supra-orbital process and shape variation of the neurocranium.     

Squirrel monkeys (Saimiri) in natural habitats in Panama,Colombia, Brazil,and Peru

Troops ofSaimiri (squirrel monkeys) were observed in 31 locations in natural habitats in Panama, Colombia, Brazil, and Peru. Troop size varied from 10 to 35 animals in Panama and Colombia and from 120 to 300 or more in the unaltered rainforests of Amazonia. Troop size correlated with forest size in all areas.Saimiri associated withCebus (capuchin monkeys) in several areas.Saimiri troops showed different responses to man, fear and flight or neutral responses or curiosity; and these correlated with local environmental conditions. Social behavior varied significantly between the large and small troops. Social interactions were much more frequent and individual distances were much smaller in the large troops than in the small troops. Several factors that may relate to these differences in sociability are discussed in light of data from this and other studies.     

Evolutionary Development in Australopithecus africanus

Evolutionary developmental biology is quickly transforming our understanding of how lineages evolve through the modification of ontogenetic processes. Yet, while great strides have been made in the study of neontological forms, it is much more difficult to apply the principles of evo-devo to the miserly fossil record. Because fossils are static entities, we as researchers can only infer evolution and development by drawing connections between them. The choices of how we join specimens together??juveniles to adults to study ontogeny, taxon to taxon to study evolution??can dramatically affect our results. Here, I examine paedomorphism in the fossil hominin species Australopithecus africanus. Using extant African apes as proxies for ancestral hominin morphology, I demonstrate that Sts 71 is most similar to a sub-adult African ape, suggesting that A. africanus is paedomorphic relative to the presumed ancestral form. I then plot ontogenetic size and shape in extant great apes, humans, and A. africanus in order to assess patterns of ontogenetic allometry. Results indicate that ontogenetic allometry in A. africanus, subsequent to M1 occlusion is similar to that in modern humans and bonobos; gorillas, chimpanzees, and orangutans share a different pattern of size-shape relationship. Combined with results from the analysis of paedomorphism plus knowledge about the developmental chronologies of this group, these findings suggest that paedomorphism in A. africanus arises relatively early in ontogeny.     

Allometric relations in three species of finches (Aves: Fringillidae)

Static nestling, adult and ontogenetic allometry were analysed in three species of finches. Static nestling allometry was very similar across age in early ontogeny and among species and could be approximated by a single matrix of phenotypic variances and covariances. The first eigenvector of this matrix showed negative allometry of bill and tarsus to mass, but positive for wing length to mass. Adult static allometry was also very similar among species, but differed from nestling pattern. In adults the bill had a positive allometry in relation to tarsus and wing, but negative to mass, while tarsus and wing were unrelated to mass. The ontogenetic allometry in each species was very similar to nestling static allometry. Viewed in relation to final size, bill characters grew more slowly than body characters, but for a longer time, which created the difference between adult and nestling allometric patterns. There were differences among species both with regard to elevation and slope of allometric coefficients, suggesting that the differences among species came about by changes in the three fundamental ontogenetic parameters namely growth rate, onset of growth and offset of growth.     

Post-natal ontogeny of the mandible and ventral cranium in <Emphasis Type="Italic">Marmota</Emphasis> species (Rodentia,Sciuridae): allometry and phylogeny

Post-natal ontogenetic variation of the marmot mandible and ventral cranium is investigated in two species of the subgenus Petromarmota (M. caligata, M. flaviventris) and four species of the subgenus Marmota (M. caudata, M. himalayana, M. marmota, M. monax). Relationships between size and shape are analysed using geometric morphometric techniques. Sexual dimorphism is negligible, allometry explains the main changes in shape during growth, and males and females manifest similar allometric trajectories. Anatomical regions affected by size-related shape variation are similar in different species, but allometric trajectories are divergent. The largest modifications of the mandible and ventral cranium occur in regions directly involved in the mechanics of mastication. Relative to other anatomical regions, the size of areas of muscle insertion increases, while the size of sense organs, nerves and teeth generally decreases. Epigenetic factors, developmental constraints and size variation were found to be the major contributors in producing the observed allometric patterns. A phylogenetic signal was not evident in the comparison of allometric trajectories, but traits that allow discrimination of the Palaearctic marmots from the Nearctic species of Petromarmota are present early in development and are conserved during post-natal ontogeny.     

Cranial ontogeny of Papio 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.