The relationships between the different component parts of organisms, such as the sharing of common development or function, produce a coordinated variation between the different traits. This ...morphological integration contributes to drive or constrain morphological variation and thus impacts phenotypic diversification. Artificial selection is known to contribute significantly to phenotypic diversification of domestic species. However, little attention has been paid to its potential impact on integration patterns. This study explores the patterns of integration in the limb bones of different horse breeds, using 3D geometric morphometrics. The domestic horse is known to have been strongly impacted by artificial selection, and was often selected for functional traits. Our results confirm that morphological integration among bones within the same limb is strong and apparently partly produced by functional factors. Most importantly, they reveal that artificial selection, which led to the diversification of domestic horses, impacts covariation patterns. The influence of selection on the patterns of covariation varies along the limbs and modulates bone shape, likely due to a differential ligament or muscle development. These results highlight that, in addition to not being constrained by a strong morphological integration, artificial selection has modulated the covariation patterns according to the locomotor specificities of the breeds. More broadly, it illustrates the interest in studying how micro‐evolutionary processes impact covariation patterns and consequently contribute to morphological diversification of domestic species.
Long bone inner structure and cross‐sectional geometry display a strong functional signal, leading to convergences, and are widely analyzed in comparative anatomy at small and large taxonomic scales. ...Long bone microanatomical studies have essentially been conducted on transverse sections but also on a few longitudinal ones. Recent studies highlighted the interest in analyzing variations of the inner structure along the diaphysis using a qualitative as well as a quantitative approach. With the development of microtomography, it has become possible to study three‐dimensional (3D) bone microanatomy and, in more detail, the form–function relationships of these features. This study focused on the selection of quantitative parameters to describe in detail the cross‐sectional shape changes and distribution of the osseous tissue along the diaphysis. Two‐dimensional (2D) virtual transverse sections were also performed in the two usual reference planes and results were compared with those obtained based on the whole diaphysis analysis. The sample consisted in 14 humeri and 14 femora of various mammalian taxa that are essentially terrestrial. Comparative quantitative analyses between different datasets made it possible to highlight the parameters that are strongly impacted by size and phylogeny and the redundant ones, and thus to estimate their relevance for use in form‐function analyses. The analysis illustrated that results based on 2D transverse sections are similar for both sectional planes; thus if a strong bias exists when mixing sections from the two reference planes in the same analysis, it would not problematic to use either one plane or the other in comparative studies. However, this may no longer hold for taxa showing a much stronger variation in bone microstructure along the diaphysis. Finally, the analysis demonstrated the significant contribution of the parameters describing variations along the diaphysis, and thus the interest in performing 3D analyses; this should be even more fruitful for heterogeneous diaphyses. In addition, covariation analyses showed that there is a strong interest in removing the size effect to access the differences in the microstructure of the humerus and femur. This methodological study provides a reference for future quantitative analyses on long bone inner structure and should make it possible, through a detailed knowledge of each descriptive parameter, to better interpret results from the multivariate analyses associated with these studies. This will have direct implications for studies in vertebrate anatomy, but also in paleontology and anthropology.
Among amniotes, numerous lineages are subject to an evolutionary trend toward body mass and size increases. Large terrestrial species may face important constraints linked to weight bearing, and the ...limb segments are particularly affected by such constraints due to their role in body support and locomotion. Such groups showing important limb modifications related to high body mass have been called “graviportal.” Often considered graviportal, rhinoceroses are among the heaviest terrestrial mammals and are thus of particular interest to understand the limb modifications related to body mass and size increase. Here, we present a morphofunctional study of the shape variation of the limb long bones among the five living rhinos to understand how the shape may vary between these species in relation with body size, body mass and phylogeny. We used three dimensional geometric morphometrics and comparative analyses to quantify the shape variation. Our results indicate that the five species display important morphological differences depending on the considered bones. The humerus and the femur exhibit noticeable interspecific differences between African and Asiatic rhinos, associated with a significant effect of body mass. The radius and ulna are more strongly correlated with body mass. While the tibia exhibits shape variation both linked with phylogeny and body mass, the fibula displays the greatest intraspecific variation. We highlight three distinct morphotypes of bone shape, which appear in accordance with the phylogeny. The influence of body mass also appears unequally expressed on the different bones. Body mass increase among the five extant species is marked by an increase of the general robustness, more pronounced attachments for muscles and a development of medial parts of the bones. Our study underlines that the morphological features linked to body mass increase are not similar between rhinos and other heavy mammals such as elephants and hippos, suggesting that the weight bearing constraint can lead to different morphological responses.
Environmental properties, and the behavioral habits of species impact sensory cues available for foraging, predator avoidance and inter/intraspecific communication. Consequently, relationships have ...been discovered between the sensory ecology and brain morphology in many groups of vertebrates. However, these types of studies have remained scare on snake. Here, we investigate the link between endocranial shape and the sensory-related ecology of snakes by comparing 36 species of snakes for which we gathered six sensory-ecology characteristics. We use µCT scanning and 3D geometric morphometrics to compare their endocranium in a phylogenetically informed context. Our results demonstrate that size is a major driver of endocranial shape, with smaller species tending to maximize endocranial volume using a more bulbous shape, while larger species share an elongate endocranial morphology. Phylogeny plays a secondary role with more derived snakes diverging the most in endocranial shape, compared to other species. The activity period influences the shape of the olfactory and optic tract, while the foraging habitat impacts the shape of the cerebellum and cranial nerve regions: structures involved in orientation, equilibrium, and sensory information. However, we found that endocranial morphology alone is not sufficient to predict the activity period of a species without prior knowledge of its phylogenetic relationship. Our results thus demonstrate the value of utilizing endocranial shape as complementary information to size and volume in neurobiological studies.
Morpho‐functional patterns are important drivers of phenotypic diversity given their importance in a fitness‐related context. Although modularity of the mandible and skull has been studied ...extensively in mammals, few studies have explored shape co‐variation between these two structures. Despite being developmentally independent, the skull and mandible form a functionally integrated unit. In the present paper we use 3D surface geometric morphometric methods allowing us to explore the form of both skull and mandible in its 3D complexity using the greater white‐toothed shrew as a model. This approach allows an accurate 3D description of zones devoid of anatomical landmarks that are functionally important. Two‐block partial least‐squares approaches were used to describe the co‐variation of form between skull and mandible. Moreover, a 3D biomechanical model was used to explore the functional consequences of the observed patterns of co‐variation. Our results show the efficiency of the method in investigations of complex morpho‐functional patterns. Indeed, the description of shape co‐variation between the skull and the mandible highlighted the location and the intensity of their functional relationships through the jaw adductor muscles linking these two structures. Our results also demonstrated that shape co‐variation in form between the skull and mandible has direct functional consequences on the recruitment of muscles during biting.
Sauropodomorph dinosaurs include the largest terrestrial animals that ever lived on Earth. The early representatives of this clade were, however, relatively small and partially to totally bipedal, ...conversely to the gigantic and quadrupedal sauropods. Although the sauropod bauplan is well defined, notably by the acquisition of columnar limbs, the evolutionary sequence leading to its emergence remains debated. Here, we aim to tackle this evolutionary episode by investigating shape variation in the six limb long bones for the first time using three‐dimensional geometric morphometrics. The morphological features of the forelimb zeugopod bones related to the sauropod bauplan tend to appear abruptly, whereas the pattern is more gradual for the hindlimb zeugopod bones. The stylopod bones tend to show the same pattern as their respective zeugopods. The abrupt emergence of the sauropod forelimb questions the locomotor abilities of non‐sauropodan sauropodomorphs inferred as quadrupeds. Features characterizing sauropods tend to corroborate a view of their locomotion mainly based on stylopod retraction. An allometric investigation of the shape variation in accordance with size highlight differences in hindlimb bone allometries between the sauropods and the non‐sauropodan sauropodomorphs. These differences notably correspond to an unexpected robustness decrease trend in the sauropod hindlimb zeugopod. In addition to forelimb bones that appear to be proportionally more gracile than in non‐sauropodan sauropodomorphs, sauropods may have relied on limb architecture and features related to the size increase, rather than general robustness, to deal with the role of weight‐bearing.
Our study investigates the limb long bone shape variation of a large sample of sauropodomorph dinosaurs in the context of the emergence of the sauropod bauplan. We highlight a differential emergence of sauropod‐related features, that appear abruptly in the forelimb bones and more gradually in the hindlimb ones. The study of the allometry shows unexpected size‐related differences of variation between non‐sauropodan sauropodmorphs and sauropods in the hindlimb zeugopod.
Beaks are among the few hard parts of coleoid cephalopods and are informative for species identification. Although mandible shape has been shown to be adaptive in many vertebrate taxa, it has been ...suggested that the shape of coleoid beaks does not bear any ecological signal. Yet, previous studies only explored beak shape in 2D and none have provided an in‐depth investigation of the potential relationship with ecological variables such as habitat use or diet.
The goal of the present study was to understand whether variation in cephalopod beak shape reflects ecology and/or is more driven by phylogenetic relatedness as suggested previously.
We imaged 101 lower and 108 upper beaks in 3D using underwater photogrammetry and micro‐CT scanning. Our 3D morphometric analysis conducted on 75 species of cephalopod shows that there is a significant but moderate phylogenetic signal. However, comparative phylogenetically informed analyses demonstrate that beak shape is also driven by ecology.
We detected significant differences in beak shape between species inhabiting different habitats (pelagic, benthic or demersal) and of different trophic levels. Our results further suggest that beak shape variation can be summarized along a continuum between two main functions: fast closing versus hard biting.
These results provide novel insights into the drivers of beak shape diversity in coleoid cephalopods and suggest that beak shape has evolved adaptively in relation to diet and habitat use.
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Read the free Plain Language Summary for this article on the Journal blog.
Abstract
Weight support is a strong functional constraint modelling limb bones in heavy quadrupeds. However, the complex relations between bone shape, mass, size and body proportions have been poorly ...explored. Rhinocerotoidea is one of the groups showing the highest body mass reached by terrestrial mammals through time. Here, we explore the evolutionary variation of shape in hindlimb stylopod and zeugopod bones and its relationship with mass, size and gracility in this superfamily. Our results show that bones undergo a general increase in robustness towards high masses, associated with reinforcements of the main muscle insertions. The shape of the femur, carrying a marked phylogenetic signal, varies conjointly with mass, size and gracility, whereas that of the tibia appears related to gracility and mass only. The shape of the fibula does not vary according to that of the tibia. Moreover, congruent variation of shape between the distal part of the femur and the complete tibia underlines the potentially strong covariation of the elements constituting the knee joint. These results, coupled with those previously obtained from forelimb study, allow a better comprehension of the relationship between bone shape and mass among Rhinocerotoidea, and a refining of the concept of ‘graviportality’ in this superfamily.
The IUCN now recognizes the savannah Loxodonta africana and forest Loxodonta cyclotis elephants to be separate species. Despite ecological, behavioural and morphological differences, and different ...habitat ranges, genetic studies confirm that the two species and hybrids coexist in forest–savannah ecotones. However, the hybrid phenotypes have not yet been described. In this survey we examined whether the phenotypes of the two species and of hybrids can be distinguished. In the first step, we used a machine learning algorithm (K-nearest neighbours) to compare 296 reference images of African elephants from five forest areas and six savannah areas where hybrids have not been recorded, confirming that six morphological criteria can be used to distinguish the species with more than 90% confidence. In the second step, we analysed 1,408 videos of elephants from 14 camera traps in Sebitoli, in Kibale National Park, Uganda, part of the main hybridization area. We used a multiple correspondence analysis and a species assignment key, highlighting the presence of three categories of phenotypes. Compared to the savannah and forest phenotypes (36.8 and 12.1%, respectively), the intermediate phenotypes, which could include hybrids, were more frequent (51.1%). Further studies combining morphology and genetics of the same individuals will be necessary to refine this species assignment key to characterize phenotypes confidently. This non-invasive, fast and inexpensive phenotypical-based method could be a valuable tool for conservation programmes.
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