Organisms are organized into suites of anatomical structures that typically covary when developmentally or functionally related, and this morphological integration plays a determinant role in ...evolutionary processes. Artificial selection on domestic species causes strong morphological changes over short time spans, frequently resulting in a wide and exaggerated phenotypic diversity. This raises the question of whether integration constrains the morphological diversification of domestic species and how natural and artificial selection may impact integration patterns. Here, we study the morphological integration in the appendicular skeleton of domestic horses and donkeys, using three-dimensional geometric morphometrics on 75 skeletons. Our results indicate that a strong integration is inherited from developmental mechanisms which interact with functional factors. This strong integration reveals a specialization in the locomotion of domestic equids, partly for running abilities. We show that the integration is stronger in horses than in donkeys, probably because of a greater degree of specialization and predictability of their locomotion. Thus, the constraints imposed by integration are weak enough to allow important morphological changes and the phenotypic diversification of domestic species.
Hybridization has been widely practiced in plant and animal breeding as a means to enhance the quality and fitness of the organisms. In domestic equids, this hybrid vigor takes the form of improved ...physical and physiological characteristics, notably for strength or endurance. Because the offspring of horse and donkey is generally sterile, this widely recognized vigor is expressed in the first generation (F1). However, in the absence of recombination between the two parental genomes, F1 hybrids can be expected to be phenotypically intermediate between their parents which could potentially restrict the possibilities of an increase in overall fitness. In this study, we examine the morphology of the main limb bones of domestic horses, donkeys and their hybrids to investigate the phenotypic impact of hybridization on the locomotor system. We explore bone shape variation and covariation to gain insights into the morphological and functional expressions of the hybrid vigor commonly described in domestic equids.
Our data reveal the occurrence of transgressive effects on several bones in the F1 generation. The patterns of morphological integration further demonstrate that the developmental processes producing covariation are not disrupted by hybridization, contrary to functional ones.
These results suggest that an increase in overall fitness could be related to more flexibility in shape change in hybrids, except for the main forelimb long bones of which the morphology is strongly driven by muscle interactions. More broadly, this study illustrates the interest of investigating not only bone shape variation but also underlying processes, in order to contribute to better understanding how developmental and functional mechanisms are affected by hybridization.
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.
The potential of artificial selection to dramatically impact phenotypic diversity is well known. Large-scale morphological changes in domestic species, emerging over short timescales, offer an ...accelerated perspective on evolutionary processes. The domestic horse (Equus caballus) provides a striking example of rapid evolution, with major changes in morphology and size likely stemming from artificial selection. However, the microevolutionary mechanisms allowing to generate this variation in a short time interval remain little known. Here, we use 3D geometric morphometrics to quantify skull morphological diversity in the horse, and investigate modularity and integration patterns to understand how morphological associations contribute to cranial evolvability in this taxon. We find that changes in the magnitude of cranial integration contribute to the diversification of the skull morphology in horse breeds. Our results demonstrate that a conserved pattern of modularity does not constrain large-scale morphological variations in horses and that artificial selection has impacted mechanisms underlying phenotypic diversity to facilitate rapid shape changes. More broadly, this study demonstrates that studying microevolutionary processes in domestic species produces important insights into extant phenotypic diversity.
The identification of domestic equid remains is a recurrent issue and an intense subject of discussion in zooarchaeological studies. Indeed, despite historical sources describing the key role of ...equids in numerous past societies, their accurate identification on archaeological sites is still problematic, and only few methods have been developed in order to distinguish the bones of horses, donkeys and their hybrids. Moreover, some of the extant published visual macroscopic criteria are considered as possibly unreliable, partly because of the absence of preliminary test on a large sample of modern specimens. In this work, we try to solve these issues by testing a set of macroscopic visual criteria, collected in the literature or newly described, on a comparative sample of 107 modern skeletons of domestic equids. We quantified the reliability of these criteria and found evidence of 26 osteological characters allowing for the identification of between 90% and 100% of the horses and donkeys of our comparative sample. A method to identify the complete or sub-complete skeletons of hybrids is also proposed using combinations of characters observed on several bones. Finally, the defined osteological criteria are observed on a set of archaeological skeletons, coming from antique to modern sites, in order to demonstrate the applicability of our approach to archaeological remains. The use of our methodology on zooarchaeological samples could allow for a better assessment of the presence of donkeys and hybrids in archaeological sites, and thus, could help improve the knowledge of their respective importance and use by human past societies.
•We studied a large sample of domestic equid skeletons to search for identification criteria of their bones.•26 osteological characters allowing reliable identification of horse and donkey bone remains are retained.•A combination of horse and donkey character states allows for the identification of hybrid archaeological skeletons.•Morphological characters allow for the reliable identification of equid remains in cases where other methodologies cannot be applied.
The first evidence for the domestication of donkeys (Equus asinus) dates back to at least 6000-5000 BP in Northeast Africa, and their dispersion is attributed to the ancient Romans. Latin authors ...described donkeys as being particularly suitable for the transport of goods and farm work. In addition, they were also bred to produce prized hybrids, particularly mules, which were perfectly adapted to the long-distance transport of people and goods. However, although the historical sources extensively describe their economic importance, both donkey and hybrid remains are surprisingly scarce in the archaeological record. This apparent contradiction is probably due to the difficulties involved in correctly identifying their bones: relatively few bones displaying morphological and metrical criteria can be used for identification, so it is often based purely on bone size. The aim of this study, therefore, is to propose solutions to identify domestic equid bones using 3D geometric morphometrics on isolated and combinations of anatomical elements. A set of 3D coordinates were registered on the 18 main skull and limb bones of 111 modern reference specimens (i.e. 42 horses, 44 donkeys and 25 hybrids). In this paper, we present the classification rate obtained on this reference sample using the k-Nearest Neighbors algorithm. The application of this method on archaeological skeletons from Roman to modern sites is also presented. The percentage of correctly classified specimens was between 77% and 95% for all 18 bones, and higher than 80% for 10 of the fragmentation patterns we defined. Using a combination of several bones enabled us to increase the rate of correct reclassification to a maximum of 97%. The application to archaeological skeletons proved the ability of this method to identify domestic horses and donkeys from archaeological samples. Correspondingly, some bones, and especially combinations of bones, provided good rates to identify hybrids. This method has proved reliable in detecting the presence of donkeys and hybrids from the archaeological samples of equid bones, and should enrich our knowledge regarding their spread across Europe.
•A 3D geometric morphometrics method to identify archaeological equid bones is proposed.•An identification method adapted to fragmented bones is proposed.•Using a combination of several bones increases the reliability of the classification.
Organisms are organized into suites of anatomical structures that typically covary when developmentally or functionally related, and this morphological integration plays a determinant role in ...evolutionary processes. Artificial selection on domestic species causes strong morphological changes over short time spans, frequently resulting in a wide and exaggerated phenotypic diversity. This raises the question of whether integration constrains the morphological diversification of domestic species and how natural and artificial selection may impact integration patterns. Here, we study the morphological integration in the appendicular skeleton of domestic horses and donkeys, using three-dimensional geometric morphometrics on 75 skeletons. Our results indicate that a strong integration is inherited from developmental mechanisms which interact with functional factors. This strong integration reveals a specialization in the locomotion of domestic equids, partly for running abilities. We show that the integration is stronger in horses than in donkeys, probably because of a greater degree of specialization and predictability of their locomotion. Thus, the constraints imposed by integration are weak enough to allow important morphological changes and the phenotypic diversification of domestic species.
The Merovingian period sees the emergence of the saddle horse as a critical component of the military forces. This increased importance is generally thought to be associated with a particular ...attention paid to horse breeding, which contributed to preserve the large size they acquired during Roman period. In spite of the Early Middle Ages being considered as a turning point in the history of their usage in Western Europe, its impact on the morphological traits of the horses remain largely unexplored. Privilege of the warrior elite, the horse is also a marker of social rank when associated to human burials. The royal tomb of Childeric the 1st (Tournai, Belgium), considered as the most lavish of the western Germanic world, revealed the presence of twenty-one skeletons of equids, deposited in peripheral pits. Here we use 3D geometric morphometrics to characterize the morphological traits and possible function of these equids using shape variation and covariation analyses of the limb bones. We found close phenotypic proximities with modern racehorse specimens. However, differences in magnitude of morphological integration between them suggest specificities in the functional features of the archaeological horses and probably reflect variation in the degree of artificial selection. Our results demonstrate the importance of accurately exploring the complete shape of bones to describe the morphological characteristics of past animals. Our results also illustrate the interest of not only investigating shape variation, but also covariation patterns, in order to address functional questions and in trying to detect possible markers of artificial selection on past horses.
•A method for studying the morpho-functional features of past horses is proposed.•Bone shape variation and covariation of the horses from Childeric's tomb are studied.•These war-horses display functional specificities which do not exist in extant horses.•Shape covariation between bones can reflect artificial selection in past horses.
PDF
