Shape variation and modularity of skull and teeth in domesticated horses and wild equids

Laura Heck,Laura A B Wilson,Allowen Evin,Allowen Evin,Allowen Evin,Allowen Evin,Allowen Evin,Allowen Evin,Madlen Stange,Marcelo R Sánchez-Villagra

doi:10.1186/s12983-018-0258-9

Abstract

BackgroundIn horses, the morphological changes induced by the process of domestication are reportedly less pronounced than in other species, such as dogs or pigs – although the horses’ disparity has rarely been empirically tested. We investigated shape differences and modularity of domesticated horses, Przewalski’s horses, donkeys and zebras. Mandibular and tooth shape have been shown to be valuable features for differentiating wild and domesticated forms in some mammals.ResultsBoth mandible and teeth, show a pattern of shape space occupation analogous to that of the cranium, with domesticated horses occupying a similar extension in shape space to that of wild equids. Only cranial shape data exhibit a tendency to separate domesticated horses and Przewalski’s horses from donkeys and zebras. Maximum likelihood model-based tests confirm the horse cranium is composed of six developmental modules, as reported for placental mammals in general. The magnitude of integration in domesticated horse skull was lower than in wild equids across all six cranial modules, and lower values of integration were associated with higher disparity values across all modules.ConclusionThis is the first study that combines different skeletal features for the description and comparison of shape changes in all living equid groups using geometric morphometrics. We support Darwin’s hypothesis that the shape variation in the skull of domesticated horses is similar to the shape variation of all wild equid species existing today. Lower magnitudes of module integration are recovered in domesticated horses compared to their wild relatives.

Highlights

In horses, the morphological changes induced by the process of domestication are reportedly less pronounced than in other species, such as dogs or pigs – the horses’ disparity has rarely been empirically tested
The shape change along principal components (PCs) 1 from negative to positive is dominated by a narrowing and straightening of the skull in combination with an elongation of the occipital-parietal region, represented by the cranial vault (Fig. 1b)
The results of the mevolCVP function suggested the reduction of the dataset to the first three PCs in all further analyses for the cranial data

Summary

Introduction

The morphological changes induced by the process of domestication are reportedly less pronounced than in other species, such as dogs or pigs – the horses’ disparity has rarely been empirically tested. We investigated shape differences and modularity of domesticated horses, Przewalski’s horses, donkeys and zebras. After being on the verge of extinction, domestication made horses one of today’s most common large animal species [1]. All living species of equids belong to the genus Equus, which is divided into the caballine taxa, including domesticated horses (H) and Przewalski’s horses (P), and non-caballine taxa, comprising the different donkey (D) and zebra species (Z). The Przewalski’s horses likely represent the sistertaxon to the extinct wild ancestor of domesticated horses [2, 3]. Like other domesticated species, have been shaped into diverse morphological types through artificial selection to fit specific functions, such as agricultural work, racing, or leisure

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