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Shape Variation and Modularity of Skull and Teeth in Domesticated Horses and Wild Equids

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Shape Variation and Modularity of Skull and Teeth in Domesticated Horses and Wild Equids

Laura Heck et al. Front Zool.

Abstract

Background: In 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.

Results: Both 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.

Conclusion: This 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.

Keywords: Cranium; Disparity; Domestication; Geometric morphometrics; Modularity.

Conflict of interest statement

Not applicable.Not applicableThe authors declare that they have no competing interests.Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Figures

Fig. 1
Fig. 1
Principal component analysis of a the cranial landmark data of 216 adult equid specimens: zebras (n = 47), donkeys (n = 31), Przewalski’s horses (n = 5), and domesticated horses (n = 133), b Shape changes in dorsal and lateral view for PC 1 and PC 2 with black lines presenting positive shape and dotted, grey lines presenting negative shape, c the mandible landmark data of 224 adult equid specimens: zebras (n = 48), donkeys (n = 33), Przewalski’s horses (n = 2), and domesticated horses (n = 141), d Shape changes in dorsal and lateral view for PC 1 and PC 2 with black lines presenting positive shape and dotted, grey lines presenting negative shape, e of the upper 3rd molar landmark data of 225 adult equid specimens: zebras (n = 42), donkeys (n = 25), Przewalski’s horses (n = 3), and domesticated horses (n = 122), f the upper 2rd premolar landmark data of 225 adult equid specimens: zebras (n = 42), donkeys (n = 25), Przewalski’s horses (n = 3), and domesticated horses (n = 122), g of the lower 3rd molar landmark data of 225 adult equid specimens: zebras (n = 29), donkeys (n = 20), Przewalski’s horses (n = 3), and domesticated horses (n = 92), h of the lower 2rd premolar landmark data of 225 adult equid specimens: zebras (n = 29), donkeys (n = 20), Przewalski’s horses (n = 3), and domesticated horses (n = 92); Symbols are circles: domesticated horses, diamonds: Przewalski’s, triangles: zebras, and squares: donkeys
Fig. 2
Fig. 2
a Scatterplot of calculated values for disparity (x-axis) and integration (y-axis) for all modules and wild horses and domesticated equids separately; b Six cranial modules in a schematic horse skull (black outline) after Goswami [20] shown in lateral and dorsal perspective; Modules are: anterior oral-nasal (AON, red), molar (M, dark green), orbital (ORB, dark blue), zygomatic pterygoid (ZP, light green), cranial base (CB, orange), and cranial vault (CV, black dashed)
Fig. 3
Fig. 3
Examples for skull shapes from lateral view of different domesticated (Draft: Shire, Belgian; Light horse: Arab; Medium horses: Hannoverian, Kladrubian, Engl. Thoroughbred; Ponies: Falabella, Shetland, Icelandic) and wild equids (Przewalski’s horse, Zebra, Donkey)
Fig. 4
Fig. 4
Landmarks on the a lateral b ventral c dorsal d posterior side of the skull and the e dorsal f lateral side of the mandible of a domesticated horse (for a detailed description of the landmarks see Table 3); Landmarks on the g upper 3rd molar h upper 2nd premolar i lower 3rd molar j lower 2nd premolar of a zebra (specimen MfN 70,299) in occlusal view (for a detailed description of the landmarks see Table 3)

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