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The monodactyl horses of the genus Equus originated in North America during the Pliocene, and from the beginning of the Pleistocene, they have been an essential part of the large ungulate communities of Europe, North America and Africa. Understanding how body size of Equus species evolved and varied in relation to changes in environments and diet thus forms an important part of understanding the dynamics of ungulate body size variation in relation to Pleistocene paleoenvironmental changes. Here we test previously published body mass estimation equations for the family Equidae by investigating how accurately different skeletal and dental measurements estimate the mean body mass (and body mass range) reported for extant Grevy's zebra (Equus grevyi) and Burchell's zebra (Equus quagga). Based on these tests and information on how frequently skeletal elements occur in the fossil record, we construct a hierarchy of best practices for the selection of body mass estimation equations in Equus. As a case study, we explore body size variation in Pleistocene European Equus paleopopulations in relation to diet and vegetation structure in their paleoenvironments. We show a relationship between diet and body size in Equus: very large-sized species tend to have more browse-dominated diets than small and medium-sized species, and paleovegetation proxies indicate on average more open and grass-rich paleoenvironments for small-sized, grazing species of Equus. When more than one species of Equus co-occur sympatrically, the larger species tend to be less abundant and have more browse-dominated diets than the smaller species. We suggest that body size variation in Pleistocene Equus was driven by a combined effect of resource quality and availability, partitioning of habitats and resources between species, and the effect of environmental openness and group size on the body size of individuals.

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Resource competition and niche partitioning among the exceptionally high number of sympatric ungulates of the Early Pleistocene site of Coste San Giacomo (Central Italy) is here examined through the study of their dietary proclivities and body size. The main aim of this study is to investigate the niche differentiation mechanisms that let the fossil ungulates coexist in the same region. We also provide information about the complementarity of two different methodologies that observe diet variation at a different time scales (inner and outer mesowear) in the study of dental wear patterns of fossil ungulates. Results from analyses of dental wear degree and body masses predictions show that a wide range of feeding behaviours were adopted by the taxonomical groups (i.e., cervids, bovids and equids) in order to avoid competition. Among larger ungulates diet ranges from strict browsing (Eucladoceros sp., Gazellospira torticornis), to mixed feeding (Gallogoral meneghinii, Leptobos sp.) to pure grazing (Equus stenonis), whereas smaller taxa are more selective feeders (Axis cf. lyra, Croizetoceros cf. ramosus) with only one exception (Gazella borbonica). When taxa with the same feeding behaviour occurred in the same habitat, competition was minimised by differences in body size.