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Ungulates are the most abundant and diverse group of mammals in arid areas. Non-ruminants, or hind gut fermenters, constitute only three extant families (horses, rhinos and tapirs); ruminants are far more dominant and form the remainder.

Much of perissodactyl evolution occurred during the Eocene, Oligocene and Miocene eras when arid savannah-type conditions prevailed. Adoption of hind gut fermentation as a digestive strategy early on in their evolution confined equids to feeding on coarse grassland, characterized by high fibre and low protein content. Hind gut fermentation of such vegetation, combined with high rates of voluntary food intake and passage of digesta, gave equids an advantage over ruminants of similar body size in digestive efficiency on high fibre grasses. This digestive strategy explains the present day ecological niche of wild equids. The large volume of water required for microbial fermentation of fibrous vegetation constitutes a fluid reservoir which could act as a potential source of fluid for the remaining body fluid pools during periods of dehydration. Evidence for such a role of the gastrointestinal tract in water homeostasis has recently been reviewed for ruminants. In this review, feeding and digestive strategies of ruminants and hind gut fermenters are compared with respect to body size. Evidence for the existence of a gastrointestinal fluid reservoir in equids is then provided. Finally, an overview of the physiological mechanisms associated with the movement of fluid across the hind gut epithelium in equids is given. (C) 1998 Academic Press Limited.