EVOLUTIONARY STRATEGY OF EQUIDAE AND ORIGINS OF RUMEN AND CECAL DIGESTION

The so-called inefficiency of the perissodactyls, resulting from the evolution of a cecal site of fermentation, arises because the strategy of selecting fibrous herbage developed early in their evolution. The ruminant artiodactyls did not adopt this sort of diet until they were sufficiently large in body size for a rumen fermentation site to be physiologically possible. Body size at the time of the adoption of a fibrous diet is the critical factor in determining the type of digestive system that will be evolved. Cecal digestion is in fact the superior adaptation for dealing with high fiber content herbage, provided that intake is not limited by the actual quantity of herbage available. The Equidae continued to be successful in the face of the radiation of ruminant artiodactyls because they could maintain themselves on herbage more fibrous than could be tolerated by a ruminant of similar body size.