title: Perspectives in perissodactyls.
abstract: The world-wide foaling rate in the mare is low and in the Hannoverian breed has remained asymptotic at about 50 per cent for over 150 years. In an attempt to discover some of the possible reasons for this low fertility rate, the evolutionary history of the perissodactyls, the odd-toed ungulates, is reviewed here. Of the 156 genera of this order, 152 are in the fossil record; this gives them the highest extermination rate of all the mammal groups. Some of the unusual and unique features of the reproductive physiology of the mare and their possible association with low fertility are considered.
mesh_terms: D000818:Animals; D005075:Biological Evolution; D005298:Fertility; D006736:Horses / Q000502:physiology; D010527:Perissodactyla / Q000502:physiology*; D012726:Sexual Behavior, Animal