An Equid Primer
by Robin Meadows
The horse family (Equidae) has nine species: two horses, four asses, and three zebras. Their next closest relatives are rhinoceroses and tapirs, the other odd-toed hoofed mammals. While found only in parts of Africa and Asia today, a great diversity of equids was abundant on all continents except Australia and Antarctica until the Pleistocene ended 10,000 years ago.
During that period, all equids died out in North America for unknown reasons; elsewhere, climate changes may have contributed to the decline of equids. The horse family is best adapted to relatively marginal habitats like savannas because these species can eat nutrient-poor, high-fiber vegetation such as tough grasses, bulbs, and rhizomes. To compensate for the low quality of their food, equids spend up to 80 percent of their time foraging, both day and night.
Highly social, equids communicate primarily by sound. In horses and plains zebras, mothers nicker to warn their foals of danger and whinny when they are separated from each other. Males squeal when about to fight for a female. While males are very competitive and may bite each others' necks and kick each others' faces with their hind legs, female groups typically engage in amicable activities such as grooming each other. However, females do have dominance hierarchies and higher-ranked mares get to graze the choicest vegetation and drink first.
Equids have two basic social systems. In the first, dominant males live with harems of females that often remain in the same group throughout their adult lives. "Extra" males live in bachelor groups. The area occupied by a particular dominant male and his harem can overlap with areas occupied by other groups. This stable social system is found in domestic horses and takhi, and plains and mountain zebras.
In contrast, the system found in Grevy's zebra and the two species of asses has fluid relationships. Adults rarely stay together for more than a few months and most adult males live alone in large territories marked with piles of dung. A male mates with any females in estrus that choose to enter his territory.
The other equids besides takhi (or Przewalski's horse) and domestic horses are the:
--African ass (Equus asinus): endangered; native to the rocky deserts of North Africa; now found only in arid areas of Ethiopia and Somalia.
--Asiatic ass (Equus hemionus): endangered; native to the arid deserts of Central Asia and the Near East; now found only in southern Mongolia and the adjacent area of China.
--Kiang (Equus kiang): endangered; limited to small areas in India, Tibet, China, and Nepal.
--Onager (Equus onager): endangered; once ranged widely from parts of Russia and the Ukraine south to northern Iraq and east to Pakistan. Now lives only in isolated populations in northwest India, Turkmenistan, and central Iran. Re-established in one site in Uzbekistan.
--Grevy's zebra (Equus grevyi): endangered, about 1,500 in the wild; native to the steppes and arid grasslands of East Africa; now found primarily in parts of Ethiopia, Somalia, and Kenya; threatened by hunting for its hide, which is the most beautiful of the three zebra species.
--Mountain zebra (Equus zebra): threatened, about 13,000 in the wild; native to arid mountain grasslands of southwestern Africa.
--Plains zebra (Equus burchelli): native to the grasslands and savanna of East Africa; now found primarily in protected areas such as the national parks and reserves from Kenya to the Cape.
During the 1880s, a yellowish-brown South African zebra called the quagga (Equus quagga) became extinct. Conservationists hope to save the remaining wild equids from a similar fate.
--Robin Meadows
Robin Meadows is a contributing editor to ZooGoer.
(ZooGoer 26(5) 1997. Copyright 1997 Friends of the National Zoo. All rights reserved.)
