Grevvy's zebra, plains zebra, and mountain zebra, are three distinct species of zebras. These handsome striped African equids (horse relatives) are threatened; some are extinct: Burchell's, quagga, North American. The zebras' striped patterns are unique to each animal with the greatest differences on the shoulders. Although herds of mixed species occur, interbreeding is not apparent. Well-adapted to extract sustenance from low quality forage, their digestive tracts range from 11 to 17 times their body length.