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Although superficially these animals might not seem related, they all belong to the order Perissodactyla, or odd-toed ungulates. The three families included (Equidae, Rhinocerotidae, and Tapiridae) have a total of 17 species between them, including the horse, donkey, ass, zebra, tapir, and rhinoceros. They are a beautiful example of related animals evolving to adapt to different lifestyles, this case animals respectively living in open areas such as grasslands and steppes (horses), dry savannas, and in Asia, wet marsh or forest areas (rhinos), and mainly tropical rainforests (tapirs).
The British zoologist Richard Owen recognised that all these animals were closely related and coined the name for this order. Their most easily recognisable shared trait is the odd number of toes, but another remarkable shared trait is that they are all hindgut fermenters. They store digested food that has left the stomach in an enlarged part of the digestive tract called the cecum (in humans this is the part at the beginning of the large intestine to which the appendix is connected) where it is digested by bacteria.