Odd toes unite horses, rhinos, and vanished giants
Horses, rhinoceroses and tapirs look nothing alike, yet they form one order, the odd-toed ungulates, with about seventeen living species. The group was far richer in the past and included Paraceratherium, a hornless rhino relative that ranks among the largest land mammals ever.
The name comes from Greek words for odd and toe, and it was the nineteenth-century zoologist Richard Owen who first linked the three families and coined it. In every species the axis of each foot runs through the third toe, always the biggest. Tapirs, suited to soft ground, walk on four front toes and three hind ones; rhinos have three on each foot; and modern horses stand on a single toe almost entirely wrapped in hoof, which limits limb mobility but aids speed. A distinctive ankle joint restricts foot movement, the collarbone is missing, and rhino horns are compacted keratin with no bony core.
Inside, all are hindgut fermenters. Food passes a simple stomach into an enlarged caecum, holding up to 90 litres in horses, where microbes begin breaking it down, and a horse’s small intestine can stretch 26 metres. There is no gallbladder. A small pouch off the tube draining the middle ear, the guttural pouch, is unique to the order and a serious infection risk in horses.
Lacking the heat-exchange network that lets even-toed ungulates keep the brain cool, odd-toed ungulates cope poorly with harsh seasonal climates and stick to places rich in food and water. From the Eocene to the Oligocene they spread over most of the world except Australia and Antarctica; today they survive only in parts of the Americas, Africa and Asia, and horses and tapirs reached South America only after Panama’s land bridge formed about 3 million years ago.
Breeding is slow. Pregnancy lasts 330 to 500 days, longest in rhinos, and usually yields one well-developed baby; foals can follow their mothers within hours. Rhinos take eight to ten years to mature. Extinct relatives included brontotheres and chalicotheres, and Paraceratherium may have reached 15 to 20 tonnes.
Source: Perissodactyla
