Odd-toed ungulates: horses, tapirs and rhinoceroses

Three families of quite different animals belong to the odd-toed ungulates. These are the horses, donkeys and zebras, together called equids on this page; the tapirs, which resemble pigs; and the rhinoceroses. They share one feature, their odd number of toes: horses have one, rhinoceroses three, and tapirs four on the forelegs and three on the hind legs. They are all hindgut fermenters, digesting a large part of their food behind the stomach. All of them have large ears. Seven species of equids, four tapirs and five rhinoceroses are distinguished.

Behaviour

Equids

The horse is discussed in detail on the page horse – pig – cow. Horses, donkeys and zebras resemble each other in many ways. All are trotting animals with long legs. Zebras populate the plains of Africa in large numbers, horses come from the plains of Asia and Europe, and donkeys live in small herds in the semi-deserts of Asia and Africa. They are closely related and can interbreed: a horse and a donkey produce a mule, a zebra and a horse a zorse, sometimes called a hebra. The offspring is infertile.

Horses and donkeys have been domesticated; with zebras this never succeeded. There is much aggression within zebra groups, which makes them hard to keep, and they accept no leader, least of all one that is not a zebra.

In sudden danger horses flee headlong. Donkeys stay where they are until they have assessed the situation and chosen the best escape route. This matters, because a donkey cannot run as fast as a horse. When the danger is small, a door slamming or something glinting in the sun, the donkey stays where it is. If the danger is real, it runs. Donkeys also put their feet down with more confidence than horses. Horses accept orders; donkeys need to be asked.

Tapirs

Tapirs look like big pigs with long legs. They live in tropical forest, mostly near water such as rivers and swamps. They spend much of their time in the water and are excellent swimmers. When threatened, some species go into the water, where they can dive and stay under for several minutes; others plunge into the undergrowth and defend themselves by biting. The Malayan tapir can walk along the bottom of a river, just like the hippopotamus. Tapirs may mate in or out of the water. They are mainly active at night and live solitary lives.

In the water their digestive activity increases, and tapirs drop their dung in or next to water, again just like hippopotamuses. They are good climbers of steep banks and mountain slopes. Tapirs follow fixed paths, which they mark with urine.

Rhinoceroses

Rhinoceroses live mainly solitary lives. The large white rhinoceros is the most social species and lives in small groups. The Asian species can behave aggressively, whereas the African species tend to avoid encounters. The white rhinoceros in particular is a good-natured, withdrawn animal. The black rhinoceros may charge at people when it smells them and stop a few metres short, apparently because only then does it see what it was charging at.

Where rhinoceroses live, dung heaps lie along the paths the animals use. They drop some dung every time they pass, and even when fleeing they take time for this.

Physical characteristics

Body of the equids

Horses, donkeys and zebras share the same body shape: the head is carried high, the neck is long and bears a mane, and the legs are long, straight and slender. The hindquarters are a little higher than the middle of the back. There are differences: horses have a relatively heavy head and a thicker neck.

Wild donkeys are lighter than the Przewalski horse, but just as long. They are also more slender, the neck included, and the head seems to be carried higher. The legs are a little longer and straighter. The donkey's mane is relatively short.

The zebra has shorter legs than the horse and a more thick-set body that looks more massive. Its body is longer than a horse's and lighter (with the exception of Grevy's zebra). The head is longer and more slender than the horse's. The mane is longer than the donkey's.

The zebra's mane is harder and stiffer than the donkey's and softer and thinner than the horse's. From donkey via zebra to horse, the mane goes from soft to hard, from flexible to stiff and from thin to thick.

Body of the tapirs

Tapirs have a massive, thick-set body; the back is a little higher than the shoulders, and the line of the back is arched. There is no distinct neck, but a gradual transition from body to head. The body is agile and mobile. Tapirs are about 2 metres long and 1 metre high and weigh 225 to 300 kilograms. Some species have a short mane on the neck.

The legs are short and stout compared with those of horses. The front legs have four toes and the hind legs three, of which the middle one is the largest. All toes carry hooves, and each foot has a callused pad that supports part of the weight, so tapirs do not walk on their hooves alone.

The upper lip is developed into a short, fleshy, prehensile proboscis, with which the tapir sniffs its way through the forest and pulls twigs towards its mouth.

Body of the rhinoceroses

Rhinoceroses, weighing 800 to 3500 kilograms, have a large, elongated body. They are up to 4 metres long and 2 metres high, and the body seems especially long. The larger species have high shoulders, so the front of the body is higher than the back. The neck is rather long and thick. The head is heavy and measures up to a third of the total body length. It is held low, beneath the line of the back; while walking, the mouth is only a few centimetres from the ground. In most species the skin is thick and nearly hairless. The skin of the Indian rhinoceros forms huge folds over its body, as if it were armoured.

The legs are short and muscular and end in three hooves. The middle hoof is larger than the other two and carries most of the weight. Despite their weight and short legs, rhinoceroses can run at up to about 50 km per hour.

On the head, behind the nostrils, there is one horn. The larger species even have two horns, one behind the other. The front horn can be as long as 1.5 metres; the rear one stays shorter. Horns are made of keratin, a product of the epidermis that also forms nails and hair. They keep growing throughout the rhinoceros's life and grow back if cut off.

Fur colour

A horse is uniformly brown, with a lighter underside. Donkeys are lighter in colour than horses, with more white on the underside and further up the flanks, with a clear boundary. Both have a white area around the mouth. Donkeys may have stripes on their legs. Zebras are striped. In some species, such as Grevy's zebra, the belly stays white; in the plains zebra the stripes run onto the underside.

Tapirs have a tough skin with short, light- to dark-brown hair. The Malayan tapir is black and white. The young are born with horizontal stripes and spots, like wild boar piglets.

Rhinoceroses are plain grey. The skin is very thick (1.5 to 5 cm) and bare.

Diet

Equids eat young, nutritious grass and leaves, which they bite off with their chisel-like incisors. They spend 60 to 80% of the day eating.

Tapirs are grazers and browsers. They eat grass, water plants, leaves, buds and fruit. They bite off grass as horses do and strip leaves by pulling a twig along their teeth with the proboscis. Occasionally they eat animal food. For hours they follow well-known paths through the vegetation with the proboscis to the ground, searching for food.

Rhinoceroses live on grass (white and Indian rhinoceros), on branches with leaves (black and Javan rhinoceros) and sometimes on fruit (Sumatran rhinoceros). They eat about 2% of their body weight in dry matter per day and drink large amounts of water, up to about 100 litres a day.

Digestive tract

Horses have a large caecum, or blind gut, and colon for fermenting their food. The digestive tract is about 15 times the body length.

Tapirs have a smaller caecum and digest their food mainly in the colon.

Rhinoceroses also digest their food in the colon. The white rhinoceros has a small caecum and an enlarged colon. Its digestive tract is 9 to 10 times its body length.

Teeth

Horses have large incisors; mares usually have no canines, stallions small ones. Male zebras have pointed canines. The molars are large and ridged.

The teeth of tapirs resemble those of horses but are not specialised for grazing. They are low-crowned. The incisors are chisel-like, the outer upper incisors are long, and the canines are conical, relatively long compared with those of horses, and stand close to the incisors.

The large African rhinoceroses lack incisors and canines. The Asian species do have incisors; in the Indian rhinoceros the lower ones form sharp tusks used in fights. All species have large molars.

Senses

Donkeys have larger eyes and longer ears than horses, and they taste small differences in their food. The ears of zebras are rounder than those of horses. All equids have a sensitive skin.

The sense of smell of tapirs is good and their hearing somewhat less developed. The eyes are small and their vision is poor.

Rhinoceroses have poor vision. At a distance of about 30 metres they cannot make out a person standing upright. Because their eyes sit at the sides of the head, they have to turn the head to see what is in front of them. Hearing is well developed: they locate sounds by turning their movable, funnel-shaped ears. The skin is not very sensitive. The most important sense organ is the nose; their sense of smell is good.

Characteristics

Equids

Among the odd-toed ungulates the equids are the animals with long, straight limbs. They have the most specialised legs, with a single hoof. They are animals of open plains that trot and run a great deal. They eat young, relatively nutritious grass, bitten off with their chisel-like incisors. Their sense organs are large too. Equids have the most powerful lower jaw, which in this approach is regarded as the limb part of the skull; the page The threefold human being explains this.

Among these animals the donkeys have the whitest underside and the largest sense organs; they can taste food well and seem to observe and analyse a situation before acting. They have the most developed nerve-sense system.

Zebras cannot be kept, behave aggressively, and the males have sharp canines. They show the characteristics of the heart-lung system, and that could also explain their stripes, since stripes and spots on the coat are a feature of carnivores and heart-lung animals in general.

Horses have less white on the underside and are sturdier. Horses and donkeys are able to learn. Horses show the characteristics of the metabolic-limb system.

Tapirs

Tapirs resemble pigs in body and behaviour; they have a plump body and a prehensile proboscis. They have relatively large canines and eat, besides grass and leaves, fruit and occasionally animal food. The young are striped.

Rhinoceroses

Rhinoceroses are enormous animals, up to 4 metres long and 3500 kilograms in weight. They have a long body with a long head carried below the line of the back. The sense organs are small and the eyes poorly developed. They eat great quantities of grass that is low in nutrients.

Three types within one group

• The equids are the nerve-sense animals of this group: the head is carried high above the back, the sense organs are large, they can learn, the legs are long, and they eat relatively nutritious young grass.
• The tapirs are the heart-lung animals, with their pig-like appearance, relatively large canines, a diet that includes some fruit and animal food, and striped young.
• The rhinoceroses are the metabolic-limb animals, with their massive, long body, low-carried head, modest senses and low-quality food.

The equids have developed their limbs the most, the rhinoceroses their metabolism. It is as if the limb process, which produces straight, long forms, works through the whole body of the rhinoceros and gives it its elongated shape.

The table sums up the features of the three families of odd-toed ungulates.

Comparison with the even-toed ungulates

The odd-toed ungulates are best understood next to their counterparts, the even-toed ungulates, which carry their weight on two toes. Among the even-toed animals the ruminants, such as the cow, digest their food in a stomach of several compartments before it reaches the intestines, whereas horses, tapirs and rhinoceroses do most of the work behind the stomach. The pigs, peccaries and hippopotamuses form a third branch, described on the page Suina: peccaries, pigs and hippopotamuses. Their resemblance to the tapirs is striking: plump bodies, a life close to water, dung dropped in the water, striped young. Yet their feet and their digestion show that they belong to a different lineage.

The page mouse – lion – cow applies the same threefold reading to three whole orders of mammals. Within the odd-toed ungulates the pattern appears again on a smaller scale: a group can be read as a whole with a head, a middle and a limb pole, and each family takes one of those places. A single toe on a horse's foot and a massive horn on a rhinoceros's nose are, in this reading, two ends of one family picture.