Odd-toed ungulates: horses, tapirs and rhinoceroses

To the odd-toed ungulates belong three famlies of quite different animals. These are the horses, donkeys and zebras (together called equus on this page); the tapirs that resemble pigs, and the rhinoceroses. They have a common feature, i.e. their odd number of toes: horses have 1, rhinoceroses 3 and tapirs have 4 on their forelegs and 3 on their hind legs. They are all hind gut fermenters; digestion is for a large part behind the stomach. All animals have large ears. 7 species of equus, 4 tapirs and 5 rhinoceroses are distinguished.

Behaviour

Equus

On the page horse - pig - cow the horse is discussed in detail. Horses, donkeys and zebras resemble each other in many ways. Horses, donkeys and zebras 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 hebra or a zorse), but the offspring is infertile.

Horses and donkeys have been domesticated, which did not succeed with zebras. There is much internal aggression with zebras, which is why they cannot be kept. Moreover, they accept no leader, especially when it is not a zebra. In imminent danger horses flight headlong. Donkeys stay where they are until they have analysed the (dangerous) situation and chosen the best escape route. This is important because a donkey cannot run as fast as a horse. When the danger is not great (just a door slamming or something that glimmers in the sun) the donkey stays where it is. Is the danger real then it runs away. Donkeys put their feet down with more confidence than horses. Horses accept orders, donkeys need to be asked.

African donkey
Zebra

Tapirs

Tapirs look like big pigs with long legs. They live in the tropical rainforest, mostly in the neighbourhood of water like rivers and swamps. They spend much of their time in the water and are excellent swimmers. When threatened, some species go into the water and they can dive and stay under for some minutes, others dive into the vegetation and defend themselves by biting. The Indian tapir can walk on the bottom of a river, just like the hippopotamus. Tapirs may copulate in - or out of the water. They are especially active during the night. Tapirs are solitary animals.

In the water the digestive activity increases and tapirs let their faeces drop 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.

Brazilian or Lowland tapir with offspring
Mountain tapir - overview

Rhinoceroses

Rhinoceroses live mainly solitary. The big white rhinoceros is the most social species living in small groups. The Asiatic species can behave aggressively, whereas the African species try to avoid contact. Especially the white rhinoceros is a good-natured and introverted animal. The black rhinoceros may charge at people when it smells them, to stop a few meters before them, because then it suddenly sees what it was charging at.

In areas where rhinoceroses live, dungheaps lie along the paths the animals use. They let some droppings fall every time they pass. Even when they are fleeing, they take some time for that procedure.

Black rhinoceros
White rhinoceros

Physical characteristics

Body

Equus

All horses, donkeys and zebras have the same bodyshape: the head is high, the neck is long with a mane and the legs are long, straight and slender. The hind part is somewhat higher then the middle of the back. There are some differences: horses have a relatively heavy head and the neck is thicker.

Wild donkeys are less heavy than the Przewalski horse, but just as long. Compared to the Przewalski horse, donkeys are also more slender, which applies to the neck as well. It seems as if the head is kept higher. The legs are a bit longer and the legs are straighter. The mane of the donkey is relatively shorter.

The zebra has shorter legs than the horse and the body is more thick-set, it seems more massive. Its body is longer than the horse's body and weighs less (with the exception of the Grevy-zebra). The head is longer and more slender than the horse's. The mane is longer than the donkey's.

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

Tapirs

Tapirs look like big pigs with long legs. Their body is massive and thick-set; the back is a bit higher than the shoulders. The backline is arched. There is no distinct neck, but there is some transitional form from body to head. The body is agile and mobile. Tapirs are about 2 meters long and 1 meter high and weigh 225 to 300 kilos. Some species have a mane in the neck.

The legs are short and stout (compared to horses). The front legs have 4 and the hind legs have 3 toes, of which 1 has grown bigger. All toes have hooves and each foot has a callous cushion, that supports part of the weight; they do not walk on the hooves only.

The upperlip is developed into a short, fleshy, prehensile proboscis, with which they sniffingly seek their way through the rainforest and pull twigs towards them.

Rhinoceroses

Rhinoceroses (800 – 3500 kilo) have a big elongated body. They are up to 4 meters long and 2 meters high. The body seems especially long. The bigger species have a high shoulder and the anterior part of the body is higher than the posterior part. The neck is rather long and thick. The head is heavy and measures up to 1/3 of the total body length. The head is kept low and is under the backline. While walking, the mouth is only a few centimeters from the ground. The skin is thick and hairless with most species. The skin of the Indian rhinoceros makes huge creases over its body, as if it is armoured.

The legs are short and muscular and have 3 hooves. The middle hoove is larger than the other two and carries most of the weight. Despite their weight and short legs, they can run up to 50 km per hour.

On the head, in the middle behind the nose, is one horn. The biggest species even have two horns behind each other. The anterior horn can be as long as 1.5 meters, de posterior stays shorter. Horns are made of keratin, a formation of the epidermis, which also makes nails and hair. Horns keep growing during the rhinoceros's whole life. If cut off, they grown back on again.

Fur colour

A horse is uniformly brown, the underside is lighter. The colour of donkeys is lighter than that of horses with more white on the underside and farther up on the sides with a clear boundary. Both have a white area around the mouth. Donkeys may have stripes on their legs. Zebras are striped. Only the Grevy-zebra has stripes on the underside as well.

Tapirs have a tough skin with short light- to dark-brown hair. The Indian tapir is black-white.
Young animals are born with horizontal stripes, like wild boar.

Rhinoceroses are solid grey. The skin is very thick (1.5 - 5 cm) and bald.

Diet

Equus eat young, highly nutricious grass and leaves. They bite it off with their chisel-like incisors. They eat 60-80% of the day.

Tapirs are grazers and browsers. They eat grass, waterplants, leaves, buds and fruit. In the same way as horses, they bite off grass. They remove leaves from twigs by pulling the twig along their teeth with their proboscis. Sometimes they eat animal food. They go for hours along well-known paths through the vegatation with their proboscis to the ground, searching for food.

Rhinoceroses live from grass (white and Indian rhinoceros) and branches with leaves (black and Javan rhinoceros) and sometimes fruit (Sumatran rhinoceros). They eat approx. 2% of their body weight in dry matter per day, that is 100 to 120 kilos, and they drink about 100 liters a day.

Digestive tract

Horses have a big blind gut (cecum) and colon (large intestine) for the fermentation of their food. The digestive tract is 15 x their body length.

Tapirs have a smaller blind gut, they digest their food in the large intestine (colon).

Rhinoceroses digest their food in the colon. The white rhinoceros has a small cecum and an enlarged large intestine (colon). The digestive tract is 9 - 10 x its body length.

Teeth

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

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

The big rhinoceros species lack incisors and canines. The Asiatic species do have incisors, but nor canines, and the Sumatran rhinoceros has both incisors and canines. All species have large molars.

Senses

Donkeys have larger eyes and longer ears than horses. They taste small differences in food. The ears of zebras are rounder than horses' ears. All species have a sensible skin.

The sense of smell of Tapirs is good, their hearing is somewhat less developed. The eyes are small and their vision is badly developed.

Rhinoceroses have a bad vision. At a distance of 30 meter they are not able to see an upright person. Because their eyes are at the sides of their head, they have to turn their head to see what is in front of them. Hearing is well developed. They can locate where sounds come from by directing their movable, funnel-shaped ears. The skin is not very sensible. The most important sense organ is the nose, their sense of smell is good.

Characteristics

Equus

In the group of the odd-toed ungulates the equus-species are the animals with the long, straight limbs. They have the most specialized legs, with only one hoof. They are animals of plains that trot and run a lot. They eat young, relative nutrient rich grass, that is bitten off with their chisel-like incisors. They have large senses, too. Equus have the most powerful lower jaw (which is the limbs part of the skull (see here for an explanation)).

Of these animals the donkeys have a white underside, the largest senses, they can taste food well and they seem to observe and analyse a situation before they do something. They have the most developed nerve-sense system.

Zebras cannot be kept, behave aggressively, the males have sharp canines. They have the characteristics of the heart-lung system. That could explain their stripes, too (as stripes and dots on the fur is a characteristic of carnivores and heart-lung animals in general).

Horses have a less white underside. They are sturdier. Horses and donkeys are able to learn. Horses have the characteristics of the metabolic-limbs system.

Tapirs

Tapirs resemble pigs in body-characteristics and behaviour; they have a plump body and a prehensile proboscis. They have relatively big canines and eat, besides grass and leaves, fruit and sometimes meat. The young are striped.

Rhinoceroses

Rhinoceroses are enormous animals with a length up to 4 meters and a weight to 3500 kilos. They have a long body with a long head, which is lower than the backline. The senses are small and the eyes are badly developed. They eat great quantities of nutrient low grass.

Conclusions

The equus-species are the nerve-sense animals of this group (head is high above the backline, big senses, can learn, long legs, eat relatively nutricious, young grass).
The tapirs are the heart-lung animals with their pig-like appearance, relatively large canines, their diet with some fruit and meat and striped young.
The rhinoceroses are the metabolic-limbs animals with their massive, long body, head kept low, not very well developed senses and low-quality food.

The equus-species have developed their limbs most. The rhinoceroses have developed their metabolism most. It seems as if the process of the limbs (which gives straight, long forms) works in the entire body of the rhinoceroses, and has an elongated form as a result.

	horses	tapirs	rhinoceroses
skin	sensitive	tough	thick, armoured
body	slender, long legs, head high	stocky, head on a level with backline	big, long body, head long en low, horns
head	long	swine-like, proboscis	long and massive
teeth	big incisors	incisors and canines	most species no incisors, big molars
food	highly nutricious, young grass	grass, leaves, fruit, some meat	grass, leaves
digestion	blind gut	colon	colon
behaviour	curious, can learn	likes water	introverted
reproduction	1 foal	1 striped young	1 young
senses	eyes, ears	smell, ears	sees badly; smell, ears
specialization	limbs	none	metabolism
conclusion	nerve-sense	heart-lung	metabolic-limbs