Zebras, the Horses of the African Savanna

But they arrived to Africa coming from North America

By Stefan Anitei on December 7th, 2007 07:36 GMT
Zebras represent a symbol of the African savanna. But, did you know that their evolution (and that of horses and asses) started 54 million years ago, in North America, with Hyracotherium, a fox sized animal with four toed fore limbs and three toed rear limbs? They lived in forests, but 20 million years ago, the clime turned dry and vast savannas appeared on North America, boosting the evolution of the ancestors of zebra-horses. The first species of Equus (the genus to which zebras, asses and horses belong) may have resembled a Grevy's zebra. In fact, zebras and horses are so much related, that it is impossible to make the difference between their skulls. Through Behring land bridge, the first Equus species passed into Eurasia, and from there into Africa.

2 million years ago, in India, there lived a type of zebra called Equus sivalensis. The extinct Equus sellardsi, which lived in North America, may have resembled a plain zebra. Later, Equus disappeared completely from North America.

2 million years ago, Equus plicatus, the direct ancestor of the Grevy's zebra, had already inhabited southern Africa. During the Ice Age, there were both zebras and asses in Europe.

The Equus populating savannas formed zebra species, the ones from the deserts - the asses, and those from the temperate steppes - the horses.

The plains zebras (Equus quagga) live in families made of one dominant male, a few females and their offspring, up to 2 years old. Some males can have up to 30 females, but usually a family has up to 16 members. Friend zebras comb each other with their incisor teeth, this being a social and affective act. A too old or ill male leaves the family, joining a bachelor group, being replaced by a younger male. The male is dominant in the group, but the bonds are strong inside the family, so that it does not fall apart with his disappearance. He defends the mares against the insistences of other males. Between females, there is a hierarchy based on age, which applies too to the offspring.

Family's movements are led always by the dominant female, while the male places in lateral or flanks, ready to intervene for defending his family.

In Grevy's zebras, males do not found families, but defend a territory up to 10 square km (4 square mi), defended against other males, but accessible to females during the mating season.

The stripe pattern is unique for each zebra, just like the spots of a giraffe. Researchers identify zebras based on the stripe pattern on the shoulders and hips. The black stripes can divide forming islands of white hair. The black stripes are less delimited in southern races of zebra.

Why do zebras have stripes? Some say that the stripes disaggregate the animals' shape, impeding predators to spot a specific individual inside the herd. Counting zebras is more difficult than counting other animals in the savanna. Other hypothesis says that the stripes ease the individual recognition.

By the age of 2, young mares are sexually mature and go out and integrate into another family (as the stallion is their father) or form another family with a stallion. Young males from the same family form bachelor groups and, by the age of 4-5, they found their own families. The difference of age between sexes' maturation impedes inbreeding. In the groups made of males, the hierarchy is established through simulated fights. When fighting, the males use their teeth and hooves. A zebra kick can fracture a lion's jaws, and there are cases of lions known to have died because of this.

If the stallion is the offspring of a dominant mare, he has high chances of becoming dominant. Usually, the dominant male is more agile and rapid. A male can be helped by a "first mate" to form his harem. Males are known to defend the herd against predators.

The mare will not accept the stallion, if she is not in heat. The male has a well developed Jacobson on the roof of his mouth, and constantly smells with it females' urine in order to assess their receptivity. Gestation lasts one year.

Birth takes place in 20 minutes, in the presence of the whole family. The laying females gets up suddenly, causing the rupture of the umbilical cord. In 15 minutes, the foal is stable on its feet and in 4 hours it can run by his mother's side. In the first days, the mother is extremely protective. In 10 days, the foal is presented to the herd and protected by the father. Weaning takes place at the age of 6-7 months. 1-2-year-old individuals are easy prey for lionesses. Females can mate soon after birth, but they have usually one offspring at two years.

Zebras move several km daily, and feeding takes 14 hours daily, both during the day and the night. During the dry season, they retreat to shade during the heat of the midday. Zebras graze in episodes of 1-3 hours, being alert to lions and hyenas. When the group stays into a somnolence during the midday, an adult always remains guarding, announcing through a specific neigh the presence of a predator.

Zebras drink at least once daily. During the dry season, the zebras dig to the table water level holes which are 50 cm (1.6 ft) deep, 1 m (3.3 ft) wide and through which water is filtered by sand. Zebras can drink salty water and complete their mineral needs by eating soil. Zebras make migrations of hundreds of kilometers during the dry season, in herds of thousands of individuals. The most famous are those made between the Lake Victoria shores and Serengeti. The outward journey takes place from January to March, and the return to northwest in June. Zebras can swim, but they do it only when necessary.

They sleep about 3 hours per day. The visual angle of a zebra is of 210o for the monocular field and 60-70o for the binocular field. Zebras have keen hearings and good olfaction. Ears also are used for social communication. Lowered ears mean aggressive attitude. Top speed of a zebra is 60 km (37 mi) per hour.

In the present, there are 3 species of zebra.

The plains zebra is the most common and widespread, being the familiar zebra of the savanna, counting with 300,000 individuals. It has several races. It has both black and brown stripes, the mane is striped too and the tail has a black tip.

-Grant's zebra (Equus quagga boehmi) from Ethiopia and Sudan to Kenya and Tanzania. It is the zebra we see in the documentaries made in Serengeti or Massai Mara.
-Selous' zebra (E q borensis) from Mozambique;
-Chapman's zebra (E q chapmani), with brown stripes interposed between two black stripes, from Zimbabwe, and South Africa;
-Crawshay's zebra (E q crawshayi) from Namibia;
-Burchell's zebra (E q burchelli) from southern Botswana to northeastern South Africa;
-quagga (E q quagga) from southern South Africa, extinct by the 19th century, was almost devoid of stripes.

The mountain zebra (Equus zebra) is restricted to meridional Africa. The heart is larger than in other zebras, weighing 3.2 kg (8 pounds). The stripes on the hips are perpendicular with those of the body. They were almost exterminated by Boers.

The Cape mountain zebra is 1.25 m (4 ft 2 in) tall and weighs 250 kg (555 pounds), being the smallest animal in the horses' family. 300 individuals live on the Mountain National Zebra Park, in South Africa, and 250 in other protected areas.

Hartmann's mountain zebra is 1.5 m (5 ft) tall and weighs 340 kg (755 pounds). 7,000 individuals live in the mountains surrounding the Namibian desert.

Grevy's zebra (Equus grevyi) from Ethiopia and northern Kenya are intermediary in look between zebras and asses, with long, rounded ears. They are also the heaviest zebras: 450 kg (1,000 pounds) and the most graceful, being 1.6 m (5.3 ft) tall at the withers. It was known in Europe since the times of the Romans, who used it in the circus games under the name hippotigris (tiger horse). The stripes are finer and closer than in other species. This species counts with about 10,000 individuals, but it is poached for skins.
Chapman's zebra stallions fighting
   Chapman's zebra stallions fighting