Waarom zebras gestreept zijn
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Het artikel werd in oktober 1989 gepubliceerd, met slechts een paar kleine wijzigingen
Zebras: Striped Horses of the African PlainsBy Johan W. Elzenga
Ever since the first explorers set foot in Africa, we have been intrigued by some of the very special animals that can been found there. One of them is still a puzzle for many scientists, because of its peculiar pattern: The striped horse called “Zebra”. The evolution of the horse is not an African story however. Hipparion, a horse-like animal that became extinct in the Pleistocene, invaded Africa in the middle of the Pliocene and Equus followed at the end of that same period. As highly evolved grazers they were able to find their place in the newly formed grasslands and probably even displaced some of the less adapted indigenous ungulates. But obviously this lead in evolution did not last forever. Other grass-eaters evolved that were even more adapted to this new kind of food. Even though Horses are able to eat all sorts of grasses at all stages of growth, their digestive systems are not as adapted to herbivoric life as that of the ruminants. Ruminants (like antelopes and cattle) store, sift, rechew and ferment their food so that most nutrients are released. Horses also break down cellulose by fermentation, but they do not rechew their food, so that more nutrients pass through their system without being used. As a result they need more food to compensate for their lesser assimilation of proteins. Although equids are fairly successful, this difference may account for the fact that many disappeared. Equus is now the only remaining equid and apart from a few remnants of wild Horses in Mongolia and the rare African Wild Ass (and our domestic Horse), there are only three Zebra species left, divided into six races.
The Grévy Zebra (E. grévyi) has no division into races. This animal lives in the semi-desert of northern Kenya, Ethiopia and Somalia and is easily recognized by its small stripes, the white belly and the large round ears. It lives in an open society: A typical Grévy family consists of a territorial male with a harem, but the bonds are fairly loose. There is a lot of movement between these families, specially when groups join to form large herds of up to 200 animals. All equids form bachelor groups, but only Zebra stallions tolerate the presence of young bachelor males in their territory. The Grévy stallion keeps company with transient bachelors, but he is defensive of his mating prerogatives.
The Common Zebra (E. quagga) is the most wide-spread of the three Zebra-species. In East Africa lives the Grant Zebra (E.q. Boehmi), often also (but wrongly) called the Burchell’s Zebra. The most abundantly striped Zebra, E.q. crawshayi of the Mozambique zone mixes in Southern Tanzania with the selousi type. All are very clearly black and white striped, but the stripes are wider than the Grévy’s and go on underneath the belly. Southern Africa is the home of the Chapman Zebra (E.q. antiquorum or E.q. chapmani). The Chapman very often has so called “shadow stripes”. In between the black stripes, brown stripes are visible (though sometimes only faintly).
The social system of the Common Zebra is completely nomadic, but the family bonds are strong. Grown up animals will hardly ever leave their family, even when the leading stallion dies. A typical family unit consists of one stallion and one to six mares with their foals, up to fifteen animals in total. The young stallions leave the family when they are between one to three years old, to live in bachelors groups. When they are old enough, they will try to separate young females from a group to form their own new family. There are no territories held by the males, probably because this type of Zebra has to migrate with the rainfall, whereas the Grévy is far less dependent of water. However, also Grévy Zebras have to drink and must sometimes move out of their territory during droughts and so it is often assumed that the Grévy is a more “ancient” animal, that did not evolve the more environment-adapted nomadic life of the other Zebras. Although the two species can be crossed in captivity, cross-breeding never occurs in the wild, even at those places where the species live very close together and sometimes have mixed herds. These mixed herds are an interesting phenomena: Mixed herds between Zebras and Gnus are also often seen, but they split up in flight from danger (in the form of Lions). The mixed herds of different Zebra species remains in tact even at the sight of danger, showing that the two species do recognize each other as both being Zebras, even though they don’t sexually mix. During flight the smallest families concentrate in the middle of the herd, making the families that “can’t afford some losses” the least vulnerable ones.
Until ±1750 two other races of E.q.were very common, both living in South Africa. The Quagga (E.q. quagga) looked more like a domestic horse than the other species, because it was brown with only a few stripes in the neck. Hundreds of thousands of these animals roamed the plains of South Africa, but then the white hunters started a slaughter second to none. In only a few decades time, the animal was decimated. The slaughter was so massive, that hunter were obliged to retrieve their bullets from the dead animals in order to cope with the shortage of ammunition. The Quagga was probably extinct in the wild at the end of the 19th century and the last living animal died in the Amsterdam Zoo in 1883. The other race that did not survive the arrival of white men was the true Burchell’s Zebra (E.q burchelii). Less than thirty years after the death of the last Quagga, the last Burchell’s Zebra died in 1911 in the Hamburg Zoo.
The third species, the rare Mountain Zebras (E. zebra) and specially the Cape Mountain Zebra (E.z. zebra) merely escaped a similar fate. In 1965 there were only 75 of these animals left, but since then most of them live protected in the Bergzebra National Park where their numbers doubled in only five years. The Hartmann Zebra (E.z. hartmannae) lives in Namibia and Angola, numbering probably around 7000 animals. Their social system is very similar to the system of the Common Zebra.
Why the stripes?
The most interesting question about zebras, that still is not answered beyond doubt, is the reason of the stripes. What is the advantage of a pattern, that would seem more of a disadvantage at first glance? There have been many speculations, some of them very odd indeed. One of the most improbable explanations is that the stripes would function as a temperature control mechanism. The white part would protect the animal against the daily heat, while the black part would conserve heat during the cold nights. It is not difficult to prove that this thesis has no scientific backbone. Clearly, the combination of two extremes is very unlikely to result in a good compromise; a grey colour would suit this purpose much better. Besides, Zebras have layers (stripes) of fat underneath the skin, that coincide only with the black stripes. Apparently they function as heat absorbers, to prevent the black part to become overheated. This proves that the stripes are certainly not a very good heat regulator and that special adaptation was necessary to cope with the temperature problems caused by the striped skin!
A more plausible explanation, that has long been accepted as the truth, is that the stripes are a form of camouflage. Although Zebras are standing out very clearly from a close distance, this isn’t an extra handicap. Lions are very able to see and smell animals at short distance, regardless of their colours. Seen from further away, something special starts to happen. Because of the heat, the air moves and this causes the stripes to “dissolve” in the blurred surroundings. Indeed, from a longer distance Zebras are less visible than plainly coloured animals of the same size. So is this then the true explanation for the stripes? Probably not, because in this theory one thing is forgotten: Lions are the main predators of Zebras and they hunt mostly at night. At night the stripes will not function as camouflage as they do during the day. On the contrary; they make the animal visible when the moon is shining and so the black skin of the Gnu or the Buffalo is clearly an advantage. The stripes must have another reason.
A more recent theory is, that the stripes do indeed provide the Zebra with a form of camouflage, but not in the way described earlier. Zebras live in herds and so any form of camouflage should be looked at from this perspective. There is no sense in trying to hide one single animal if it lives in a big group. The stripes could cause confusion, when a lion would want to pick one individual out of the large herd. When the zebras stampede, the moving stripes will make it difficult to tell where one body ends and the next one begins and that would make it more difficult for the lions to concentrate all their efforts on one individual Zebra. This theory does take into account that lions hunt at night (the camouflage would only fail in a pitch dark night, but then the lions will have troubles anyway), but it doesn’t answer one important question: If black and white stripes are such a good form of camouflage, then why did no other herd-living animal develop a similar skin pattern? And why wasn’t the Quagga also striped?
That last question also remains unanswered in yet another theory, although there is a lot of proof that supports it. Zebra skins are different in every animal, just as our fingerprints. It has often been suggested that these different patterns act in the recognition between individuals. Just like we recognize people by their unique facial expressions, a Zebra will recognize its herd members by their stripes. The interesting difference between mixed herds of Zebras and Gnus, compared to mixed herds of different Zebra species, would also prove this phenomena. Besides that, the stripes also function in the spacing between animals and socialization between individuals. Experiments with captive animals have proven that indeed Zebras react to black and white patterns quite remarkably. For example, Zebras will stand themselves in front of a striped panel much more often than in front of a plainly painted panel, the distance closely correlated with the narrowness of the stripes. It seems that indeed the stripes also play a very important role in the complex social behaviour of the Zebra. Comparison of Zebras with Horses shows that the typical social behaviour of the latter, called “grooming” is less frequent in Zebras. The evolution of stripes could have been as a focus for grooming (neck and shoulders area), later developing into a more general promotion of sociability and therefore extending over the entire body. If its ancestors were (only partly) brown/black striped, then the Zebra would have optimized this pattern and that may be the reason why it is the only truly successful wild Equid today. Clearly this is the strongest theory discussed so far and there are lots of facts supporting it, but it still doesn’t answer a few questions: Why didn’t the Quagga evolve the same stripes? Was it, because the Quagga had a more “horse-like” social structure with grooming as an important way of showing bonds? But why then do we see that the stripes diminish when travelling from East Africa to the south, not only in the Quagga, but also in the shadow-stripes of the Chapman Zebra?
There is one theory that may answer these last remaining questions, even though it also seems unlikely as the sole reason for the stripes. Large parts of Africa are the home of the Tsetse-fly, a blood sucking insect that transmits the dangerous “sleeping sickness”. Just like domestic cattle, Zebras are more susceptible to this illness than the other animals of the plains, that are normally immune. It has often been said, that the Tsetse-fly is Africa’s most effective nature conservationist, as it keeps humans and cattle away. Tsetse-fly do not see stationary objects in the same way we do, they mainly react to movement and the darkness of colours. The fly bites through dark clothes, even when bare white limbs are available, something I can confirm from my own painful experience, while my black African friends have experienced Tsetse-flies always biting in parts of their bodies that are uncovered. It seems however, that the fly needs a relatively large plainly coloured subject in order to see it. It has been postulated that the Tsetse-fly cannot correctly see Zebras and consequently a Zebra is not bitten as much as other animals. The stripes would thus be a form of camouflage against a very odd “predator”. Although this theory seems a bit far-fetched as the sole explanation for the stripes, it could answer why the Quagga didn’t need this peculiar marking: It lived in an area that is free from Tsetse-flies. The Cape Mountain Zebra does too, but this animal might have invaded South Africa long after it developed its stripes in Central Africa. It obviously also explains why Horses from other continents or other African animals didn’t need to develop stripes.
It is unlikely that we will ever be able to prove beyond any doubt why Zebras are striped. The most likely explanation seems the social behaviour one, whereas the Tsetse-fly could have added an extra need for stripes and could have been the natural selection that eradicated the non-striped forms of Equus. One thing is absolutely clear though: The Zebra stripes did not evolve because they are really perfect for the modern autofocus cameras of the tourists, that nowadays roam the plains of Africa in their minibusses!
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