Posts Tagged ‘Grévy’s zebra’

From the Journal of Experimental Biology:

If there was a ‘Just So’ story for how the zebra got its stripes, I’m sure that Rudyard Kipling would have come up with an amusing and entertaining camouflage explanation. But would he have come up with the explanation that Gábor Horváth and colleagues from Hungary and Sweden have: that zebra’s stripes stave off blood-sucking insects…?

Horseflies (tabanids) deliver nasty bites, carry disease and distract grazing animals from feeding. According to Horváth, these insects are attracted to horizontally polarized light because reflections from water are horizontally polarized and aquatic insects use this phenomenon to identify stretches of water where they can mate and lay eggs. However, blood-sucking female tabanids are also guided to victims by linearly polarized light reflected from their hides. Explaining that horseflies are more attracted to dark horses than to white horses, the team also points out that developing zebra embryos start out with a dark skin, but go on to develop white stripes before birth. The team wondered whether the zebra’s stripy hide might have evolved to disrupt their attractive dark skins and make them less appealing to voracious bloodsuckers, such as tabanids.

Travelling to a horsefly-infested horse farm near Budapest, the team tested how attractive these blood-sucking insects found black and white striped patterns by varying the width, density and angle of the stripes and the direction of polarization of the light that they reflected. Trapping attracted insects with oil and glue, the team found that the patterns attracted fewer flies as the stripes became narrower, with the narrowest stripes attracting the fewest tabanids.

The team then tested the attractiveness of white, dark and striped horse models. Suspecting that the striped horse would attract an intermediate number of flies between the white and dark models, the team was surprised to find that the striped model was the least attractive of all.

Finally, when the team measured the stripe widths and polarization patterns of light reflected from real zebra hides, they found that the zebra’s pattern correlated well with the patterns that were least attractive to horseflies.

‘We conclude that zebras have evolved a coat pattern in which the stripes are narrow enough to ensure minimum attractiveness to tabanid flies’, says the team and they add, ‘The selection pressure for striped coat patterns as a response to blood-sucking dipteran parasites is probably high in this region [Africa]’.

So it was horseflies that created the selection pressures on the ancestral zebra population that created these three species of striped equines.

From my reading of this analysis, the narrowest stripes are the least attractive to horseflies, so this means that the Grévy’s zebra is the one that is most adapted to living in horsefly-infested regions.

Grevy's zebra.

I wonder if this means that horse owners should put stripes on their horses during the summer.

I would had to think that people might white-wash their horses in this manner, but maybe the fly sheets that some horses wear in the summer could utilize this design.

Maybe striped fly sheets are in the offing!


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Grévy’s Zebra

Grevy's zebra

Equines are among the best-known animals. Most of us know about the donkeys and horses, as well as the mules that are created through hybridizing the two species, and one would be hard pressed to find a child who hasn’t heard of a zebra. However, most people would be unaware how many interesting species of equine exist. I’m sure that the average person is unaware of the Przewalski’s horse or the onager. Further, I doubt that many people are aware that there are three species of zebra: the plains, the mountain, and the Grévy’s.

Although I’m sure the plains and mountain species have their fans, my favorite species of zebra is the Grévy’s.

It is a large thin-striped zebra that bears some resemblence to a donkey. It is the largest of zebras, with the largest stallions weighing nearly 1,000 pounds. It is adapted to high plains of East Africa, where it exists as the transition species between the arid living wild donkeys of North Africa and the more southerly species of mountain and plains zebras.

This species has a rather interesting history with the West. You see, the révy’s was the first zebra species to be known by Europeans, for it was the “hippotigris” that appeared in the Roman Circuses.  After all, this species lives in areas that were known to the Ancient Egyptians, as well as the peoples living in the Middle East and Mesopotamia. It would make sense that some of these animals arrived in these areas under Rome’s control, and when they did, they wound up in bestiaries and circuses as examples of the vastness of Rome’s empire.

However, the Romans were not expert taxonomists, and when Europeans began to expand into Africa, they assumed that the zebras they saw were the same species as the one that appeared in the Roman circuses.

Of course, Europeans settled and explored the southern  and eastern parts of Africa first, and that’s why the plains and mountain zebra are so much better known. Of these two, the plains zebra makes it into wildlife documentaries and zoos, and it is by far the better known of the two species.  The main difference between the two is the mountain zebra has a small dewlap– the only equine to have one. However, none of this information was known to the Romans or Medieval Europeans. If a zebra showed up in Europe, it was assumed they were all the same species.

Because of this ignorance of zebra taxonomy, the Grévy’s zebra was not documented until the seventeenth century, when Europeans began to have normal relations with Ethiopia. Interestingly, the species does not get its name from the person who discovered it. Jules Grévy was president of France in the 1880’s and was given one as a present from the government of Ethiopia. The animal was very different from other zebras that were in Europe at the time, and because the specimen was brought over as Grévy’s possession, the animals were called Grévy’s zebras.

These animals are the “missing link” between zebras and donkeys.  The domestic donkeys that we know today come from North African wild donkeys that are very well adapted for arid climates. Wild jack donkeys maintain large territories, which they use to attract bands of females. Wild horses and mountain and plains zebras live very differently. In their societies, the mares live in a closely-bonded band that has a definite dominance heirarchy. It is led by a dominant mare and a single harem stallion.

The Grévy’s zebra has the donkey social structure. However, the males maintain much smaller territories than wild jack donkeys do, and the males are not aggressive to bachelor males that have no set territory. As with all equines, the young males are driven from their natal bands, where they form these young male bands called “bachelor groups.”  In virtually all species of equine, the males with females or established territories tend to show a lot of aggression towards these bachelor bands. However, the Grévy’s  stallions tend not to have such aggression towards these bands. Their aggression is much more heightened when other stallions with truly established territories encroach upon their terriotories.

The striped pelt of the Grévy’s is in demand in the fur trade, and many of these animals have been killed for their skins.  Habitat destruction is also taking its toll upon the species, for people now graze their livestock in the zebra’s native ranges, further denuding the high plains. These domestic animals also drive the zebra from waterholes, and herdsmen are not above chasing them from water sources to ensure that their stock has enough to drink. Such pressures have led to the IUCN to declare the species endangered.

It would be a shame if the zebra that once graced the circuses and bestiaries of Rome disappeared. These animals are vital to the biodiversity of their high plains ecosystem, and they tell us so much about how zebras evolved from the same ancestors as the donkeys.

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