Zebra affairs – flies and stripes

Whether you think zebras are white with black stripes, or black with white stripes, will depend much on your way of looking at things, although DNA tests confirmed that zebras were originally a dark color and evolved the lighter shades.

You would surely agree that the most obvious characteristic of zebras are their striking striped patterns. The striped patterns of the zebra must have reminded the Romans of tigers, as they named the zebras Hippotigris, to describe these horses with tiger-like patterns.

Much like human fingerprints, zebra stripes are personal, and no two zebras have been found to bear the same markings.

The stripes run vertically on the head, neck, forequarters and the main body, while running horizontally in the rear and legs. How can such conspicuous markings be favorable in any way, if the stripes on zebras are as striking as a “turd in a punchbowl”?

For many years it was clear to most that the zebra had evolved stripes as a predator defense mechanism, and many explanations have supported this. The stripes of several individuals together will confuse the predator as the pattern merges to form a large optic illusion where individuals disappear in the stripes of the crowd, thus making it difficult for predators to focus on a solitary individual.

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stripes zebras hamiltonmarler
Just for the fun of it, you can step away from the screen while looking at the above image, and you will appreciate how one of the zebras slowly blends into the surroundings depending on the distance from which it is seen, or the eyes that see it.

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Of course, if one considers that lions (Panthera leo) are the zebra’s main predator, and these felines are allegedly color blind, it makes more sense that zebras blend into a background that is not black and white. It is also thought that lions have difficulty in seeing zebras that are inmobile in tall grass. If  truly colorblind,  the picture above is likely to give a small “lion’s” glimpse of the optical ilusion created between the striped horses and the backdrop.

For most, when predators are mentioned in relation to zebras, one almost immediately thinks of lions, hyenas, Lycaon and suchlike, but two recent experiments support stripes as an effective means of confusing the visual system of predators through disruptive coloration; however the predators studied are the much smaller and possibly more persistent blood-sucking tsetse fly and horse flies. Both these flies are  Tabanidae, or tabanids as they are more commonly known. After all,  the notoriety of ungulates as targets for flies of all varieties is common knowledge, and pests like these are likely to pose potential feeding difficulties, blood loss, and transmission of disease.

The “fly” hypothesis was tested in a farm near Budapest plagued with horseflies, wherein the experimenters painted horse-shaped surfaces with black and white stripes of varying widths and angles, these striped surfaces were then covered in glue to see which pattern attracted (by number of flies stuck to the glue) the most flies. The results revealed that flies were drawn less to narrower stripes. They later painted all black, all white and striped surfaces just to check other variables, finding that flies were least likely to be attracted to striped surfaces. These findings are in line with earlier studies of the tsetse flies that showed these flies had a preference for large dark moving objects.

In fact, if stripe formation was linked to pest avoidance it would make sense in the light of inter-specific grazing, as between zebra and wildebeast or zebra and antelope. As flies are attracted to the darker colors of the other grazing ungulates, zebras would undoubtedly suffer less bites per capita that the darker species grazing alongside the zebra.

As mentioned earlier, no two zebra marking are the same, and this has led to another hypothesis whereby stripes are likely to play an important role in individual recognition, important for most social mammals.

Another hypothesis favors the role of stripes in thermo-regulation, the striped pattern helping to dissipate the heat of the African plains. A curious observation is that as you go further south on the African plains, the farther apart is the patterning, from narrower  to wider stripes.

Three species of zebra remain, plains zebra (Equus quagga), Grevyi’s zebra (Equus grevyi) and mountain zebra (Equus zebra), of these the seriously endangered Grevyi’s zebra has the narrowest stripes.

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While searching the web for information regarding zebras, I came across a curious African Bush tale of how Zebras got their stripes.

Long ago, when animals were still new on earth, the weather was very hot, and what little water there was remained in pools and pans. One of these was guarded by a boisterous baboon, who claimed that he was the ‘lord of the water’ and forbade anyone from drinking at his pool. When a zebra and his son came down to have a drink, the baboon, who was sitting by his fire, jumped up.

‘Go away, intruders,’ he barked. ‘This is my pool and I am the lord of the water.’

‘The water is for everyone, not just for you, monkey-face,’ shouted back the zebra’s son.

‘If you want it, you must fight for it,’ returned the baboon in a fine fury, and in a moment the two were locked in combat. Back and forth they went, until with a mighty kick, the zebra sent the baboon flying high up among the rocks of the krantz behind them.

The baboon landed with a smack on his seat, and to this day he carries the bare patch where he landed.

The zebra staggered back through the baboon’s fire, which scorched him, leaving stripes across his white fur. The shock sent the zebra galloping away to the plains, where he has stayed ever since.

The baboon and his family, however, remain high up among the rocks where they bark defiance at all strangers, and hold up their tails to ease the smarting of their bald patches.

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Read more on zebras:

Equus Evolution: The Security of Stripes in the Zebra Population

Hoedspruit Endangered Species Centre

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Biblioraphy

Bard, J. B. L. 1977. A unity underlying the different zebra striping patterns. J. Zool. (London) 183: 527-539.

Bard, J. B. L. 1981. A model for generating aspects of zebra and other mammalian coat patterns. J. Theoret. Biol. 19: 363-385.

Egri, A. et al. Polarotactic tabanids find striped patterns with brightness and/or polarization modulation least attractive: an advantage of zebra stripes. Journal of Experimental Biology 215, 736-745 (2012).

Horváth, G. et al. An unexpected advantage of whiteness in horses: the most horsefly proof horse has a depolarizing white coat. Proceedings of the Royal Society B: Biological Sciences 277, 1643-1650 (2010)

Marler, P. and Hamilton, W. J. 1968. Mechanisms of Animal Behavior. Wiley, New York.

Ruxton, G. D. 2002. The possible fitness benefits of striped coat coloration for zebra. Mammal Review 32:237-244.

Vale, G. A. 1974. The response of tsetse flies (Diptera, Glossinidae) to mobile and stationary baits. Bull. Entom. Res. 64: 545 – 588.

Waage, J. K. 1981. How the zebra got its stripes: biting flies as selective agents in the evolution of zebra coloration. J. Entom. Soc. South Afric. 44: 351 – 358.

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