Equus Evolution: The Security of Stripes in the Zebra Population
The evolutionary phenomenon of zebra stripe patternation has continued to elude the scientific community for the past century, and has culminated in intense debate about the reasons behind their formation. A multitude of hypotheses have been formulated to explain their existence, and what beneficial trait they might attribute to the zebra (Equus quagga). The predominant hypotheses alluded to the evolutionary mechanisms acting upon social function, thermoregulation, and protection (Ruxton 2002), but a variety of these hypotheses were nullified because of their inability to be tested, or because of recently conducted research. A recent experiment conducted on a Hungarian horse-farm indicated that zebra stripes were formed by convergent evolution as a response to the incessant biting of flies of a tabanid origin.
The earliest hypothesis as to why the zebra developed stripes suggested that the stripes were an identification marker. Much like a snowflake, the unique, natural variation found in the stripes of a zebra allows no two zebras to have exactly the same patterning. Earlier studies proposed that this was how zebras recognized each other; however, horses without striped coloration are able to recognize each other with as much ease. Thermoregulation is the ability of an organism to keep its temperature in a homeostatic state. A scientist hypothesized that a slight breeze could be created due to the different cooling rates between the black and white colored skin, but the hypothesis was never experimentally tested. Several ideas for the origin of stripes developed under the protection hypothesis. The stripes were thought to make the zebra appear larger, “dazzle” predators, and allow for camouflage. Stripes can create an optical illusion that may allow a zebra to appear larger, and a stampeding herd of zebra may be able to successfully elude predators due to the dizzying effects of blurring stripes, but there is not enough evidential support to make a valid claim. Using stripes for camouflage among tall grasses might sound like a perfectly valid and well-founded declaration, but zebra graze out on the open plain, in visible, unobstructed sight.
The hypothesis increased the support behind their use as camouflage using an alternative, and slightly modified version of the original hypothesis; the stripes were thought to have made the zebra appear “ghost-like,” and allow it to graze undisturbed during dawn and dusk when its biggest natural predators, the lion (Panthera leo) and the hyena (Crocuta crocuta), are on the predatory advance. But much like the other hypotheses, the evidential support needed for the validation of the hypothesis is missing. The last plausible hypothesis, and most recent, that could be tested claimed that the stripes were a product of convergent evolution, selected for by the pestilent, diseased presence of tsetse (‘si: tsi) flies (Glossina) (Ruxton 2002).
The idea that stripes formed as natural pest repellant has been around since the 80s, and has been tested twice since its inception. An experiment had been conducted previously using only tsetse flies, but the newest experiment was conducted using horseflies (both are of the family Tabanidae, commonly referred to as “tabanids”). Both tsetse and horseflies can carry diseases (i.e. sleeping sickness) that affect zebras as strongly as it does a human, and their incessant biting can lower body fat and reduce milk production, not only aggravating the current generation, but the future generation as well. The most recent experiment, led by scientist Gábor Horváth, was conducted in Hungary, and chose to use horseflies due to their over-abundance, and lack thereof, of tsetse flies, which are indigenous to Africa. The scientists used monochromatically painted boards as controls, painted black, brown, gray, and white. Earlier conducted research indicates that tabanids prefer dark-coats to light coats (Horváth et al 2010). Another board was painted in a varying stripe pattern similar to that of a zebra. Going into the experiment, the horseflies were expected to land in relative amounts varying from largest to smallest along the four monochromatically painted boards. The boards were covered in sheer amounts of olive oil or glue to capture the horseflies and allow for a qualitative measurement. The board most similar to the pattern coloration of the zebra had the fewest number of flies alighted upon it (Egri et al 2012).
The scientists expected more flies to be attracted to the dark monochromatic board simulations of horse pigmentation because of the way dark coloration reflects light. Dark coloration reflects horizontal, polarized light, and light or white coloration reflects unpolarized light. Polarized light emits light waves that have vibrations occurring in a single plane, but unpolarized light emits light waves that have vibrations occurring on multiple planes. Specific animals are capable of perceiving polarized light to some extent, tsetse flies and horseflies being two of them. Tabanids are more attracted to polarized light because it is easiest viewed by them, and is similar to that of light reflected off of water, which is where their eggs are laid (Lee 2012). The horseflies were readily available to land upon the darker colored boards, and even the unpolarized lighted boards. While lighter colors like gray and white emit unpolarized light, tabanids will still land upon them. Unpolarized light is repugnant to tabanids, but they are not completely subjective and dismissive of it (Horváth et al 2010). The black and white striped painting of the experimental simulation “zebra” board is thought to disorient the tsetse and horseflies due to the fact that it emits both polarized and unpolarized light (Figure 1) (Doyle-Burr 2012).
The experiment suggested that the production of a white, striped coloration (zebra base-coats are black, and their stripes are white) may offer greater fitness and advantageous benefit to the zebra, but it has inherent flaws. The experimental and laboratory conditions were not equivocal to natural conditions. In the wild, zebras emit strong odors, that more than likely over-power any benefit that a striped pattern may offer (Lee. 2012); however, it does not necessarily mean that the varying emittance of alternating polarized and unpolarized light is not enough to keep the majority of pests at bay.
Evolution does not promise perfection of fitness, after all. The experiment must be conducted again in a better simulated habitat, introducing the odor variable, and must experiment upon actual zebras and horses if it wishes to offer sound, scientific validity.
Even though the emergence of zebra stripes is not explicitly answered without a doubt, it is strongly supported by evidential and experimental support that can be duplicated if necessary. However, the hypothesis that stripes evolved to directly combat the tsetse flies and horseflies is an excellent demonstration of the effects of natural selection presented by the parasitic flies and of the environment itself (horseflies and tsetse flies are most pestilent and abundant in Africa, and explain why not all species of horse have developed stripes to combat their similar foes in different regions), and a well-encompassed representation of convergent evolution. A variety of animals also mimic zebra striation as well (zebra finches and zebra spiders are two examples), but their uses are purely analogous. If the stripes are a defense mechanism, in the zebra spider (Salticus scenicus), they might possibly be used for camouflage among loamy soil. The bright coloration and markings of the zebra finch (Taeniopygia guttata) are likely a tool in a male of that species’ arsenal used to improve his chances at being selected by a female for mating. Even stationary organisms have adapted a “loosely” striping pattern. While not necessarily considered a mimic of the zebra stripe, the Tiger Lily (Lilium lancifolium) uses its darkened markings to lure pollinators to spread its gametophytes, thus continuing the life cycle of the plant and its alternation of generations. Sexual and natural selection clearly contribute to the continuance of not necessarily a “perfect species,” but a species that is better adapted to its environment than it would be without the advantageous benefits offered by their markings. Analogous or not, all forms of the stripe pattern of the zebra, and those replicated in nature by other animals, are all bred by evolution to reap an advantageous benefit of some sort.
- 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).
- Ruxton, G.D. The possible fitness benefits of striped coat coloration for zebra. Mammal Review 32, 237-244 (2002).
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