Rudyard Kipling’s collection of “Just So Stories” lends the origins of everything from the camel’s hump to the leopard’s spots, transporting us along the way to the lair of a whale and far away sun-laden lands steeped in “more-than-oriental splendour.” His stories are origin stories, and while they are poetic, fantastical, and oftentimes unforgettable, they do not pretend to be anything else.
It is interesting, therefore, in researching how the zebra came to have its stripes, that the vast majority of popular explanation and scientific consensus is in fact nothing more than conjecture. The mystery is far from settled, yet an exploration of the many thoughts and theories regarding the topic is worth illuminating.
The families Rhinocerotidae and Equidae, members of the odd-toed ungulate order, split on the evolutionary tree around 50 million years ago, and while the family Equidae was once diverse, it is now reduced to the single genus of Equus, containing four subgenera and currently 10 extant species of horse, ass, hemione, and zebra. Of those 10 species, three of them are recognized as zebras: the mountain zebra (Equus zebra hartmannae), the plains zebra (E. quagga burchelli), and the Grevy’s zebra (E. grevyi), all with varying sizes, striping patterns, habitat preferences, and allopatric/sympatric ranges. (Myka et al., 2003)
Yet given the success of the asses, horses, and hemiones, why develop striping patterns in the first place? Responses to this question highlight the presence of four potential and distinct evolutionary mechanisms: predator avoidance, social benefits, thermoregulation, and protection from biting tsetse flies.
The first explanation highlights predator avoidance. Importantly, supporting evidence is solely anecdotal and oftentimes actually contrary to fact. The idea that stripes make a zebra look bigger so as to cause a leaping predator to misjudge his jump, has never been substantiated by data; and in fact, the animals that predate on zebras do not primarily leap. (Ruxton, 2002)
Stripes have also been suggested to cause confusion among predators, however this also finds no substantiation in any field or experimental research; and zebras are not killed proportionally less than animals lacking this adaptation. (Ruxton, 2002)
The idea that stripes help zebras blend in with tall grass receives even less validation when it is noted that zebras never attempt to conceal themselves from predators, but instead prefer to congregate in exposed groups. Mathews (1971, as cited by Ruxton, 2002) notes that they graze openly, appearing to be completely indifferent as to whether or not they are seen. In fact, when compared to other African hoofed animals they are exceptionally noisy and mobile. And lastly, the tall grass theory was discredited when a group using Fourier decomposition on a Grevy’s zebra photograph against the typical type of country found in Kenya found that the zebra’s pattern actually stands out at spatial frequencies similar to those to which feline eyes are adjusted. (Ruxton, 2002)
The second posited evolutionary reasoning for stripes is social benefits. Some suggest that zebras stay close to and groom according to an affinity to striped coloration. While this has not been formally tested, it has been informally supported by sightings of extraordinary-looking zebras. Yet, perhaps more important is the point that the three extant species of zebra has varying social structures, which they share respectively with other non-striped equids: why would stripes be advantageous to some equids and not others with similar social structures? (Ruxton, 2002)
Equally untested is the claim that stripes are markers for grooming, indicators of preferred grooming locations, and that their location all over the body would create instant affection and friendship. The variety of equid types over overlapping locations and living at similar densities seems to refute this claim. And what of the social bonding of those equids with lesser or no stripes, such as horses? Serious empirical data needs to be collected before these questions can be addressed. (Ruxton, 2002)
The third hypothesis is that of thermoregulation. Proposed in the 1980s and 1990s, it is thought that the white and black stripe patterns could create rotary breezes through differential cooling. This finds some support from an equid known as the quagga’s cool southern habitat and unique half-striped pelage. It is called into question, however, by the contradictory cases of the stripe-less North African ass as well as the heavily striped mountain zebra in the south. Some semblance of lab experiments should be able help this determine the validity, if any, of this claim. (Ruxton, 2002)
The last, and most satisfying, explanation for the evolution of zebras’ striped coats is not for protection against predators, but rather protection against pests. Ungulates are notorious targets for flies of all varieties, and the potential for feeding difficulties, blood loss, and disease infection are very real (Waage 1981). All in all, it is postulated that the reduced fitness caused by prolonged and persistent insect parasitism could be enough to require multiple evolutionary adapted responses. Previously hypothesized adaptations include specific grooming responses (Klingel, 1972 as cited by Waage, 1981) as well as possible herd responses, such as size and movement (Duncan & Vigne, 1979; Duncan & Cowtan, 1980, respectively; as cited by Waage, 1981). Waage hopes to add the spectacular coat coloration of zebras.
Biting flies are by and large attracted to large, dark forms, especially those conspicuous to their environment and especially those moving. In the field, zebras have been observed to be much less bothered by tsetse and other flies than other neighboring ungulates in their vicinity; and in fact, zebra-identified tsetse blood-meals are astonishingly rare. Experimentally, where host-finding was predominated by visual stimuli, it was shown that solid colors (black or white) were far more attractive to the biting flies than striped stationary or moving targets; however, the proportion of flies landing on both striped and solid moving models was equivalent. This suggests that the stripes do not interfere with the landing response so much as the flies’ attraction from a distance. Waage posits that the stripes could at distance either blur the body edge of the animals or be too narrow of landing pads to register attraction; at longer distances, perhaps they merely create the illusion of a grey zebra, contrasting less with the surrounding environment. (Waage, 1981)
The main wild card in this hypothesis involves olfactory stimuli, which is known to be a key player in pest attraction. Experimentally, Waage demonstrates that the coat pattern (whether solid or striped) did not affect the number of flies attracted to odor-laden models’ vicinity; however, coat pattern did affect dramatically the number of flies that actually landed. If anything, this suggests that when inter-specific grazing occurs, such as zebra-antelope, the surrounding flies will be attracted to the more visually attractive hosts. His theory is also supported by the fact that the three zebra subspecies have varying striped pelages and differing relationships to tsetse flies. (Waage, 1981)
Waage (1981) shows an interesting coincidence, which may help answer the question, why do the three species of zebra have varying striping patterns? He notes that only one zebra lives largely sympatrically with tsetse flies, and that this zebra, Equus burchelli, is also the zebra with “the most boldly obliterative pattern” (Waage, 1981: 356). Those zebras near the boundary of tsetse distribution or beyond, E. grevyi and E. zebra, have white bellies and appear greyer due to narrower stripes, and those E. burchelli found south of tsetse distribution are noted to have yellow ground color, shadow stripes, and in fact a loss of striping on both the legs and belly. All in all, Waage posits that perhaps it is the proximity to tsetse predation that sustains strong stripe delineation and thus the disruptive pattern.
A final theory for why all three zebra species have differing coat patterns was suggested by Morris (1990 as cited by Ruxton, 2002) in which the difference in striping patterns could quickly and efficiently allow for group separation while inter-specifically grazing and mingling. This is supported by the most conspicuous striping differences being located on the rear. (Ruxton, 2002)
In the end, there hasn’t been enough research to definitively put this issue to rest; however, it is important to note that the sole hypothesis supported by experimental evidence is Waage’s regarding tsetse fly proximity as a pelage determinant. More in depth research on the nature between visual, olfactory, and tsetse fly repulsion needs to occur before the exact evolutionary nature of zebra stripes can be determined; however, out of a history of explanations muddled in conjecture and lacking scientific rigor, his work appears to be a critical starting point in moving the debate out of the “just so” realm and into the light of science.
Myka, J. L.; Lear, T. L.; Houck, M. L.; Ryder, O. A.; Bailey, E. 2003. Homologous fission event(s) implicated for chromosomal polymorphisms among five species in the genus Equus. Cytogenetic and genome research 102:217.
Ruxton, G. D. 2002. The possible fitness benefits of striped coat coloration for zebra. Mammal Review 32:237-244.
Waage, J.K. 1981. How the Zebra Got its Stripes-Biting Flies as Selective Agents in the
Evolution of Zebra Coloration. Journal of the Entomological Society of Southern Africa 44:351-358.