There are many hypotheses for striping in zebras: (i) camouflage against predators through (a) background matching or (b) disruptive colouration; (ii) avoiding being killed by predators through confusion by (a) misjudging the number of zebras in the group, (b) the size of the target, (c) flight speed, or through (d) quality advertisement or (e) aposematism; (iii) cooling the animal; (iv) social factors, namely, (a) individual recognition, (b) social stimulation of conspecifics, (c) causing individuals to group together, (d) directing grooming at certain parts of the body or (e) mate choice; and (v) avoidance of biting flies, be they (a) glossinids or (b) tabanids. Our comparative analyses that pit the five principal hypotheses against each other indicate that biting flies (hypotheses v(a) and v(b)) are the evolutionary drivers for striping in equids, at least on most regions of the body (Table 1). Many of our independent variables were highly intercorrelated; for instance, spotted hyaenas inhabit the hot tropics and subtropics25 and tsetse flies rest on trees26. Therefore, we can only be confident of our findings stemming from a model selection procedure that pits factors against each other to see the relative importance of each in explaining variation across all possible models.

Regarding the possible effect of biting flies as evolutionary drivers for striping in equids, our significant findings were (i) that presence of body stripes is associated with tsetse fly distribution and between 5 and 11 consecutive months of tabanid activity at the species level, and with tsetse fly distribution at the subspecies level; (ii) that aspects of face, neck, flank, rump striping and shadow stripes are associated with between 6 and 9 consecutive months of tabanid activity at the subspecies level; and (iii) and that belly stripe number is associated with tsetse fly distribution at the subspecies level and with tsetse flies and marginally with woodlands at the species level. Indeed, the ranges where tabanids and tsetse flies are active matches that of striped equids well (Fig. 2). We had estimated 15–30 °C and 30–85% humidity as ideal conditions for tabanid activity and reproduction (see Methods) but recognize that Culicidae (mosquitoes), Stomoxys (Muscidae, stable flies) and Simuliidae (blackflies) are abundant in the wet season too27. While the association between striping and warm humid months might imply stripes being a means of reducing heat load, this seems unlikely, given the lack of association with any maximum temperature in any multivariate model.

Figure 2: Distributions of equids, tabanids and glossinids. (a) Map of the Old World showing distribution of 7 species of equids. (b) Distribution of tsetse flies (Glossina) and location of 7 consecutive months of tabanid activity (see methods). Full size image

Instead, our results lend strong ecological and comparative support to experiments that show that some tsetse and tabanid species avoid black and white striped surfaces11,12,13,14. Biting flies are attracted to hosts by odour, temperature, vision and movement that may act at different stages during host seeking, but vision is thought to be important in the landing response27 with dark colours being particularly attractive28. Waage11 found that black and white striped models caught fewer Glossina morsitans and G. pallidipes in the field than black or white models whether moving or stationary, and similar results were found for tabanids. He argued that stripes might obliterate the stimulus of the body edge or reduce the amount of contrast of the animal’s form against its background or be too narrow to elicit attraction. He believed that attraction from a distance rather than the landing response was affected by striping. Brady and Shereni13 found similar results for G. morsitans and Stomoxys calcitrans demonstrating reduced landings as stripe number and thinness increased. Gibson12 showed that horizontal stripes of 5 cm were actually avoided by tsetse and that vertical stripes were much less attractive than black or white surfaces (although similar to grey). She suggested that horizontal stripes might appear to a tsetse fly to be inconspicuous patches of dark and light, while the edges of the animal perpendicular to the alignment of stripes might be difficult to detect because of the absence of lateral inhibition. It is therefore noteworthy that stripes on all parts of the body except the forehead lie perpendicular to the outline of the zebra’s body. Egri et al.14 showed that tabanids are less likely to land on black and white striped surfaces than uniform black or (arguably) white surfaces, that this effect is more marked as stripe width declines and that the most effective deterrent stripe widths used in their experiments matched the range of stripe widths found on the three species of zebra. From the literature, we recalculated glossinid, tabanid and stomoxys preferences for varying stripe widths. Zebra stripe widths on most parts of the body fall in a range lower than those preferred by all three groups of biting flies (Fig. 3) extending the analysis by Egri et al.14 to two other families of biting flies. It is noteworthy that stripes nearer the ground (on the legs and head when grazing) are especially thin and biting flies prefer to alight on shaded parts of animals low on the body28. Specifically, tabanids cruise at around 30 cm above ground and prefer alighting on lower legs of cows29, although the particular area differs by species; stable flies concentrate low on cows’ bodies as do blackflies, and tsetse flies prefer to feed on the hocks of cattle30. Interestingly, faint leg striping is sometimes seen on some Przewalski’s horse individuals, although on none in our sample.

Figure 3: Biting fly preferences and stripe widths. Percent of biting flies landing on stripes of varying widths. Stomoxys and Glossina from ref 13, Tabanidae from ref 14. Vertical lines show average stripe widths taken from (left to right) face, legs, neck, flank and rump averaged across the three species of zebra. Full size image

Is there evidence that zebra striping deters biting flies from landing? Indirect observations are suggestive. First, incidence of tsetse blood meals derived from zebras is low31 particularly in G. morsitans. Second, the incidence of trypanosomiasis carried by tsetse flies is either comparable to sympatric mammals or lower32, whereas domestic horses (unstriped) suffer badly from this disease in many parts of Africa33. Elsewhere, feral and semidomesticated horses can be plagued by flies34. Many herbivores and feral horses are subject to intense fly annoyance that can reduce time spent feeding, and ameliorate attack by moving to different habitats35, resting36, retreating to shade37 or grouping together38.

Why, therefore, should African equids be so sensitive to biting flies and have evolved morphological as well as behavioural defenses? At a proximal level, hair depths and lengths of zebras are shorter than sympatric African artiodactyls and Przewalski’s horse (Fig. 4) either potentially making them more susceptible to biting flies probing perpendicularly or burrowing into and against the hair shafts, or that stripes are a sufficient deterrent to allow zebras to have cropped pelage. Moreover, unlike Asiatic equids, zebras do not have thick winter coats. Biting fly mouthpart lengths are markedly longer than average hair depths of zebras and approximately the same as zebra hair lengths (Fig. 4), perhaps making zebras particularly susceptible to annoyance.

Figure 4: Pelage characteristics of herbivores. Mean and s.e. hair depths (above) and mean and s.e. longest hair lengths (below) of zebra species, non-striped equids and artiodactyls living sympatrically with zebras measured using standard techniques. E. burchelli (5, 14 pelts, respectively) E. grevyi (3, 6), E. zebra (1, 1), E. hemionus (0, 7), E. ferus przewalskii (1, 2), blue wildebeest Connochaetes taurinus (1, 8), roan antelope Hippotragus equinus (4, 18), Oryx Oryx gazella (3, 9), hartebeest Alcelaphus buselaphus (1, 7), waterbuck Kobus ellipsiprymnus (3, 9) and topi Damaliscus lunatus (7, 13). Lengths and depths calculated from taking the averages of at 7 points on the body (neck, flank, belly, rump, upper and lower foreleg and hindleg) for each pelt and averaging across pelts. Also shown are lengths of mouthparts of biting flies. Full size image

At an ultimate level, blood loss from biting flies can be considerable. Calculations show that blood loss from tabanids alone can reach 200–500 cc per cow per day in the United States39. For example, in Pennsylvania, mean weight gain per cow was 37.2 lbs lower over an 8-week period in the absence of insecticide that prevented horn fly (Siphona), stable fly (Stomoxys) and horse fly (Tabanus) attack40, and in New Jersey, milk production increased by 35.5 lbs over a 5-week period with insecticide41. Milk loss to stable flies was calculated at 139 kg per cow per annum in the United States42. Similarly, blood-sucking insects have been shown to negatively affect performance in draft horses43.

Alternatively or additionally, striped equids might be particularly susceptible to certain diseases that are carried by fly vectors in sub-Saharan Africa44. We collated literature on diseases carried by biting flies that attack equids in Africa (Supplementary Table 1) and note that equine influenza, African horse sickness, equine infectious anaemia, and trypanosomiasis and are restricted to equids, all are fatal and all are carried by tabanids. Currently, we are unable to distinguish whether zebras are particularly vulnerable or susceptible to biting flies because they carry dangerous diseases or because of excessive blood loss, but we are inclined towards the former because Eurasian equids are not striped, yet demonstrably subject to biting fly annoyance37.

Background matching, heat management, facilitation of social interactions and predation have been also suggested as evolutionary drivers for striping in equids. Regarding background matching, we found no evidence for stripes being associated with woodland habitats, a lack of support for background matching against large mammalian predators in this environment. For subspecies, there was a marginal association between number of belly stripes and living in woodland habitat, but we suspect this derives from the close association between tsetse flies and woodlands rather than striping being a form of crypsis. Indeed, we found no evidence of striping elsewhere on the body being associated with woodland habitat, and it is difficult to envisage belly striping as a form of background matching, given its ventral position. Many artiodactyls have light or white stripes and spots set on a light or dark brown background that differ greatly from the sharp high contrast markings on zebras. Adult artiodactyls that have striped coats live in lightly wooded forests and those with striped young hide their offspring at birth45. Equids, on the other hand, are an open country clade with asses inhabiting deserts, zebras inhabiting tropical grasslands and horses living in temperate grasslands. Striped equids spend much of their time in open habitats and their foals follow their mothers from birth, which, respectively, demote the importance of striping being a form of background matching against trees and being a form of crypsis for young; background matching against grassland has been discounted by other authors46.

Heat management does not appear as a driver for equid stripes either. There were no significant associations between striping at species or subspecies levels and inhabiting hot areas using six maximum temperature measurements. In fact, all equids live in open areas and temperatures can be very high in parts of the geographic range inhabited by E. burchelli and E. grevyi (east and southern Africa, northeast Africa, respectively) and unstriped E. africanus and E. hemionus (northeast Africa and central southern Asia, respectively) and both striped and unstriped equids manage heat stress by seeking shade if available. While black and white stripes might set up convection currents, E. zebra and E. burchelli often have thick black stripes running transversely along the dorsum, which would absorb heat especially when the sun is overhead.

A social function cannot explain striping in equids either. We found no significant associations between striping and group size measures, although we acknowledge that these are coarse measures of the relative intensity of social interactions that unfortunately have received uneven attention across equid species. Generally, equids have two sorts of social system: a single stallion that lives with a small harem of mares and non-harem holding stallions that form small bands. This (type I) system47 is characteristic of E. zebra, E. burchelli and E. przewalskii. The type II system (E. grevyi, E. africanus, E. kiang and E. hemionus) consists of loose bands of animals that collect together in medium sized to large aggregations at certain times of the year, long-lasting associations only occurring between mare and foal. Here individual recognition based on stripe pattern might be advantageous but three out of these four species do not have body stripes. Given that domestic horses are capable of recognizing other individuals48 using visual, auditory and olfactory cues49, it seems unlikely that stripes are a critical means of identification.

Predation has been the predominant factor included in a number of hypotheses for body striping in equids centering on avoiding predation through confusing predators, tricking them into misjudging the size of their prey or the speed of the prey, thereby miscalculating the final leap, by making it difficult to single out a member of the herd or through aspects of striping being a signal of individual quality tied to flight speed10,19,21,50,51. All indicate that stripes should have evolved in the presence of large predators, yet there was no strong congruence between striping on any part of the animal at the species or subspecies level and close sympatry with lions, except for leg stripe intensity at the species level, driven particularly by the extinct Barbary lion and extinct Altas African wild ass co-occurring in the Maghreb. Across contemporary ecosystems in Africa, where lions have been subjects of repeated detailed study, lions capture zebra in significantly greater proportions to their abundance52, suggesting that stripes are not an antipredator defence against lions acting through confusion, dazzle or other means. Despite the popularity of various sorts of confusion hypotheses, our data provide little support for this idea.

In contrast, spotted hyaenas, a smaller predator than lions, capture fewer zebras than predicted by their relative abundance53. Hyaenas find it difficult to pull down an adult zebra because zebras deliver sharp kicks and bites to these predators; instead, hyaenas concentrate mostly on foals25. We found significant associations between rump striping and leg stripe intensity at the species level, so the possibility that striping in these areas of the body (where coursing predators might have prolonged views) is an aposematic signal to warn spotted hyaenas (and perhaps leopards P. pardalis and cheetahs Acinonyx jubatus) to keep away cannot be dismissed entirely. Nonetheless, there were no significant species associations between hyaena distribution and striping measures on other areas of the body, and none at the subspecific level: aposematic deterrence of hyaenas may be a secondary benefit of stripes but cannot explain the presence of stripes on zebras more generally. In parts of Asia once inhabited by tigers and wolves, equids are not striped.

These analyses tease apart long-standing but mainly untested hypotheses for the functions of striping in equids and lend strong ecological support for striping being an adaptation to avoid biting flies. Nonetheless, caution is warranted. First, our attempts to extract variables to test each hypothesis are inexact: plotting the distribution of tabanids was very difficult as there are no range maps for the >4,000 species and estimates of abundance are restricted to isolated study sites often in the New World. We hope that future studies can generate better quantitative spatial measures of tabanid annoyance. However, we are confident about historical tsetse fly distributions, historical predator ranges and annual temperatures. A second issue is that the sample size available with which to test hypotheses for striping in equids is small because there are only 7 extant species and 23 subspecies for which we have coarse environmental information, 3 of the latter had to be dropped owing to inadequate resolution of our phylogenetic tree. Furthermore, tree reconstruction at the species level shows that striping evolved only once in this clade. These caveats notwithstanding, it is reassuring that the sequence of studies indicating that biting flies shun striped surfaces, first put forward by Harris22 in 1930, has been given ecological validity by our finding that striping on equids is perfectly associated with increased presence of biting flies.