Over the past three years I’ve written about research, mostly by Tim Caro’s group, dealing with the perpetual question: “Why do zebras have stripes?” (See earlier posts here, here, and here.) There used to be lots of hypotheses, including confusing predators like lions, aiding thermoregulation, camouflaging these equids, keeping groups together, and so on, but research has demolished these hypotheses one by one (for example, striped animals are no more camouflaged to predators than are solid-colored animals.)
Rather, evidence has been accumulating that stripes help deter biting flies, particularly tabanid flies (horseflies) and tsetse flies that not only draw blood but, more dangerously, carry disease that can kill zebras. It’s been shown, for instance, that the ranges of striped equids all fall within the ranges of biting flies, and that these flies can carry diseases like equine influenza, African horse sickness, equine infectious anaemia, and trypanosomiasis, all fatal. Other evidence is that tabanids appear averse to landing on striped patterns, as well as the evidence against the alternative hypotheses mentioned above.
Now, however, we have more direct evidence from Caro and his colleagues in a paper published in PLoS ONE. Click on the screenshot below, or get a free pdf here (the reference is at bottom).
What the authors did here was look at the behavior of horseflies around zebras and different-colored horses, all in a pasture in Somerset in the UK. They also dressed up the horses in zebra suits as well as in monochrome white or black suits, and watched the behavior of tabanids around the equids and the behavioral responses of zebras and horses to flies. Here’s a horse in a zebra suit from the experiment:
There were a number of sub-studies, and I’ll give the results briefly. The conclusion that tabanids are averse to crawling on striped patterns seems pretty solid.
Direct observations of flies. These were done by observers watching equids and flies directly, and also by video that could follow the trajectory of flies as they approached zebras and horses.
The non-video observations showed that there was no significant difference in the rates at which horseflies circled zebras versus horses, or touched them briefly, but significantly fewer flies landed on zebras than on horses. The probability of this occurring by chance, while significant, was not very impressive (p = 0.041). This could, however, reflect a differential behavior of the animals, for horses respond to flies by twitching their skin while zebras more often switched their tails (and ran away). Perhaps skin-twitching helps deter landing.
The videos showed that the trajectories of flies approaching zebras differed from that approaching horses: flies approached the horses more slowly, and decelerated very little while approaching zebras, so that they simply bumped into the zebras rather than landing on them. This difference in the proportion of approaching flies that landed was highly significant (p < 0.0001). Further, flies flew away from zebras significantly faster than from horses.
Further, once landed, a tabanid spent considerably more time on the horse than on the zebra: 10.1 versus 1.2 seconds respectively. This again is significant at the 0.01 level, but it’s not clear whether this reflected differential behavior of the equids toward flies (e.g., skin twitching versus tail switching).
Experiment with colored coats on horses. This is the most convincing part of the results. They used cloths of three types, as shown in the diagram, and you can see that the rate of horseflies landing on the cloths or touching the clothes was significantly lower (p < 0.0001) for the striped coat versus the white or black coat, with the latter two not differing from each other. In contrast, there was no difference in the rate of flies touching or landing on the horse’s heads, which were not covered by the cloth. That shows that the pattern is probably what’s important. (Sample sizes are given in the figure; they are not large.)
The upshot: Stripes don’t appear to deter tabanids at a distance, but, once close to the horse or zebra, appear to deter landing. The authors say that tabanids find equids at a distance using odor rather than visual ues, so this is understandable.
In the case of the horse-coat experiment and of the comparison of horses versus zebras, tabanids don’t land as often on stripes as on solid colors, and, once landed, they spend less time on zebras then on horses. The flies also do not decelerate so much when approaching zebras, perhaps because they don’t see a striped substrate as an appropriate place to land.
The conclusion is that if you have stripes, you’re more likely to deter flies from landing or, if they land, less likely to get bitten and thus to get any diseases carried by the flies. As the authors note,
In summary, multiple lines of evidence indicate that stripes prevent effective landing by tabanids once they are in the vicinity of the host but did not prevent them approaching from a distance. In addition, zebras appear to use behavioural means to prevent tabanids spending time on them through constant tail swishing and even running away. As a consequence of both of these morphological and behavioural defenses, very few tabanids are able to probe for a zebra blood meal as evidenced by our data.
Now we still don’t really know what it is about stripes that deter flies or drive them away. Perhaps their visual system gets confused or flummoxed. But the coincidence of the ranges of striped equids and biting flies (see here) is pretty remarkable, and supports this experiment in suggesting that stripes evolved in equids to protect them from the bites of disease-carrying flies. I’ll put up those range maps again, which are historical rather than present-day ranges (after all, selection for stripes operated in the past), and here’s what I wrote about them:
Here’s the association between the historical (not present!) ranges of equids and of tabanids and tsetse flies; equids at top (zebra ranges striped!) and flies at bottom. Note that tsetse flies (Glossina) aren’t found outside Africa. E. kiang is an unstriped wild ass, E. africanus is the African wild ass, having thin stripes on its legs, E. hemionus is the onager, an unstriped wild ass, and E. ferus przewalskii is Prezewalski’s horse, a rare wild horse thought to be the closest living relative of the domestic horse.
The correspondence is pretty good, although not perfect, since flies live in some areas where zebras don’t. The crucial observation, though, is that biting flies always occurred in areas where zebras lived.
Note, too, that unstriped equids don’t generally coexist with either kind of fly, though the African wild ass, which does have thin striping on its legs, does live in areas with horseflies.
Caro, T., Y. Argueta, E. S. Briolat, J. Bruggink, M. Kasprowsky, J. Lake, M. J. Mitchell, S. Richardson, and M. How. 2019. Benefits of zebra stripes: Behaviour of tabanid flies around zebras and horses. PLOS ONE 14:e0210831.