Image: Derek Mead
Zebras are obviously the chillest animals on Earth, but how did they get that way? As it turns out, their signature stripes may not have evolved as camouflage, but instead are largely a deterrent to blood-sucking flies.
It's easy to wonder what possible value a zebra's striking coloration could have, especially since many other mammals, and more specifically equids, sport coats that do a better job of blending in with their surroundings. But zebras' stripes wouldn't have evolved without a purpose, and as you probably heard in grade school, the most obvious explanation is that it serves as camo that confuses predators with the dazzle effect.
But according to research published today in Nature Communications, there's another likely reason: Biting flies don't like to land on stripes. The study team, led by Tim Caro of UC Davis, compared the habitats, predators, and other factors that might potentially make stripes useful for the seven extant equid species in Africa and Asia.
Two things stood out: The three species with stripes are the only ones located in the same spot as blood-sucking, disease-carrying tsetse flies, and their width of their stripes match previous models showing the optimal stripe size for deterring flies. And to be clear, biting flies can be a huge problem: studies have shown that cattle in the US can lose 200-500cc of blood per day if biting flies aren't controlled with pesticides, and they can also be serious disease vectors.
"Our data shows forcefully that the function (i.e. current usefulness) of stripes is to deter flies from landing," Caro told me in an email.
This figure shows that equid species with stripes (including E. africanus, which has leg striping) are found in regions with far higher tabanid fly activity. Image: Caro et. al
Various studies have shown that tsetse ﬂies, stomoxys stable ﬂies, and tabanid biting flies land on striped surfaces far less than uniform ones, but that alone doesn't discount a number of other hypotheses for the utility of stripes. Broadly, they fit into five categories:
- What we'd think of as traditional camouflage, in which the stripes help a zebra blend into a woodland background or shadows from tall grass.
- Stripes confuse predators, by making it hard to judge an animals size, speed, trajectory, location in a herd, or something similar.
- Stripes help with heat management, especially on the hot savannah.
- Zebras use stripes as social cues or for mate choice, as is often the case for flamboyant coloration, especially in birds.
- Stripes act as a deterrent to ectoparasites.
After testing for each factor, Caro's team found that "there is no consistent support for camouﬂage, predator avoidance, heat management or social interaction hypotheses." As they note, many factors are interrelated. Hyenas are located in both the hot tropics and subtropics, combining temperature and predation variables; meanwhile, tabanid flies sometimes rest in trees, combining parasite and background blending.
But the key correlation remained stripes and biting flies. Previous studies have shown that biting flies are much less likely to land on surface with stripes of less than 5 centimeters in width than surfaces with wider stripes or uniform colors; all zebra stripes fall in that range.
"There may be some equivocation depending on the species of fly and their landing behavior, but the majority of field experimental studies find that striped targets receive less flies," Caro wrote.
There's also a clear geographic correlation. "lndeed, the ranges where tabanids and tsetse ﬂies are active matches that of striped equids well," they write. Take a look for yourself:
Comparison between equid ranges and biting fly ranges. Image: Caro et. al
Note that the ranges of the three striped zebra species all overlap in regions where tabanid flies (Glossina) are known to be found, or where tabanid flies (Tabanus) are located at least seven consecutive months out of the year. In contrast, the three Asian species, the onager (E. hemionus), Przewalski’s horse (E. ferus przewalksii), and kiang (E. kiang) all aren't located in the ranges of the biting flies included in the study, and none of the trio have stripes.
I asked Caro about the puzzle-like lack of overlap between Asian equid species and tabanid fly ranges, and he said it might be a bit of a coincidence. "I saw that too, but remember we just took one measure of tabanid biting fly annoyance—seven consecutive months," he wrote. "If we had taken eight or six, the fly range boundaries would have moved a bit, so dont read too much into it."
Additionally, zebras are hardly the only large mammals susceptible to biting flies. "Why, therefore," the authors ask, "should African equids be so sensitive to biting ﬂies and have evolved morphological as well as behavioural
defenses?" While it's a bit of a chicken-egg scenario, the authors found that zebras have far shorter hair than their non-striped relatives, and also don't have the added benefit of growing thick winter coats like those equids found in cold Asian regions.
So that means flies likely have an easier time biting zebras than species with thicker coats. And regardless of whether it's largely blood loss or disease that's proving negative, it's clear that zebras would need an alternate defense.
"We discuss whether zebras are trying to avoid blood loss, or to avoid fatal diseases carried by African biting flies, and we err towards the latter explanation," Caro wrote in an email. "But certainly domestic livestock suffer badly from weight loss and low milk production in the southeast US without insecticide application , so blood loss might also impact fitness. Disease surely will."
The authors note that it's not possible to say that stripes solely evolved to combat flies, as there are too many intermixed variables, and stripes may provide benefits that didn't initially drive their evolution. For example, zebras are fearsome kickers, and rump stripes may also serve as a warning to predators—especially hyenas, which have trouble taking down zebras—to back off.
There's also the question of sample size: With only a few extant equid species and thousands of tabanid flies, developing complete range maps for comparison is difficult.
Still, the influence seems clear. As the authors conclude, "striping on equids is perfectly associated with increased presence of biting ﬂies."