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."
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