Image: Derek Mead
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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
- 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.
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Comparison between equid ranges and biting fly ranges. Image: Caro et. al
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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."