Why are zebras striped? – Biting flies read between the lines

Featured Image Caption: Zebras have high-contrast black and white stripes, but the purpose that they serve remains a mystery (Image Source: “~Zebra~” by ~Sage~ is licensed under CC BY-NC-ND 2.0).

Reference: Tombak, K. J., Gersick, A. S., Reisinger, L. V., Larison, B., & Rubenstein, D. I. (2022). Zebras of all stripes repel biting flies at close range. Scientific Reports, 12, 18617. https://doi.org/10.1038/s41598-022-22333-7

Why the Long Stripes?
Can you find differences in the stripe patterns between these two zebras? Scientists hypothesize that stripes could function as a way for zebras to recognize each other (Image Source: “Zebra” by mape_s is licensed under CC BY 2.0).

Why have zebras developed such intense stripes? The question has been racking the brains of scientists for ages – with each experiment presenting more questions than answers. Stripes are certainly not unique to zebras, as many other animals, like chipmunks and cats, have stripes. What makes zebra stripes such an anomaly, however, is how conspicuous and contrasting the stripes are. Now, scientists have new leads on potential explanations.

Perhaps the stripes function to identify that an individual belongs to a particular species or help zebras distinguish between individuals. Just like our fingerprint, each zebra’s striping pattern is unique to that individual, but it is unclear if they actually use their stripes to recognize each other. On the other hand, zebras need to avoid recognition by predators, which would be extremely difficult with their highly visible stripes. Camouflage is an unlikely function when viewed from the eyes of a large mammal. However, zebras face many predators with a smaller bite but equal risk: biting flies. A recent study has shown that what may not fool a large mammal may trick a small insect.

What Doesn’t Bite You Makes You Stronger
Biting flies survive on the blood of animals, but the zebra doesn’t seem to be a preferred meal. The zebra’s stripes may make navigation difficult from a fly’s eye view (Image Source: “Stomoxys calcitrans” by Marcello Consolo is licensed under CC BY-NC-SA 2.0).

Although biting flies are small, they pack a mighty punch, as bites can lead to the transfer of diseases, the loss of blood, and the reduction of feeding time for zebras. Zebras live in African woodlands and savannas that host many types of biting flies. Zebras – like some other large, hooved mammals – can produce some anti-fly odors that help deter the flies. Still, zebras are attacked far less than these animals with similar fly-repelling super-scents. Scientists have now tested whether the zebra’s stripes might add another layer of protection against the disease-ridden flies.

Flies have eyes that differ considerably from our eyes and the eyes of other mammals. What seems conspicuous to us may be visually challenging to a fly’s compound eye. For a fly, a zebra may be difficult to distinguish from its background, making them appear blurred. As the fly gets closer, a transition from a blurry background to floating bars may be visually confusing enough to deter a fly. Furthermore, a fly within a meter of a zebra may have difficulty orienting and landing on the stripes.  

Dr. Kaia Tombak, with her team of researchers, tested whether biting flies within a meter of a zebra were repelled by its stripes. Of the three species of zebras, they tested two species that had the greatest difference in the width of their stripes – as there is some evidence that narrower stripes are better repellents. The researchers used real animal pelts from the Plains zebra (wide stripes), Grevy’s zebra (narrow stripes), and impala (a hooved mammal that also produces anti-fly scent). The pelts were collected from animals found dead of natural causes in Kenya and salt-washed to remove any scent.

The Plains zebra (left) and the Grevy’s zebra (right) differ in the width of their stripes (Image Source: left – “ONTARIO-00509 – Plains Zebra” by archer10 (Dennis) is licensed under CC BY-SA 2.0; right – “Grevy’s Zebra at Chester Zoo” by bombhead is licensed under CC BY-NC 2.0).
A Disguise from the Flies

The biting flies strongly preferred the impala hide over either zebra hide – 20% landed on the impala while only 4% landed on each of the zebras.

The arena that the researchers used to test fly preference for zebra (left) or impala skin (right) (Image Source: Figure 3 from cited paper taken by Lily V. Reisinger under license CC BY 4.0).

A clear preference for impala skins over zebra skins within a meter of each other suggests that biting flies are sufficiently deterred at close range. Anti-fly odor may not be the only repellent since pelts scrubbed of scent could deter biting flies. However, they equally disliked landing on either the Plains or Grevy’s zebra, which suggests that the width of the stripes made no difference. Thus, the function of zebra stripes – especially pertaining to the differences in stripe width between species – remains an unsolved mystery. Of the flies that did land on zebras, three-fourths of them landed on the black stripes rather than the white – a result that opens yet another question surrounding the mysterious stripes.

The researchers found that zebra stripes are sufficient to evade attacks by biting flies, but in true research fashion, the experiment left researchers with more questions. Time may tell, but can scientists put the pieces together before the zebra’s time runs out?

For more information about threats to zebras and other precious African wildlife, visit https://www.awf.org/wildlife-conservation/zebra.

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Brandi Pessman

I am a third-year Ph.D. student at the University of Nebraska-Lincoln in the School of Biological Sciences. Growing up on a farm in a small town in Illinois, I developed an early love for animals and a fascination with their behaviors. When I was younger, however, it never crossed my mind that I would be using spiders to investigate how human presence affects animal behavior, but I am loving every second of it. Studying the behaviors of animals can tell us a lot about the role that we play in their survival (or death), which is becoming increasingly important as human populations continue to grow. When I am not studying spiders, I enjoy playing with my cat or being outdoors!

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