- Scientists have long wondered why zebras wear striped coats and a 2014 study might have finally supplied the answer: biting flies like glossinids (tsetse flies) and tabanids (horseflies) appear to be the “evolutionary driver” of the zebra’s stripes.
- Finding the answer to how zebras got their stripes led to another question: How exactly do stripes help zebras avoid biting insects?
- Tim Caro, a professor of wildlife biology at the University of California, Davis in the US, and Martin How, a researcher at the University of Bristol in the UK, led a new study to examine how stripes might deter biting flies as they attempt to land on zebras.
Scientists have long wondered why zebras wear striped coats — none other than Alfred Russel Wallace and Charles Darwin are known to have debated the question well over a century ago. Theories have held that a zebra’s stripes might provide camouflage or are otherwise helpful in disrupting predatory attacks, that they are a means of thermal regulation for the animals, or that they might have some social function.
A 2014 study might have finally supplied the answer, however, concluding that biting flies like glossinids (tsetse flies) and tabanids (horseflies) appear to be the “evolutionary driver” of the zebra’s stripes. Tim Caro, a professor of wildlife biology at the University of California, Davis, led a research team that mapped the geographic distributions of seven different species of zebras, horses, and asses that have stripes of various intensities on different parts of their bodies. The team then looked at where the animals’ geographic ranges overlapped with a number of different environmental variables, such as woodland areas where the stripes might camouflage the animals, the ranges of large predators, temperatures, and the geographic distributions of glossinids and tabanids.
Through their analysis, the researchers were able to rule out every theory for the origin of zebra stripes except one: the stripes help the animals deter blood-sucking flies. “I was amazed by our results,” Caro said in a statement when the research was released. “Again and again, there was greater striping on areas of the body in those parts of the world where there was more annoyance from biting flies.”
Finding the answer to how zebras got their stripes only leads to another question: How exactly do stripes help zebras avoid biting insects? Caro and Martin How of the UK’s University of Bristol led a new study to examine how stripes might deter biting flies as they attempt to land on zebras. Their results are detailed in a paper published in the journal PLOS ONE las week.
Caro, How, and team conducted a series of experiments in order to observe the behaviors of horseflies when they attempted to prey on captive zebras and domestic horses at a livery in North Somerset, England. They found no significant difference in the rate at which flies circled and even touched zebras compared to uniformly colored horses held in similar enclosures, but they did discover that the flies successfully landed on the zebras far less frequently.
In order to rule out zebras’ different smells and movements compared to horses as the potential fly-deterring mechanism, the researchers placed a striped coat on several horses and observed the results. They found that flies landed far less often on the striped coat, but just as often on the uncovered heads of the horses. “In summary, the zebra cloth coat had beneficial effects for the horse but the naked head suffered the same frequency of landings by tabanids,” the researchers write in the study.
“Horse flies just seem to fly over zebra stripes or bump into them, but this didn’t happen with horses,” Caro said in a statement about the new research. “Consequently, far fewer successful landings were experienced by zebras compared to horses.”
By analyzing videos of the flies interacting with the zebras and horses included in the experiment, the researchers discovered that flies decelerated prior to landing on horses but approached zebras at faster speeds and failed to slow down as they got close to the animals, causing the flies to simply bump into the zebras and fly away again.
“This reduced ability to land on the zebra’s coat may be due to stripes disrupting the visual system of the horse flies during their final moments of approach,” How said. “Stripes may dazzle flies in some way once they are close enough to see them with their low-resolution eyes.”
According to the researchers, zebras make a much greater effort to keep flies off of themselves by swishing their tails continuously and running away when annoyed by flies. By contrast, horses primarily twitch and only occasionally swish their tails to fend off flies. That means that even those flies that do successfully land on zebras are more likely to be shooed away before they can bite than they are when they land on horses.
Altogether, these findings suggest that stripes do not deter flies from approaching zebras but do prevent them from landing, providing further support for the theory that reducing the number of bites from predatory flies is the evolutionary benefit of zebra stripes.
“As a consequence of zebras’ striping, very few tabanids successfully landed on zebras and, as a result of zebras’ changeable behaviour, few stayed a long time, or probed for blood,” the researchers write.
It’s possible that these “morphological and behavioural anti-parasite defense strategies” help zebras avoid diseases, the authors note in the study: “In Africa where zebras live, tabanids carry diseases fatal to zebras including trypanosomiasis, equine infectious anemia, African horse sickness and equine influenza and zebras are particularly susceptible to infection because their thin pelage allows biting flies to probe successfully with their mouthparts.”
• Caro, T., Izzo, A., Reiner Jr, R. C., Walker, H., & Stankowich, T. (2014). The function of zebra stripes. Nature communications, 5, 3535. doi:10.1038/ncomms4535
• Caro, T., Argueta, Y., Briolat, E. S., Bruggink, J., Kasprowsky, M., Lake, J., … & How, M. (2019). Benefits of zebra stripes: Behaviour of tabanid flies around zebras and horses. PLOS ONE, 14(2), e0210831. doi:10.1371/journal.pone.0210831