Stripes reduce controlled landing by biting flies, supporting the leading hypothesis of their utility.
The stripes of a zebra deter horse flies from landing on them, according to a new study.
Zebra stripes have been posited to provide camouflage, visually confuse predators, signal to other zebras, or help control heat gain, but none of these hypotheses have withstood rigorous experimentation.
An alternative, that stripes somehow reduce the likelihood of being bitten by predatory flies, has gained adherents, but the mechanism has been unclear.
In the new study, the authors compared behavior of horse flies as they attempted to prey on zebras and uniformly colored horses held in similar enclosures.
Flies circled and touched horses and zebras at similar rates, but actually landed on zebras less than one-quarter as often.
Additionally, zebras were at greater pains to keep flies off through tail swishing and running away.
Stripes do not deter flies from approaching zebras, but do prevent effective landing, and thus, reduce the number of flies successfully feeding.
This finding provides further support for the hypothesis that the evolutionary benefit of zebra stripes is to reduce biting by predatory flies.