For over a century, scientists have been trying to solve the mystery of why zebras evolved to have stripes. Theories have ranged from deterring predators and social interactions to keeping cool and avoiding flies, all of which have been disproved or lack sufficient proof. Now, a new study has provided some evidence that one of these hypotheses may be true ― the pattern may, in fact, have evolved to deter biting flies. Findings of the study published on 20 February in PLoS ONE suggest the stripes confuse approaching flies and disrupt their landing on these large striped animals (1).
In Africa, tabanids carry infectious diseases that are particularly detrimental to zebras, owing to their thin pelage (hair) which allows insects to easily pierce their skin. This has, in part, led to an emerging consensus among biologists that zebra stripes have evolved to foil incoming attacks from horseflies, as well as other potentially dangerous biting insects.
To gain more insight into this possible evolutionary driver of zebra stripes, an international team of researchers studied the behaviour of tabanids (horseflies) around captive zebras compared to domestic horses by direct observations and video footage. The study involved a total of nine horses and three zebras.
While they found no differences in the number of flies circling the animals, they discovered that the rate of successful landings was significantly lower on zebras compared to the horses. To control for differences in the smell of zebras and horses, some of the horses were cloaked in striped coats. The researchers discovered that, like zebras, the horses in striped disguises also experienced fewer horse fly landings.
Interestingly, the effect only seems to work as the flies get close to the animals. As the flies approach, instead of a nice controlled landing, stripes cause the horse flies either quickly turned away quite quickly or collide with the animal. Furthermore, zebras behaved differently to the horses, swishing their tails and at times even running away in an effort to escape the flies. The authors suggest this could be yet another species-specific trait adopted to thwart biting assailants.
In addition, the researchers found that horseflies circled and touched a single grey horse less frequently than zebras, although there was no significant difference. But this is potentially important since flies are likely unable to see a zebra’s stripes from a distance so a zebra will look like a grey horse at distances greater than two meters. Thus, the results provide “multiple lines of evidence” indicating that stripes prevent horseflies from landing effectively.
Why this is the case remains unknown. The sudden appearance of stripes on closer approach might surprise the insects causing them to change their trajectory, or they might alter the insect’s perception of how fast things are moving, according to the authors. Based on the video footage, it would appear the flies were unable to slow down steadily they got closer to the zebras. The authors also write that different amount of heat given off by the white and black stripes could confuse the flies, however, there is no direct evidence of this.
The study had a number of limitations. The experiment was performed in the UK and not in Africa. Moreover, zebra and horse coats are made of different materials. Nevertheless, perhaps with this new knowledge, patterned clothing will become the next insect repellent?
(1) Caro, T. et al. Benefits of zebra stripes: Behaviour of tabanid flies around zebras and horses. PLoS ONE (2019). DOI: 10.1371/journal.pone.0210831