A recent paper in PLOS ONE has offered evidence for one explanation of why zebras have stripes - namely that the pattern confuses the motion perception systems of flies.
Being able to avoid bites from horseflies or tsetse flies would arguably confer an evolutionary advantage, because of the diseases that these flies often carry.
The researchers observed the behaviour of horseflies in the vicinity of, respectively, zebras and domestic horses kept on a single farm in the UK, recording the flight paths of horseflies as they approached the animals and attempted to land on them.
While the stripes were not sufficient to deter flies from approaching (roughly equal numbers of flies circled around the horses and the zebras), when the flies attempted to land on the zebras, they were far less successful in doing so than on the horses, often flying past or colliding with the zebras. Analysis of flight paths showed that the flies slowed down prior to landing on the horses, but failed to decelerate in time to successfully complete controlled landings on the zebras.
The paper also refers to earlier work by their group (https://doi.org/10.1016/j.zool.2013.10.004), in which they demonstrated that their simulation of a previously-suggested algorithm of motion detection in the fly could be confused by a pattern of moving stripes, into miscalculating the direction of movement of the pattern. Essentially, the mechanism is that, when correlating stripes from one frame of the moving stimulus to the next, the model matches up a stripe in one frame with an adjacent stripe in the next frame, rather than with the displaced original stripe. At certain speeds of motion, the whole pattern appears to move in the opposite direction from its actual motion. In the PLOS ONE paper, the authors suggest that this may be one mechanism by which the striped pattern of the zebra disrupts optic flow perception in the fly, and its subsequent landing behaviour.
Putting striped coats on the horses greatly protected them against horsefly landings, while solid black or white coats did not. Meanwhile, the flies still managed to land at the same high rate on the horses' heads, which were uncovered.
Behaviour also played a later role: in the rare event that a horsefly did manage to land on a zebra, the zebra typically responded by flicking its tail or moving away, while these behaviours were relatively absent in the horses. The authors argue that it was a consequence of both the striped pattern and behaviour of the zebra that very few horseflies were able to successfully feed on the blood of any of the zebras in this study.
Very interesting work. This almost seems to be crying out for an electrophysiological study….!