Why do zebras have stripes? The long-assumed answer that herds of striped beasts confuse predators, making it hard for them to focus on a single individual, has taken a hit.
When you are trying to hide on the plains of Africa, having black or brown and white stripes seems more likely to advertise your presence than to hide it. The fact that the big cats and other predators (humans excluded) are color-blind has been proposed as an explanation but, even to a creature with dichromatic vision, stripes may make a meal on legs more visible, not less.
A more sophisticated theory is that the chaotic movement of vertical lines during a stampede makes it hard for a lion to pounce. Testing is rather hard to do in this case. Zebras probably wouldn't take kindly to researchers chasing them around with pots of paint, and their kick is lethal.
Anna Hughes of the University of Cambridge decided to reverse the usual flow of research and conduct studies on humans to learn about animals. She had people play a computer game with either striped or grey targets that needed to be picked out in a moving crowd.
Hughes says that the idea of “motion dazzle” as a form of camouflage was so popular that ships in World War I and II were painted with geometric shapes in the hope that they would confuse enemies. However, the idea was based more on theory than experimental research. Seventy years later Hughes tested the idea by having 60 participants chase moving objects with differing patterns around a screen, either singly or in groups. Her findings are reported in Frontiers of Zoology.
“We found that when targets are presented individually, horizontally striped targets are more easily captured than targets with vertical or diagonal stripes,” said Hughes. “Surprisingly, we also found no benefit of stripes when multiple targets were presented at once, despite the prediction that stripes should be particularly effective in a group scenario. This could be due to how different stripe orientations interact with motion perception, where an incorrect reading of a target's speed helps the predator to catch its prey.”
“For an isolated target, plain gray carried a similar risk to lines perpendicular or diagonal to the direction of motion (vertical on a zebra), while parallel lines assisted the predator. With multiple targets any form of stripes made the prey more vulnerable, although at least the horizontal pattern on a zebra's hindquarters no longer made them more of a target than stripes running in other directions would."
Given how successful zebras have been, Hughes is not ready to give up the motion dazzle theory entirely. “Motion may just be one aspect in a larger picture,” she said. “Now we need to consider whether color, stripe width and spatial patterning, and a predator's visual system could be important factors for animals to avoid capture.” Moreover, in an earlier version of her research published last year, Hughes found that stripes are at least better protection than spots or other patterns.
Hughes told IFLScience that her findings so far are “counter-intuitive”, indicating that stripes may actually be more useful in isolation than in groups, but this may reflect the preliminary nature of the work. “The single target game was time-limited, whereas the subjects had as much time as they liked to catch all the targets in the multiple target game. So it could be that it's only a very 'split second' effect.”
Alternatively, Hughes says, stripes may not be about camouflage at all. No less a source than Darwin doubted this explanation, and theories such as bug repellants and temperature control have some support today. "I think it's probably still too early to say with any certainty which of the many competing hypotheses about the zebra stripes is correct! It's also quite possible the stripes evolved for multiple functions," Hughes told IFLScience.