7 26 1
Scientists have puzzled for decades over why zebras have such a startling array of stripes, with theories that range from staying cool and repelling biting insects to confusing predators on the hunt.
Now, researchers at the University of Cambridge in England have produced evidence suggesting that the dazzling array of moving stripes when a herd moves off might not protect zebras from predators after all.
They found that humans playing a computer game captured striped targets more easily than uniform grey targets when multiple targets were present. This rebukes assumptions that stripes evolved to make it difficult to capture animals moving in a group.
The university’s Anna Hughes said: “We found that when targets are presented individually, horizontally striped targets are more easily captured than targets with vertical or diagonal stripes.
“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.”
Stripes, zigzags and high contrast markings make animals highly conspicuous, which you might think would make them more visible to a predator. Researchers have wondered if movement is important in explaining why these patterns have evolved.
Striking patterns may confuse predators and reduce the chance of attack or capture. The concept was termed “motion dazzle”, where high contrast patterns cause predators to misperceive the speed and direction of the moving animal. It was suggested that motion dazzle might be strongest in groups, such as a herd of zebra.
“Motion dazzle” is a reference to a type of camouflage used on ships in World War 1 and 2, where ships were patterned in geometric shapes in contrasting colors. Rather than concealing ships, this dazzle camouflage was believed to make it difficult to estimate a target’s range, speed and heading.
The researchers, whose findings have been published in the journal, Frontiers in Zoology, had 60 human participants play a game to test whether stripes influenced their perception of moving targets.
They performed a touch-screen task in which they attempted to “catch” moving targets, both when only one target was present on screen and when there were several targets present at once.
When single targets were present, horizontal striped targets were easier to capture than any other target, including uniform color, or vertical or diagonal stripes. However, when multiple targets were present, all striped targets, irrespective of the orientation, were captured more easily than uniform grey targets.
Hughes continued: “Motion may just be one aspect in a larger picture. Different orientations of stripe patterning may have evolved for different purposes. The evolution of pattern types is complex, for which there isn’t one over-ruling factor, but a multitude of possibilities.
“More work is needed to establish the value and ecological relevance of ‘motion dazzle’. 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.”
Hughes, in a separate blog on the research, noted that, until recently, there had been little scientific evidence for motion dazzle effects.
She said her study was preliminary in many ways. “Human eyes are different to those of the real predators of zebras, and therefore the effects of stripes on visual perception may also be different.
“Similarly, our model of group movement was very basic, with all the targets moving independently from each other: it may be the case that more complex designs, where the targets try to ‘stick together’ in the manner of a real herd would give a different result.
“Finally, the evidence for motion dazzle effects of stripes needs to be pieced together with the evidence for the other functions of the zebra’s distinctive pattern, as it is possible that the stripes have evolved for multiple purposes.
“Overall, while our study provides evidence that some stripe patterns may provide protection when in motion, the effects seem to depend upon both the orientation of the stripes and whether there are multiple targets present.
“Whether motion dazzle is a plausible explanation for the mystery of the zebra’s stripes is certainly not solved yet.”
Hughes was joined in the research by Richard Magor-Elliott, who is also from the University of Cambridge, and Martin Stevens, from the University of Exeter, in Cornwall, England.
Anna E. Hughes, Richard S. Magor-Elliott, Martin Stevens. The role of stripe orientation in target capture success. Frontiers in Zoology, 2015; 12 (1) DOI: 10.1186/s12983-015-0110-4
The full study can be read here.