Zebra’s stripes remain a mystery
Long-held belief that pattern keeps animals cool is challenged by scientists
LONDON A fondly held theory that zebras evolved their distinctive black and white stripes in order to stay cool under the African sun has been debunked by new research.
It was thought the heat-retaining black stripes of the animal got warmer than the white areas, creating small vortexes when the hotter air above the dark fur met the adjacent cool air.
However, the idea that the zebra had developed one of nature’s most ingenious air-conditioning units and was a marvel of natural selection, has been dismissed as nothing but an old wives’ tale by Swedish biologists.
A team at Lund University set out to discover if the theory translated to the real world by filling large metal barrels with water and then covering them with the skins of horses, cattle and zebras with various black, white and grey striped patterns. The barrels were placed in the sunshine and the water temperatures measured.
Not surprisingly, the black barrel became the hottest and the white barrel the coolest. However, the striped barrel and a grey barrel of equivalent overall whiteness reached similar levels.
Using thermography, the temperature distributions of the barrel surfaces were then compared to those of living zebras. The sunlit zebra-striped barrels were found to accurately reproduce the surface temperature characteristics of sunlit zebras.
The study, published in the journal Scientific Reports, found no significant core temperature differences between the striped and grey barrels, even on many hot days, independent of the air temperature and wind speed.
Prof. Susanne Akesson, who led the research, said: “All these experimental findings provide evidence against the hypothesis of the cooling effect of zebra stripes because striped coats do not keep the core temperature of the body any cooler than homogeneous grey coats with a similar average whiteness.
“The stripes didn’t lower the temperature. It turns out stripes don’t actually cool zebras.”
The apparent failure of the theory is the latest turn in a centurieslong academic debate.
Other prominent explanations claim the zebra fur thwarts attacks from biting flies, that the stripes help protect the animal from predators by visually confusing them, or that they are the result of natural “sex selection.”
The latter two theories formed the basis of a robust exchange between Charles Darwin and rival naturalist Alfred Russel Wallace.
In The Descent of Man, Darwin rejects the idea the hide could act as camouflage, quoting the explorer William Burchell’s description of a herd in support of the sex selection argument: “Their sleek ribs glistened in the sun, and the brightness and regularity of their striped coats presented a picture of extraordinary beauty, in which probably they were not surpassed by any other quadruped,” he said.
Wallace, however, supported the camouflage argument, saying zebras were at most risk of attack at watering holes at night and their black and white stripes could help the animal blend in.
Akesson explained how modern technology had added weight to the air-conditioning theory.
“This hypothesis seems reasonable as in sunshine the black zebra stripes are warmer due to their stronger absorption of sunlight compared to the cooler white stripes of higher reflectance,” she said.
“Infrared photography of zebras showed sunlit black stripes are warmer than sunlit white stripes and that the difference between them increases with rising air temperature.
“At night, however, temperature differences are reversed, with black stripes being cooler than white ones.”
London Daily Telegraph