Why do zebras have stripes? Writing in the Journal of Natural History, husband-and-wife team Alison and Stephen Cobb claim that the iconic colour pattern helps regulate body temperature.
The black stripes on a white background were seen traditionally as a form of camouflage. That might seem an odd suggestion when seeing these brightly coloured animals in zoos, but it’s entirely plausible on encountering them in their native African habitat. Zebras blend into the savannah background, their profiles broken up by stripes mimicking shafts of vegetation.
The pattern tricks the human eye, but seems unlikely to fool lions and cheetahs whose vision is much more acute than ours. The fluctuating pattern of stripes on a fleeing zebra, however, might confuse a pursuing predator. But is camouflage the whole story?
Each animal has its own unique stripe pattern, akin to a fingerprint in humans. A wildlife ranger in Africa claimed that the stripes serve as a bar code. A zebra leaves the herd to give birth. Hidden in grassy vegetation, she ‘drops’ her foal. The baby is on its feet in minutes. According to my ranger friend, the mother walks around the infant and imprints her stripe pattern on the foal. Then she leads the youngster, unsteady on its wobbly legs, back into the herd. Having memorised the bar code, it can locate the mother if they become separated in the throng.
I asked several mammal experts about this. None of them had heard of the bar-code theory and were sceptical concerning it. A baby zebra, they thought, would rely on smell, not vision, to locate its mother. Wildlife and tourist guides are not beyond embellishing their comments to give extra ‘colour’ to the items, natural or cultural, they feature. Was my ranger friend in Umfolosi ‘making it up’? Fake news is not confined to politics.
Twelve years ago, another idea emerged; the stripes, it was suggested, acted as a defence against tsetsi flies, scourge of humans and animals alike. These blood-suckers, transmitters of sleeping sickness, target mainly large patches of uniform colour on a victim’s pelage. Striped animals lack such patches and so receive fewer bites, or so the theory claimed. Support for the suggestion comes from an extinct type of zebra, known as the ‘quagga’. Living in areas where there were no tsetsies, it had few, and fainter, stripes.
The Cobbs measured temperatures on the hides of two wild zebras in Kenya and studied a skin under experimental conditions. They recorded temperature differences between the striped areas and the white background. Black objects absorb heat, white ones reflect it. During the heat of the day, the black hairs were up to 15C hotter than white background ones, resulting in flows of air between the two colour areas. Zebras, like all horses, sweat to cool down. The tiny convection currents, generated by the temperature gradient, help dissipate the animal’s body heat.
Zebras, the Cobbs discovered, can erect their black hairs, something not previously known. This may enhance the air-cooling mechanism. In the early morning, the hairs remain flat, trapping air under them to keep the animal warm. Later in the day, the hairs rise up, activating the cooling mechanism.
The various theories don’t contradict each other. Some, or all, of them may be valid.
Alison Cobb & Stephen Cobb. ‘Do zebra stripes influence thermoregulation?’ Journal of Natural History, 2019.