How did Zebras Evolve with Black and White Stripes?

Zebras are numerous species of African equids coupled with their distinctive black-and-white striped coats. Their stripes come in diverse patterns, unique to each individual. They are usually social animals that live in small stables to large herds.

Let’s first see the evolution of Zebra in brief

Zebras evolved among the Old World horses within the last 4 million years. It has been proposed that zebras are paraphyletic and that striped equids evolved more than once. Wide stripes are posited to have been of little use to equids that live in low densities in deserts or ones that live in colder environments with shaggy coats and annual shading (like some horses). However, molecular evidence supports zebras as a monophyletic lineage. The zebra has between 32 and 46 chromosomes, depending on the species.

Stripes

It was earlier thought that zebras were white animals with black stripes since some zebras have white underbellies. Embryological data, however, shows that the animal’s background color is black and the white stripes and bellies are accessories. It is likely that the stripes are caused by a blend of factors.

The stripes are typically vertical on the head, neck, forequarters, and main body, with horizontal stripes at the rear and on the legs of the animal.

A wide variety of hypotheses have been proposed to account for the evolution of the striking stripes of zebras. The more widespread of these relate to camouflage. A currently leading hypothesis is that the stripes confuse the vision of biting flies.

  1. The upright striping may help the zebra hide in the grass by disrupting its outline. In extension, even at moderate distances, the striking striping merges to an apparent grey. However, the camouflage has been struck with arguments that most of a zebra’s predators (such as lions and hyenas) cannot see well at a distance, and are more likely to have smelled or heard a zebra before seeing it from a distance, especially at night.
  2. The stripes may help to distract predators by motion dazzle—a group of zebras standing or moving close together may appear as one large mass of flickering stripes, making it more difficult for the lion to pick out a target. It has been suggested that when moving, the stripes may confuse predators and/or biting insects by two visual illusions: the wagon-wheel effect, where the perceived motion is inverted, and the barber pole illusion, where the perceived motion is in a wrong direction.
  3. The stripes may serve as visual signs and identification among zebras themselves. Although the striping pattern is unique to each individual, it is not known whether zebras can distinguish one another by their stripes, but comparable non-striped species can differentiate individuals in some other way.
  4. The stripes may deter flies, including blood-sucking tsetse flies and tabanid horseflies. A 2012 experiment in Hungary showed that zebra-striped models were unattractive to tabanid horseflies. These flies are attracted to linearly polarized light, and black and white stripes disrupt the attractive pattern. Further, attractiveness increases with stripe width, so the relatively narrow stripes of the three living species of zebras should be unattractive to horseflies. In 2019 scientists performed an experiment putting striped blankets on horses, and found that flies were largely unable to make a controlled landing on the striped patterns, providing strong evidence for the fly-deterrence theory.
  5. Stripes may cool the zebra. Air may move more quickly over black light-absorbing stripes while moving more slowly over white stripes. This would create convection currents around the zebra that would cool it. One study found that zebras have more stripes in hotter habitats. On a sunny day in Kenya, the temperature difference between the black and the white stripes has been measured to be about 10 degrees C. This establishes small convective air movements which allow beads of sweat to evaporate more quickly.

Source: Wikipedia, BBC Future and American Society of Mammalogists.

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