Why is a Zebra Black and White?
When observing zebras in South Africa a few years ago, I began pondering. Why is a zebra black and white? What is the function of those zebra stripes? Why are zebras so unique within the Equus genus, which includes stripe-less horses, asses, and donkeys?
To my surprise, there is no consensus. Why are the stripes of a zebra black and white? What is the reason for zebras evolution? In fact, there are several theories as to why zebras evolved to exhibit such vivid stripe patterns.
Hey, you there with the black and white stripes (Attraction for procreation)
Charles Darwin was the first to offer an explanation by stating, “He who attributes the white and dark vertical stripes on the flanks of various antelopes to sexual selection, will probably extend the same view to the… beautiful zebra.”
This implies the purpose of the zebra stripes is to attract a mate.
Although it is true that each zebra has a unique black and white stripe pattern, the overall stripe pattern is quite similar within its own kind. Also, rare non-striped zebras have been observed to have little problem finding mates.
I know you! The one with the black and white stripe pattern (Recognition)
As noted above, each zebra stripe pattern is unique, similar to our fingerprints. One theory suggests these black and white patterns, help zebras recognize each other. A key aspect of zebra socialization.
This theory’s weakness is that non-striped horses have no problem recognizing each other through sight and sound.
Can’t see me and my zebra pattern! (Camouflage)
We humans may have no problem spotting a zebra black and white striped amongst green and brown trees and tall grasses. However, predators such as colour-blind lions, will have a more difficult time seeing their potential prey.
A vulnerability to this theory is that zebras run when threatened rather than hide. This could negate the camouflage advantage of the zebra stripes pattern.
You’ve seen one zebra, you’ve seen them all (Optical illusion)
This is the zebra black and white theory I admit growing up believing. Zebras often stay in a group and the zebra stripe patterns can blend into one another.
Thus, either giving the impression of a larger animal, or preventing the identification of any one individual zebra. The striped zebra herd may very well confuse predators as they attempt to attack their prey.
The counter to this theory, is that when fleeing, zebras do not stay tightly together which nullifies the optical illusion advantage.
Also, key predators including lions and spotted hyenas, have poor eyesight. This reduces the effectiveness of the optical illusion idea as the predators need to be close before making out black and white stripe patterns.
Keeping it cool (Temperature control with black and white stripes)
One theory suggests the stripes zebras have help regulate the internal temperature of the zebras. This being so important in the hot African environments in which they reside.
Some interesting experiments have tested this theory of why do zebras have stripes. One study included adding black and white stripes to water bottles and exposing them to the heat of the day.
In this case, the water temperature in the striped bottles was slightly lower than non-striped bottles. Personally, I think this is a weak experiment. Comparing static, plastic water bottles with black and white stripes to live, moving black and white zebras seems a stretch.
As with humans and horses, zebras sweat through pores in their skin. Observation of the zebras noted that hairs on the black stripes tended to become more erect in the morning and around noontime.
It has been interpreted, this hair action traps heat in the cool air of the morning and facilitates evaporation in the noontime sun.
Also, the difference in temperature between the black and white stripes in the heat of the day, may cause some localized air turbulence (mini-weather events). This in turn may enhance evaporation of the zebras’ perspiration.
A drawback to this theory is the question why the zebra black and white stripes would create this unique temperature control feature. Why would other grazing mammals do not require a stripe pattern?
Shoo fly don’t bother me (stripes of a zebra defend against flying insects)
The anti-insect theory suggests the striped patterns on zebras deter insects, like horseflies and tsetse flies, from landing on their hides.
Some interesting experiments have been performed to test this theory. In one test, horse mannequins were painted with a variety of patterns then coated them with a sticky substance (think of fly strips).
The horseflies seemed to avoid the black and white striped mannequins.
In another small experiment, six cows were painted with zebra black and white stripes. Then the cows were interspersed with unpainted cattle.
The study results revealed the striped cows suffered about 50% fewer insect bites than the unpainted cows.
But now the question is why the insects will be less likely land on black and white striped animals?
One theory is that flies are attracted to dark surfaces such as water or mud (their favourite places for breeding). Light surfaces are less attractive, and striped surfaces are even worse for attraction, perhaps confusing the flies’ sense of place.
Another theory suggests the black and white stripes create a type of depth perception problem for the flying pests. In one experiment, black horses, white horses, horses with zebra jackets, and zebras were mingled together.
Horseflies were observed hovering over all the animals equally, but fewer horseflies landed on the zebras and stripe-coated horses.
In these cases, the horseflies tried to land, but tended to come in too fast and bounce off. This suggests the insects’ depth perception may be affected by the color of zebra, the black and white stripes.
One pushback to this theory, is that insect annoyance may not be a strong enough reason for this amazing evolutionary feature of zebra stipes.
Why do zebras have stripes? (No black and white conclusion)
Several competing theories try to explain why zebras have vivid black and white-striped coats. Each theory has merits and challenges, so which one is it? Or is it a combination of two or more?
When observing zebras in South Africa a few years ago, I began pondering. Why is a zebra black and white? What is the function of those zebra stripes? Why are zebras so unique within the Equus genus, which includes stripe-less horses, asses, and donkeys? To my surprise, there is no consensus. Why are the stripes of a…
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