Spotlight: Grevy’s Zebra
Or maybe it’s white on black. But why have those stripes anyway?
The zebra: A kind of “horse,” yes, but a horse of a very different stripe!
Zebras are hooved mammals, members of the greater equine family that also includes horses and donkeys. They resemble horses, but they are stockier, closer to donkeys. And of course, unlike horses or donkeys, zebras are covered with those dazzling black and white stripes.
Three different species
The three species of zebras — Grevy’s zebras, plains zebras, and mountain zebras — differ in size and coloration. Grevy’s, the kind at the Reid Park Zoo, are the largest, at about 900 pounds, 5 feet tall at the shoulder, and 8 feet long. Plains and mountain zebras are 1–2 feet shorter and about 200 pounds lighter. Within each zebra species, males and females are about the same size.
How about those stripes?
Like fingerprints in humans, no two zebras have the same pattern of stripes. But does a zebra have black stripes on white fur or white stripes on black fur? The fact that a zebra’s fur is white in regions that don’t have stripes (often, the belly and parts of the legs) makes many people say that the pattern is black stripes on white fur. But under those stripes in its fur, a zebra’s skin is all black, and that makes other people say that the pattern is white stripes on black. What do you think?
Grevy’s, plains, and mountain zebras wear their stripes in different ways. They all have a dark dorsal stripe that extends from forehead to tail. Beyond that, Grevy’s zebras’ stripes are the narrowest of the three; they are all black and white, and they extend over the zebra’s head, neck, back and sides, and legs down to the hooves. A mountain zebra has vertical stripes on its neck and torso, but wider and fewer stripes on its haunches and rump. Many plains zebras have “shadow” stripes: dark and white stripes where their dark stripes alternate color between black and brown.
The enduring mystery
Don’t you wonder why zebras have stripes, though? Maybe you learned an answer from the children’s book, “How the Zebra Got Its Stripes.” The San people of the Kalahari Desert and the Bush People of Kenya tell an ancient story about a long-ago fight between a baboon and an all-white zebra, with the zebra ending up getting scorched by a fire in a striped pattern. (The same story explains why baboons have no hair on their rumps!)
Scientists have wondered about zebra stripes for years, too. An early idea was that the stripes help to camouflage the animals, particularly in a herd, where the stripes might make it harder for a predator to pick out individual animals to attack. Maybe this is why a herd of zebras is called a “dazzle!” Apparently, if you paint vertical stripes on a wall, zebras will tend to stand next to it, so the zebras themselves might vote for this “camouflage” hypothesis. But scientists have found that predators like lions and hyenas can only see a zebra’s stripes when they are less than 10–20 feet away, so stripes probably wouldn’t make zebras less visible to predators that were farther away.
Another idea — that the unique patterns of different individuals could help other members of their herd to tell them apart — might be right, but we know that other species of social animals can distinguish individuals in their herd without any of them having stripes. The stripes could function as a “name tag,” but it is not clear whether that function would drive the evolution of something as unusual as the zebra’s stripes.
Some recent experiments with flies, zebras, and horses wearing black-and-white-striped covers support a stranger hypothesis: that the stripes on a zebra create an optical illusion that makes it difficult for flies to gauge distance and land properly on the zebra’s hide. If flies can’t land, they can’t bite the zebra, so maybe the stripes are a novel kind of insect repellant. Whatever hypothesis you favor, it is clear that the debate about the zebra’s stripes is not over yet!
Zebras are African animals, but the original ancestors of zebras, horses, and donkeys first arose in North America about 4 million years ago, then spread to Europe and Asia. The zebra lineage separated from the others and spread into Africa about 2–2½ million years ago.
Zebras live in savannahs, grasslands, shrublands, and some woodlands. Wild Grevy’s zebras are now found only in protected game reserves in east Africa, mainly in Kenya. Plains zebras are much more abundant than Grevy’s, and they are found in wide areas of eastern and southern Africa. Mountain zebras are between the others in numbers and they are found mainly in southwestern Africa and in several scattered locations in South Africa.
Going where the grass is greener
Zebras in east Africa participate in what is called the Great Migration — a seasonal movement of millions of wildebeest and hundreds of thousands of zebras and antelopes between their winter range in Kenya and their summer calving grounds in Tanzania, over 100 miles away. Mountain zebras in southwestern Africa seasonally migrate even greater distances. These migrations are a natural result of seasonal changes in rainfall and vegetation — the animals go to where the grass is green!
It’s all about grazing
Zebras mostly graze on grass, but also will eat leaves, stems, and the bark of bushes. During the Great Migration with wildebeest and antelopes, the zebras often get to new pastures first. Rather than eating the grass down to its roots, though, the zebras bite off just the tops of the grasses, leaving plenty of food for the wildebeest and antelopes that “get to the table” after them. When not migrating, zebras spend more than half of their day grazing.
The different species of zebras have different social structures. Grevy’s zebras form herds whose membership is loose and changeable. Males usually establish territorial domains, and females roam through them, from one male’s domain to another’s. In contrast, plains and mountain zebras form into more stable herds, with one dominant male (the stallion) and several females (mares) and foals. All zebra species also form some all-male “bachelor” herds.
After a gestation of about 13 months, a pregnant zebra gives birth to a single foal. Foals stand and walk within minutes after birth. Foals’ stripes are brown and white at birth and darken to black and white with age. Grevy’s foals stay with their mother until they mature, and then leave the group. Zebras are preyed upon by lions, leopards, wild dogs, and hyenas. When they cannot outrun a predator (their top speed is about 40 mph!), they defend themselves with powerful kicks of their extremely hard hooves. Grevy’s zebras live for 20–25 years in the wild and for 25–30 years in human care in.
Grevy’s zebras are listed as Endangered by the International Union for the Conservation of Nature (IUCN), with fewer than 2,000 of them left in the wild. Plains zebras are much more abundant (150,000–250,000 in the wild), so the IUCN classifies them as Near Threatened, but their numbers are declining. Mountain zebras are classified as Vulnerable, but the good news is that their numbers are estimated to be increasing!
Stripes in Tucson
Advisory: a real groaner of a pun is coming up!
The Reid Park Zoo currently has two Grevy’s zebras, a male and a female that are named Ben and Anna. Together, they are “Ben-Anna” — like the yellow fruit you peel to eat, get it?
As part of the Species Survival Plan established for Grevy’s zebras by the Association of Zoos and Aquariums, the RPZ is allowing Ben and Anna to breed, in the hope that they will produce offspring and maintain diversity in the gene pool of Grevy’s zebras in human care. In fact, on July 4, 2020, Anna gave birth to a male foal, which was very exciting. Sadly, though, the new foal died suddenly a few days after birth, as regularly happens in the wild, in this case probably from a spinal injury. Zoo visitors and the Zoo’s staff hope that Anna will birth a new foal in the future. Zebras’ 13-month gestation will allow plenty of warning for everyone to think about possible names for a new zebra foal!
And once the Reid Park Zoo expansion opens, you might be interested to compare the striping pattern on zebras with the stripes on another beautiful and threatened species, the Malayan tigers! Whichever patterns you prefer, you’ll be helping the species just by visiting the Zoo.
Originally published at http://reidparkzooexpansion.org on July 9, 2021.