Zebra

Zebras are African equines with distinctive black-and-white striped coats. There are three living species: Grévy's zebra, the plains zebra, and the mountain zebra. Zebras share the genus Equus with horses and asses, the three groups being the only living members of the family Equidae. Zebra stripes come in different patterns, unique to each individual. Several theories have been proposed for the function of these patterns, with most evidence supporting them as a deterrent for biting flies. Zebras inhabit eastern and southern Africa and can be found in a variety of habitats such as savannahs, grasslands, woodlands, shrublands, and mountainous areas.
Zebras are primarily grazers and can subsist on lower-quality vegetation. They are preyed on mainly by lions, and typically flee when threatened but also bite and kick. Zebra species differ in social behaviour, with plains and mountain zebra living in stable harems consisting of an adult male or stallion, several adult females or mares, and their young or foals; while Grévy's zebra live alone or in loosely associated herds. In harem-holding species, adult females mate only with their harem stallion, while male Grévy's zebras establish territories which attract females and the species is polygynandrous. Zebras communicate with various vocalisations, body postures and facial expressions. Social grooming strengthens social bonds in plains and mountain zebras.
Zebras' dazzling stripes make them among the most recognisable mammals. They have been featured in art and stories in Africa and beyond. Historically, they have been highly sought by exotic animal collectors, but unlike horses and donkeys, zebras have never been completely domesticated. The International Union for Conservation of Nature lists Grévy's zebra as endangered, the mountain zebra as vulnerable and the plains zebra as near-threatened. The quagga, a type of plains zebra, was driven to extinction in the 19th century. Nevertheless, zebras can be found in numerous protected areas.

Etymology

The English name "zebra" derives from Italian, Spanish or Portuguese. Its origins may lie in the Latin equiferus, meaning "wild horse". Equiferus appears to have entered into Portuguese as ezebro or zebro, which was originally used for a mysterious equine reported in the wilds of the Iberian Peninsula during the Middle Ages. In 1591, Italian explorer Filippo Pigafetta recorded "zebra" being used to refer to the African animals by Portuguese visitors to the continent. In ancient times, the zebra was called hippotigris by the Greeks and Romans.
The word zebra was traditionally pronounced with a long initial vowel, but over the course of the 20th century the pronunciation with the short initial vowel became the norm in British English. The pronunciation with a long initial vowel remains standard in American English.

Taxonomy

Zebras are classified in the genus Equus along with horses and asses. These three groups are the only living members of the family Equidae. The plains zebra and mountain zebra were traditionally placed in the subgenus Hippotigris in contrast to the Grévy's zebra which was considered the sole species of subgenus Dolichohippus. Groves and Bell placed all three species in the subgenus Hippotigris. A 2013 phylogenetic study found that the plains zebra is more closely related to Grévy's zebras than mountain zebras. The extinct quagga was originally classified as a distinct species. Later genetic studies have placed it as the same species as the plains zebra, either a subspecies or just the southernmost population. Molecular evidence supports zebras as a monophyletic lineage.
Equus originated in North America and direct paleogenomic sequencing of a 700,000-year-old middle Pleistocene horse metapodial bone from Canada implies a date of 4.07 million years ago for the most recent common ancestor of the equines within a range of 4.0 to 4.5 mya. Horses split from asses and zebras around this time and equines colonised Eurasia and Africa around 2.1–3.4 mya. Zebras and asses diverged from each other close to 2 mya. The mountain zebra diverged from the other species around 1.6 mya and the plains and Grévy's zebra split 1.4 mya.
A 2017 mitochondrial DNA study placed the Eurasian Equus ovodovi and the subgenus Sussemionus lineage as closer to zebras than to asses. However, other studies disputed this placement, finding the Sussemionus lineage basal to the zebra+asses group, but suggested that the Sussemionus lineage may have received gene flow from zebras.
The cladogram of Equus below is based on Vilstrup and colleagues and Jónsson and colleagues :

Extant species

Fossil record

In addition to the three living species, some fossil zebras and relatives have also been identified. E. oldowayensis is identified from remains in Olduvai Gorge dating to 1.8 mya. Fossil skulls of E. mauritanicus from Algeria which date to around 1 mya appears to show affinities with the plains zebra. E. capensis, known as the Cape zebra, appeared around 2 mya and lived throughout southern and eastern Africa.

Hybridisation

Fertile hybrids have been reported in the wild between plains and Grévy's zebra. Hybridisation has also been recorded between the plains and mountain zebra, though it is possible that these are infertile due to the difference in chromosome numbers between the two species. Captive zebras have been bred with horses and donkeys; these are known as zebroids. A zorse is a cross between a zebra and a horse; a zonkey, between a zebra and a donkey; and a zoni, between a zebra and a pony. Zebroids are often born sterile with dwarfism.

Characteristics

As with all wild equines, zebras have barrel-chested bodies with tufted tails, elongated faces and long necks with long, erect manes. Their thin legs are each supported by a spade-shaped toe covered in a hard hoof. Their dentition is adapted for grazing; they have large incisors that clip grass blades and rough molars and premolars well suited for grinding. Males have spade-shaped canines, which can be used as weapons in fighting. The eyes of zebras are at the sides and far up the head, which allows them to look over the tall grass while feeding. Their moderately long, erect ears are movable and can locate the source of a sound.
Unlike horses, zebras and asses have chestnut callosities present only on their front legs. In contrast to other living equines, zebras have longer front legs than back legs. Diagnostic traits of the zebra skull include: its relatively small size with a straight dorsal outline, protruding eye sockets, narrower rostrum, less conspicuous postorbital bar, separation of the metaconid and metastylid of the tooth by a V-shaped canal and rounded enamel wall.

Stripes

Zebras are easily recognised by their bold black-and-white striping patterns. The coat appears to be white with black stripes, as indicated by the belly and legs when unstriped, but the skin is black. Young or foals are born with brown and white coats, and the brown darkens with age. A dorsal stripe acts as the backbone for vertical stripes along the sides, from the head to the rump. On the snout they curve toward the nostrils, while the stripes above the front legs split into two branches. On the rump, they develop into species-specific patterns. The stripes on the legs, ears and tail are separate and horizontal.
Striping patterns are unique to an individual and heritable. During embryonic development, the stripes appear at eight months, but the patterns may be determined at three to five weeks. For each species there is a point in embryonic development where the stripes are perpendicular to the dorsal line and spaced apart. However, this happens at three weeks of development for the plains zebra, four weeks for the mountain zebra, and five for Grévy's zebra. The difference in timing is thought to be responsible for the differences in the striping patterns of the different species.
Various abnormalities of the patterns have been documented in plains zebras. In "melanistic" zebras, dark stripes are highly concentrated on the torso but the legs are whiter. "Spotted" individuals have broken up black stripes around the dorsal area. There have even been morphs with white spots on dark backgrounds. Striping abnormalities have been linked to inbreeding. Albino zebras have been recorded in the forests of Mount Kenya, with the dark stripes being blonde. The quagga had brown and white stripes on the head and neck, brown upper parts and a white belly, tail and legs.

Function

The function of stripes in zebras has been discussed among biologists since at least the 19th century. Popular hypotheses include the following:

The crypsis hypothesis suggests that the stripes allow the animal to blend in with its environment or break up its outline. This was the earliest hypothesis and proponents argued that the stripes were particularly suited for camouflage in tall grassland and woodland habitat. Alfred Wallace also wrote in 1896 that stripes make zebras less noticeable at night. Biologist Tim Caro notes that zebras graze in open habitat and do not behave cryptically, being noisy, fast, and social and do not freeze when a predator is near. In addition, the camouflaging stripes of woodland living ungulates like bongos and bushbucks are much less vivid with less contrast with the background colour. A 1987 Fourier analysis study concluded that the spatial frequencies of zebra stripes do not line up with their environment, while a 2014 study of wild equine species and subspecies could not find any correlations between striping patterns and woodland habitats. Melin and colleagues found that lions and hyenas do not appear to perceive the stripes when they are a certain distance away at daytime or nighttime, thus making the stripes useless in blending in except when the predators are close enough by which they could smell or hear their target. They also found that the stripes do not make the zebra less noticeable than solidly coloured herbivores on the open plains. They suggested that stripes may give zebras an advantage in woodlands, as the dark stripes could line up with the outlines of tree branches and other vegetation.
The confusion hypothesis states that the stripes confuse predators, be it by: making it harder to distinguish individuals in a group as well as determining the number of zebras in a group; making it difficult to determine an individual's outline when the group runs away; reducing a predator's ability to keep track of a target during a chase; dazzling an assailant so they have difficulty making contact; or making it difficult for a predator to deduce the zebra's size, speed and direction via motion dazzle. This theory has been proposed by several biologists since at least the 1970s. A 2014 computer study of zebra stripes found that they may create a wagon-wheel effect and/or barber pole illusion when in motion. The researchers concluded that this could be used against mammalian predators or biting flies. The use of the stripes for confusing mammalian predators has been questioned. Caro suggests that the stripes of zebras could make groups seem smaller, and thus more likely to be attacked. Zebras also tend to scatter when fleeing from attackers and thus the stripes could not break up an individual's outline. Lions, in particular, appear to have no difficulty targeting and catching zebras when they get close and take them by ambush. In addition, no correlations have been found between the number of stripes and populations of mammal predators. Hughes and colleagues disputed the idea of motion dazzle and concluded that moving objects that are solidly grey or have less contrasted patterns are actually more likely to escape being caught.
The aposematic hypothesis suggests that the stripes serve as warning colouration. This hypothesis was first suggested by Wallace in 1867 and discussed in more detail by Edward Bagnall Poulton in 1890. As with known aposematic mammals, zebras are recognizable up close, live in more open environments, have a high risk of predation and do not hide or act inconspicuous. However, Caro notes that stripes do not work on lions because they frequently prey on zebras, though they may work on smaller predators, and zebras are not slow-moving enough to need to ward off threats. In addition, zebras do not possess adequate defenses to back up the warning pattern.
The social function hypothesis states that stripes serve a role in intraspecific or individual recognition, social bonding, mutual grooming or a signal of fitness. Charles Darwin wrote in 1871 that "a female zebra would not admit the addresses of a male ass until he was painted so as to resemble a zebra" while Wallace stated in 1871 that: "The stripes therefore may be of use by enabling stragglers to distinguish their fellows at a distance". Regarding species and individual identification, Caro notes that zebra species have limited range overlap with each other and horses can recognise each other using visual communication. In addition, no correlation has been found between striping and social behaviour or group numbers among equines, and no link has been found between fitness and striping.
The thermoregulatory hypothesis suggests that stripes help to control a zebra's body temperature. In 1971, biologist H. A. Baldwin noted that heat would be absorbed by the black stripes and reflected by the white ones and in 1990, zoologist Desmond Morris suggested that the stripes create cooling convection currents. A 2019 study supported this, finding that where the faster air currents of the warmer black stripes meet those of the white, air swirls form. The researchers also concluded that during the hottest times of the day, zebras erect their black hair to release heat from the skin and flatten it again when it gets cooler. Larison and colleagues determined that environmental temperature is a strong predictor for zebra striping patterns. Others have found no evidence that zebras have lower body temperatures than other ungulates whose habitat they share, or that striping correlates with temperature. A 2018 experimental study which dressed water-filled metal barrels in horse, zebra and cattle hides concluded that zebra stripes had no effect on thermoregulation.
The fly protection hypothesis holds that the stripes deter blood-sucking flies. Horse flies, in particular, spread diseases that are lethal to equines such as African horse sickness, equine influenza, equine infectious anemia and trypanosomiasis. In addition, zebra hair is about as long as the mouthparts of these flies. This hypothesis is the most strongly supported by the evidence. It was found that flies preferred landing on solidly coloured surfaces over those with black-and-white striped patterns in 1930 by biologist R. Harris, and this was proposed to have been a function of zebra stripes in a 1981 study. A 2014 study found a correlation between striping and overlap with horse and tsetse fly populations and activity. Other studies have found that zebras are rarely targeted by these insect species. Caro and colleagues studied captive zebras and horses and observed that neither could deter flies from a distance, but zebra stripes kept flies from landing, both on zebras and horses dressed in zebra print coats. There does not appear to be any difference in the effectiveness of repelling flies between the different zebra species; thus the difference in striping patterns may have evolved for other reasons. White or light stripes painted on dark bodies have also been found to reduce fly irritations in both cattle and humans. How the stripes repel flies is less clear. A 2012 study concluded that they disrupt the polarised light patterns these insects use to locate water and habitat, though subsequent studies have refuted this. Stripes do not appear to work like a barber pole against flies since checkered patterns also repel them. There is also little evidence that zebra stripes confuse the insects via visual distortion or aliasing. Takács and colleagues suggest that, when the animal is in sunlight, temperature gradients between the warmer dark stripes and cooler white stripes prevent horseflies from detecting the warm blood vessels underneath. Caro and colleagues conclude that the high colour contrast and relative thinness of the patterns make it difficult for the insects to find a place to land.