Grévy's Zebra

Grévy's Zebra
Grévy's Zebra
Conservation status
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Perissodactyla
Family: Equidae
Genus: Equus
Species: E. grevyi
Binomial name
Equus grevyi
Oustalet, 1882
Grévy's zebra range
(blue — native, red — introduced)

The Grévy's zebra (Equus grevyi), also known as the Imperial zebra, is the largest extant wild equid and one of three species of zebra, the other two being the plains zebra and the mountain zebra. Named after Jules Grévy, it is the sole extant member of the subgenus Dolichohippus. The Grévy's zebra is found in Kenya and Ethiopia.[2] Compared with other zebras, it is tall, has large ears, and its stripes are narrower. It is more ass-like in appearance as compared to other zebras, which are more horse-like.

The Grévy's zebra lives in semi-arid grasslands where it feeds on grasses, legumes, and browse; it can survive up to five days without water. It differs from the other zebra species in that it does not live in harems and has few long-lasting social bonds. Male territoriality and mother–foal relationships form the basis of the social system of the Grévy's zebra. This zebra is considered to be endangered. Its population has declined from 15,000 to 3,000 since the 1970s. However, as of 2008 the population is stable.[1]

Contents

Taxonomy and naming

The Grévy's zebra was first described by French naturalist Émile Oustalet in 1882. He named it after [[Jules Grévy], then president of France, who, in the 1880s, was given one by the government of Abyssinia. It is the only extant species of the subgenus Dolichohippus. The plains zebra and mountain zebra belong to Hippotigris. Fossils of Dolichohippus zebras have been found throughout Africa and Asia in the Pliocene and Pleistocene deposits.[3] Notable examples include E. sanmeniensis from China, E. cautleyi from India, E. valeriani from central Asia and E. oldowayensis from East Africa.[3] The latter, in particular is very similar to the Grévy's zebra and may have been its ancestor.[3] The modern Grévy's zebra arose in the early Pleistocene.[3] Recent phylogenetic evidence suggests that Grevy's zebras are with asses and donkeys in a lineage separate from plains zebras, but perhaps not from mountain zebras.[4] In areas where Grévy's zebras are sympatric with plains zebras, it is not unusual to find them in the same herds[5] and fertile hybrids do occur.[6]

Description

From left to right: a cranium, a complete skeleton, a left forefoot frontal, and a left forefoot lateral from a Grévy's zebra.

Grévy's zebra is the largest of all wild equines. It is 2.5–2.75 m (8–9 ft) from head to tail with a 38–75 cm (15–30 in) tail, and stands 1.45–1.60 m (4'7"–5'3") high at the shoulder. These zebras weigh 350–450 kg (770–990 lb).[5] The stripes are narrow and close-set,[5] being broader on the neck, and they extend to the hooves. The belly and the area around the base of the tail lack stripes. Grévy's zebra differs from the other two zebras in its more primitive characteristics.[7]:147 Its head is large, long, and narrow with elongated nostril openings.[7]:147 It is particularly mule-like in appearance with a brown muzzle. The ears are very large, rounded, and conical.[8] The mane is tall and erect; juveniles have a mane that extends to the length of the back and shortens when they reach adulthood. Foals are born with brown and white striping, with the brown stripes darkening to black as they grow older.[8]

Range and ecology

Zebra on the barren plains of Kenya.

The Grévy’s zebra once ranged though most of Kenya, Eritrea, Ethiopia and Somalia. Today it now largely inhabits northern Kenya, with some isolated populations in Ethiopia.[7]:147[8] Its status in Sudan is uncertain.[1] This zebra fills an ecological niche between the African wild ass, which prefers a more arid habitat, and the more water-dependent plains zebra.[7]:147[5] It has adapted to the thornbush country of the Acacia-Commiphora bushlands and the Somali Arid Zone's barren plains.[5] Lactating females and non-territorial males use areas with green, short grass and medium, dense bush more often than non-lactating females and territorial males.[9]

Grévy's zebras rely on grasses, legumes, and browse for nutrition.[8] They commonly browse when grasses are not plentiful.[5][10] Their hindgut fermentation digestive system allows them to subsist on diets of lower nutritional quality than that necessary for ruminant herbivores. Grevy's zebras can survive up to five days without water, but will drink daily when it is plentiful.[11] They often migrate to better watered highlands during the dry season.[5] Females require significantly more water when they are lactating.[12] During droughts, the zebras will dig water holes and defend them.[5] Grévy's zebras are preyed on by lions, hyenas, wild dogs, cheetahs and leopards.[8] In addition, they are susceptible to various gastro-intestinal parasites, notably of the Trichostrongylus genus.[13]

Behavior

Herd of zebras.
Closeup of a zebra grazing.

Behaviourally, the Grévy's zebra differs from the other two zebra species as it does not live in harems.[14] The basic social units of the species are adult females or mares with their immature offspring or foals. Numerous groups of females and young often gather into herds which are open and fluid and have no strict dominance hierarchies.[5] Adult males or stallions will establish territories that average 5.75 km²,[7]:152[14] through vocalizations and by marking them with dung piles.[5][7]:151 They mostly live in territories during the wet seasons but some may stay in them year round provided local water sources last.[5] Stallions that are unable to establish territories are free-ranging[7]:151 and are known as bachelors. Females, young and non-territorial males wander though large home ranges. The females will wander from territory to territory preferring the ones with the highest-quality food and water sources.[15] Up to nine males may compete for a female on neutral ground.[8] Territorial stallions will tolerate other stallions who wander in their territory, however when an estrous female is present the territorial stallion keeps other males at bay.[5][7]:151 Non-territorial males may avoid territorial ones because of harassment.[9] When females are not around, a territorial stallion will seek the company of other stallions. Dominance is asserted with an arched neck and a high-stepping gait and the least dominant stallions submit by extending their tails and lowering their heads.[7]:151 The call of the Grévy's zebra has been described as "something like a hippo's grunt combined with a donkey's wheeze".[5] Grévy's zebras do not perform mutual grooming. To get rid of flies, they roll in dust, water or mud or twitch their skin. They also rub against trees, rocks and other objects to get rid of irritations like itchy skin, hair or parasites.[8]

Reproduction

Mother zebra with foals.

Grévy's zebras can mate and give birth year-round, but most mating takes place in the early rainy seasons and births mostly take place in August or September after the long rains.[8] An estrous mare may wander though as many as four territories a day[15] and will mate with the stallions in them. Among territorial stallions, the most dominant ones control territories near open watering points, which mostly attract mares with foals,[16] while more subordinate stallions control territories away from water with more abundant vegatation, which mostly attract mares without foals.[16] The resident stallions of territories will try to subdue the entering mares with dominance rituals and then continue with courtship and copulation.[5] Grévy's zebra stallions have large testicles and can ejaculate a large amount of semen to replace the sperm of other males.[15] This is a useful adaptation for a species whose females mate polyandrously. Bachelors or outside territorial stallions sometimes "sneak" copulation of mares in another stallion’s territory.[15] While female associations with individual males are brief and mating is promiscuous, females who have just given birth will reside with one male for long periods and mate exclusively with that male.[15] Lactating females are harassed by males more often than non-lactating ones and thus associating with one male and his territory provides an advantage as he will guard against other males.[17]

Zebra foal resting.

Gestation of the Grévy's zebra normally lasts 390 days,[8] with a single foal being born. A newborn zebra will follow anything that moves, so new mothers prevent other mares from approaching their foals while imprinting their own striping pattern, scent and vocalization on them.[8] This prevents the foal from imprinting on another female as its mother. From the time their foals are born until the foals reach an age of 3 months, females form small groups (three females and their foals).[12] Mares may leave their foals in "kindergartens" while searching for water.[12] The foals will not hide, so they can be vulnerable to predators.[5] However, kindergartens tend to be guarded by an adult, usually a territorial male.[12] A female with a foal stays with one dominant territorial male who has exclusive mating rights to her. However, the male must look after a foal which is likely not his.[18] For these males, infanticide is not an option as without her foal, the mother will leave the territory.[18] Thus the territorial stallion must assist another male's offspring in order to ensure his own reproduction.[18] To adapt to a semi-arid environment, Grévy's zebra foals take longer intervals between suckling bouts and do not drink water until they are 3 months old.[12] Foals become independent of their mothers after half a year but will continue to follow them for up to three years.[5]

Relationship with humans

Grévy's zebra at the Cincinnati Zoo.

The Grévy's zebra was the first zebra to be discovered by the Europeans and was used by the Romans in circuses.[3] However, it was largely forgotten in the Western world for a thousand years.[3] It was rediscovered in the seventeenth century, when the king of Shoa (now central Ethiopia) sent one to the Sultan of Turkey and another to the Dutch governor of Jakarta.[3] A century later in 1882, the government of Abyssinia sent one to French president Jules Grévy. Around the same time, French naturalist Alphonse Milne-Edwards recognized it as distinct from the better known zebras of southern Africa and it was named in Grévy’s honor.[3]

Status and conservation

Grevy's zebras in Samburu National Reserve.

The Grévy's zebra is considered endangered.[1] Its population was estimated to be 15,000 in the 1970s and by the early 21st century the population was lower than 3,500, a 75% decline.[19]:11 It is estimated that there are less than 2,500 Grévy's zebras still living in the wild.[1] There are also an estimated 600 Grévy's zebras in captivity.[19]:20 The Grévy's zebra population trend is considered stable as of 2008.[1]

The Grévy's zebra is legally protected in Ethiopia. In Kenya it is protected by the hunting ban of 1977 but is still listed as a "Game Animal". In the past, Grévy's zebras were threatened mainly by hunting for their skins which fetched a high price on the world market. However hunting has declined and the main threat to the zebra is habitat loss and competition with livestock. Cattle gather around watering holes and the Grévy's zebras are fenced from those areas.[19]:17 Community-based conservation efforts have shown to the most effective in preserving Grévy's zebras and their habitat. Less than 0.5% of the range of the Grévy's zebra is in protected areas.[1] In Ethiopia, the protected areas include Alledeghi Wildlife Reserve, Yabelo Wildlife Sanctuary, Borana Controlled Hunting Area and Chalbi Sanctuary. In Kenya, the Buffalo Springs, Samburu and Shaba National Reserves and the private and community land wildlife conservancies in Isiolo, Samburu and the Laikipia Plateau provide a core and crucial protection of the southern population of Grévy's zebra. On the Laikipia Plateau, zebra numbers have increased thanks to protection and reduced competition with domestic livestock.[1]

References

  1. ^ a b c d e f g h Moehlman, P.D., Rubenstein, D.I. & Kebede, F. (2008). Equus grevyi. In: IUCN 2008. IUCN Red List of Threatened Species. Downloaded on 10 April 2009. Database entry includes a brief justification of why this species is endangered.
  2. ^ Grubb, Peter (16 November 2005). "Order Perissodactyla (pp. 629-636)". In Wilson, Don E., and Reeder, DeeAnn M., eds. Mammal Species of the World: A Taxonomic and Geographic Reference (3rd ed.). Baltimore: Johns Hopkins University Press, 2 vols. (2142 pp.). p. 631–632. ISBN 978-0-8018-8221-0. OCLC 62265494. http://www.bucknell.edu/msw3/browse.asp?id=14100019. 
  3. ^ a b c d e f g h Prothero D.R, Schoch R. M (2003). Horns, Tusks, and Flippers: The Evolution of Hoofed Mammals'. Johns Hopkins University Press. pp. 216–18. ISBN 0-801-87135-2. http://books.google.co.uk/books?id=kWpQX-sfsLgC&printsec=frontcover&dq=Horns,+Tusks,+and+Flippers:+The+Evolution+of+Hoofed+Mammals&hl=en&ei=nFtdTbv2EIO0hAeQ5-GqCA&sa=X&oi=book_result&ct=result&resnum=1&ved=0CC8Q6AEwAA. 
  4. ^ Orlando, Ludovic; et al. (2009). "Revising the recent evolutionary history of equids using ancient DNA". PNAS 106: 21754–21759.  Abstract
  5. ^ a b c d e f g h i j k l m n o p Estes, R. (1991). The Behavior Guide to African Mammals. University of California Press. pp. 240–242. ISBN 0-520-08085-8. 
  6. ^ J. E. Cordingley, S. R. Sundaresan, I. R. Fischhoff, B. Shapiro, J. Ruskey, D. I. Rubenstein (2009). "Is the endangered Grevy's zebra threatened by hybridization?", Animal Conservation, 12(6): 505 - 513. Full Article
  7. ^ a b c d e f g h i Kingdon, J. (1988). East African Mammals: An Atlas of Evolution in Africa, Volume 3, Part B: Large Mammals. University of Chicago Press. ISBN 9780226437224. 
  8. ^ a b c d e f g h i j Churcher, C.S. 1993. Equus grevyi. Mammalian Species. 453:1–9.
  9. ^ a b Sundaresan, S., I. Fischhoff, H. Hartung, P. Akilong, D. Rubenstein. (2008) "Habitat choice of Grevy’s zebras (Equus grevyi) in Laikipia, Kenya", African Journal of Ecology, Vol. 46(3): 359-364. Full Article
  10. ^ Bauer, I. E., McMorrow, J. and Yalden, D. W. 1994. "The Historic Ranges of Three Equid Species in North-East Africa: A Quantitative Comparison of Environmental Tolerances", Journal of Biogeography 21(2): 169–182. Abstract
  11. ^ Youth, H. (2004). Thin Stripes on a Thin Line. ZooGoer. 33(6).
  12. ^ a b c d e Becker, C. D., J. R. Ginsberg (1990). "Mother-infant behaviour of wild Grevy's Zebra." Animal Behavior 40(6): 1111–1118. Abstract
  13. ^ Muoria. P. K., Muruthi. P, Rubenstein. D, Oguge N. O, Munene E. (2005) "Cross-sectional survey of gastro-intestinal parasites of Grevy's zebras in southern Samburu, Kenya", African Journal of Ecology, 43(4): 392-395. Full Article
  14. ^ a b Klingel, H. (1972). "Social behavior of African equidae." Zoologica Africana 7(1): 175–185
  15. ^ a b c d e Ginsberg, R., D. I. Rubenstein (1990). "Sperm competition and variation in zebra mating behavior." Behavioral Ecology and Sociobiology 26(6): 427–434. Full Article
  16. ^ a b Rubenstein, D. I. (2010) "Ecology, social behavior, and conservation in zebras". Pp. 231-258. In: Advances in the Study Behavior: Behavioral Ecology of Tropical Animals, Vol. 42. R. Macedo, ed. Elsevier Press, Oxford, UK. Full Article
  17. ^ Sundaresan, S., I. Fischhoff, D. Rubenstein. (2007) "Male harassment influences female movements and associations in Grevys zebra (Equus grevyi)", Behavioral Ecology, 18(5): 860-865. Full article
  18. ^ a b c Rubenstein, D. I. (1986) "Ecology and sociality in horses and zebras". Pp. 282-302. In: Ecological Aspects of Social Evolution. D. I. Rubenstein & R. W. Wrangham, (eds.). Princeton University Press, Princeton, NJ. Full Article
  19. ^ a b c Moelman, P.D (2002). "Status and Action Plan for the Grévy's Zebra (Equus grevyi) by Stuart D. Williams". Equids. Zebras, Assess and Horses. Status Survey and Conservation Action Plan'. IUCN/SSC Equid Specialist Group. pp. 11–27. ISBN 2-831-70647-5. 

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