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Groves (2011) recognizes six Oryx species. The East African oryxes have traditionally been treated as a single species, Oryx beisa (and often even considered conspecific with the Gemsbok, O. gazella, of southwest Africa). According to Groves (2011), however, although they are very similar in appearance they are best treated as three distinct species: Beisa Oryx (O. beisa), found in northern and central Somalia and the Ogaden region of Ethiopia north to Berbera, west to Eritrea, and south into the Awash Valley; Galla Oryx (O. gallarum), found in northern Kenya and northeastern Uganda and extending into Somalia and southeastern Ethiopia; and Fringe-eared Oryx (O. callotis), found in southeastern Kenya and northeastern Tanzania. The fourth oryx species still living in the wild is the Gemsbok.
In addition to these four species are two species that went extinct in the wild, but persist in captivity and are the focus of reintroduction efforts. The Scimitar-horned Oryx (Oryx dammah)--both sexes of which have long, slender, hollow horns that are annulated (i.e. with ring-like divisions) for the basal third and curve over the back--used to be found on the southern and northern edges of the Sahara Desert. They did not inhabit the desert interior, as does the Addax (Addax nasomaculatus). The former range of the Scimitar-horned Oryx, which encompassed over 4 million square km, experiences prolonged droughts, the most recent of which extended from the 1960s to the early 1990s! The ongoing southward spread of the Sahara Desert likely contributed to the decline of this species. When sedentary, herds consisted of 10 to 30 or even 100 individuals. During migration, groups of 1000 or more would aggregate (an aggregation of 10,000 was reported from Chad in 1936). It is estimated the the wild population of Scimitar-horned Oryx once numbered around a million individuals. In addition to the expansion of the Sahara, the main causes of extinction were human population growth, motorized access to the desert, overhunting, and increased use of key habitats by livestock. The last known wild individuals were in Chad and Niger in the 1980s. Fortunately, captive populations were established beginning in the 1960s (around 4,000 captive animals are in the United Arab Emirates in a private collection and around 2,000 on private ranches in Texas, U.S.A.), so re-establishing wild Scimitar-horned Oryx populations is a possibility that is being actively pursued.
The only native oryx species outside Africa is the Arabian Oryx (Oryx leucoryx). This species was extinct in the wild by 1972, but since then free-ranging populations have been established in Israel, Oman, and Saudi Arabia. This species was formerly present throughout the Arabian Peninsula, extending north to Israel, Jordan, Kuwait, Iraq, Syria, and Sinai. Poaching and overhunting in Oman eliminated the last wild individuals. Fortunately, captive breeding efforts had begun in the 1950s and reintroduction efforts began in the early 1980s and are ongoing. The world captive population is around 6,000 to 7,000, but the re-introduced free-ranging populations include only around 250 mature individuals.
(Kingdon 1997; Groves 2011)