The wildebeests, also called gnus, are a genus of antelopes which includes two species, both native to Africa: the black wildebeest, or white-tailed gnu and the blue wildebeest, or brindled gnu. The most obvious way of telling the two species apart are the differences in their colouring and in the way their horns are oriented. Wildebeest is Dutch for "wild beast" or "wild cattle" in Afrikaans.
Both species of wildebeest are even-toed, horned, greyish-brown ungulates resembling cattle. Males are larger than females and both have heavy forequarters compared to their hindquarters. They have broad muzzles, Roman noses, shaggy manes and tails. The most striking morphological differences between the black and blue wildebeest are the orientation and curvature of their horns and the color of their coats. The blue wildebeest is the bigger of the two species. The horns of blue wildebeest protrude to the side then curve downwards before curving up back towards the skull, while the horns of the black wildebeest curve forward then downward before curving upwards at the tips. Blue wildebeest tend to be a dark grey color with stripes, but may have a bluish sheen. The black wildebeest has brown-coloured hair, with a mane that ranges in color from cream to black, and a cream-coloured tail.
The blue wildebeest lives in a wide variety of habitats, including woodlands and grasslands, while the black wildebeest tends to reside exclusively in open grassland areas. In some areas the blue wildebeest migrates over long distances in the winter, whereas the black wildebeest does not. The two species of the wildebeest are known to hybridise. The differences in social behaviour and habitats have historically prevented interspecific hybridisation between the species, however hybridisation may occur when they are both confined within the same area. The resulting offspring is usually fertile. A study of these hybrid animals at Spioenkop Dam Nature Reserve in South Africa revealed that many had disadvantageous abnormalities relating to their teeth, horns and the wormian bones in the skull. Another study reported an increase in the size of the hybrid as compared to either of its parents. In some animals, the tympanic part of the temporal bone is highly deformed and in others the radius and ulna are fused.
Not all wildebeest are migratory. Black wildebeest herds are often nomadic or may have a regular home range of 1 km2. Blue wildebeest have both migratory and sedentary populations. In the Ngorongoro most animals are sedentary and males maintain a network of territories throughout the year, even though breeding is seasonal in nature. Females and young form groups of about ten individuals or join together in larger aggregations, and non-territorial males form bachelor groups. In the Serengeti and Tarangire ecosystems, populations are mostly migratory, with herds consisting of both sexes frequently moving, but resident subpopulations also exist. Each year, some East African populations of blue wildebeest have a long-distance migration, seemingly timed to coincide with the annual pattern of rainfall and grass growth. The timing of their migrations in both the rainy and dry seasons can vary considerably (by months) from year to year.
