The nilgai or blue bull (aɪ/; literally meaning "blue cow"; Boselaphus tragocamelus) is the largest Asianantelope and is endemic to the Indian subcontinent. The sole member of the genusBoselaphus, the species was described and given its binomial name by German zoologist Peter Simon Pallas in 1766. The nilgai stands 1–1.5 metres (3.3–4.9 ft) at the shoulder; males weigh 109–288 kilograms (240–635 lb), and the lighter females 100–213 kilograms (220–470 lb). A sturdy thin-legged antelope, the nilgai is characterised by a sloping back, a deep neck with a white patch on the throat, a short crest of hair along the neck terminating in a tuft, and white facial spots. A column of pendant coarse hair hangs from the dewlap ridge below the white patch. Sexual dimorphism is prominent – while females and juveniles are orange to tawny, adult males have a bluish-grey coat. Only males possess horns, 15–24 centimetres (5.9–9.4 in) long.
The nilgai is diurnal (active mainly during the day). The animals band together in three distinct kinds of groups: one or two females with young calves, three to six adult and yearling females with calves, and all-male groups with two to 18 members. Typically tame, the nilgai may appear timid and cautious if harassed or alarmed; it flees up to 300 metres (980 ft)-or even 700 metres (2,300 ft), galloping away from the source of danger. Herbivores, nilgai prefer grasses and herbs, though they commonly eat woody plants in the dry tropical forests of India. Females become sexually mature by two years, while males do not become sexually active until four or five years old. The time of the year when mating takes place varies geographically, but a peak breeding season lasting three to four months can be observed at most places. Gestation lasts eight to nine months, following which a single calf (sometimes twins or even triplets) is born. As typical of several bovid species, nilgai calves stay hidden for the first few weeks of their lives. The lifespan of the nilgai is around ten years.
Nilgai prefer areas with short bushes and scattered trees in scrub forests and grassy plains. They are common in agricultural lands, but hardly occur in dense forest. Major populations occur in the Terai lowlands in the foothills of the Himalayas (northern India), but the antelope is sparsely found in Nepal and Pakistan and is extinct in Bangladesh. Nilgai were first introduced to Texas in the 1920s and the 1930s. As of 2008, the feral population in Texas is nearly 37,000. The nilgai is categorised as Least Concern by the IUCN. The nilgai has been associated with Indian culture since the Vedic period (1500–500 BCE). Hindus revere the nilgai as sacred and associate it with the cow, the mother animal in Hinduism, through its name and loosely similar physical features. They were hunted in the Mughal era (16th to 19th centuries) and are depicted in numerous miniatures. Nilgai have been considered a pest in several north Indian states, as they ravage crop fields and cause considerable damage. In Bihar, authorities have classified the nilgai as vermin.
The vernacular name "nilgai" aɪ/ comes from the fusion of the Hindi words nil ("blue") and gai ("cow"). The word was first recorded in use in 1882. Alternative origins could be from the Persian gaw ("cow"). The nilgai has been referred to by a variety of names: neelghae, nilgau, nilgo, nylghau, and nylghai, constructions referring to other "blue" animals. They are also known as white-footed antelope. During Mughal emperorAurangzeb's reign (1658–1707) of India, the nilgai was known by the name nilghor ("nil" for "blue" and "ghor" for "horse").