bull mastiff (măst´Ĭf), breed of powerful working dog developed in England in the second half of the 19th cent. It stands from 24 to 27 in. (61–68.6 cm) high at the shoulder and weighs from 100 to 130 lb (45.4–59 kg). Its dense, short coat may be fawn, red, or brindle, with a darker shading on the ears and muzzle. Because of the increasing need to protect game preserves and large estates from poachers, English gamekeepers began to cross existing breeds in an attempt to produce a dog that would possess the required speed, strength, aggressiveness, good night vision, and the capacity to remain silent at the approach of the poacher. It would be the task of the desired dog to knock down the intruder and keep him down until he was captured, rather than simply alarming him into running away. After many breeds were tried, mastiff and bulldog stock were crossed, producing a dog with all the necessary qualities, the bull mastiff. Today it is raised as a guard and show dog and as a pet. See dog.