Impalas range between 73 and 92 cm (29 and 36 in) tall. Average mass for a male impala is 46 to 76 kg (100 to 170 lb), while females weigh about 37 to 50 kg (82 to 110 lb). They are normally reddish-brown in color (hence the Afrikaans name of "Rooibok"), have lighter flanks and white underbellies with a characteristic "M" marking on the rear. Males, referred to as rams, have lyre-shaped horns, which can reach up to 90 centimeters in length. Females, referred to as ewes, have no horns. The black impala, found in very few places in Africa, is an extremely rare type. A recessive gene causes the black colouration in these animals.