The Rhesus Macaque (Macaca mulatta), often called the Rhesus Monkey, is one of the best known species of Old World monkeys. It is a typical macaque, common throughout Afghanistan to northern India and southern China. Rhesus Macaques are sexually dimorphic. Adult male Rhesus Macaques measure approximately 53 centimeters on average and weigh an average of 7.7 kilograms. Females are smaller, averaging 47 centimeters in length and 5.3 kilograms in weight. They are brown or grey in color and have pink faces which are typically bereft of fur. Their tails are of medium length and average between 20.7 and 22.9 centimeters. They typically have a lifespan of about 25 years. The Rhesus Macaque is well known to science owing to its relatively easy upkeep in captivity, and has been used extensively in medical and biological research.