Sand-coloured macaque (Macaca mulatta), widespread in South and Southeast Asian forests. Rhesus monkeys are 17–25 in. (43–64 cm) long, excluding the furry 8–12-in. (20–30-cm) tail, and weigh 10–24 lb (4.5–11 kg). They eat fruits, seeds, roots, herbs, and insects. They are held sacred in some parts of India. Hardy in captivity, highly intelligent, and lively, they make good pets when young but may become bad-tempered as adults. They have been used frequently in medical research. The determination of the Rh (from rhesus) factor in human blood involves reaction with the blood of this species. See alsoRh blood-group system.