With unusually small territories for their size, grey-legged night monkeys are relatively sedentary primates, but will defend their range with vigour. Males display and call to intruders, particularly during the breeding seasons, and mark their territory by rubbing a gland at the base of the tail to release a brown, oily and smelly substance. However, although this species is highly monogamous, and exists usually in pairs with up to three dependent young, aggregations of these family units have been seen together, both feeding and sleeping, numbering up to around 30 individuals (3).
Each female will only give birth once a year after a gestation of about 130 days. A single infant is born and is lavished with parental care. However, the mother will only have contact with the infant to suckle it. The male is responsible for carrying, defending, playing with and instructing the young, and the mother will actively pull an infant off her and bite it should it try to clamber aboard. The young stay with their parents for two or three years, after which time they become temporarily nomadic as they search for a mate (3). Both males and females urinate on their hands and rub it on branches in order to track down a mate and two individuals will sniff each other for an extended period upon meeting (5).
The grey-legged night monkey feeds most on moonlit nights, leaving the sleeping sites shortly after sunset and returning before sunrise (4). It takes fruits, insects and leaves for much of the year, but during July and August when most foods are scarce, it will mainly eat nectar (3). Night monkeys are extremely efficient at snatching insects from leaves, branches (3), and even out of the air with lightening speed (4). They may also feed occasionally on bats, small birds, eggs and lizards. They are rarely preyed upon, but potential predators include cats and large owls, who would likely only manage an infant (3).