Most monkeys can breed at any time of the year, so their troops will contain young of all different ages. Most kinds of monkeys give birth to a single baby at a time. The gestation period for female monkeys range from 4 to 8 months. Most baby monkeys depend entirely on their mother for food and security. Depending on the species, babies will nurse on their mother's milk anywhere from a few weeks to two years.
Most monkeys survive largely by using their intelligence.
Capuchins will sometimes fend off predators by urinating on them from high above in the trees or by jumping up and down to make dead branches fall on the predators. The South American marmosets, face the danger of being captured and sold as pets. The guenons and colobus monkeys of Africa, face the problems of deforestation as well as being hunted for their pelts. In Asia, the macaques and langurs are endangered. In fact, the lion-tailed macaque which lives in southwest India is thought to be the rarest Old World monkey and is in extreme danger of extinction.