Females of the Colobus monkey subfamily mature at about 4 years of age, and males mature at about 5 years of age. There is no strict seasonal limit on mating of colobus monkeys, but there is generally a peak of birth, and the young monkeys are weaned just when food is plentiful. Mating behavior is generally initiated by females. A fertile female proboscis monkey who catches a fancy to a male monkey will close her mouth and purse her lips. If the male looks back at the female, the female will shake her head quickly. The male monkey will respond with a pouting expression and then either get close to the female or let the female get closer to it. If the male fails to respond to the female's initiative, the female proboscis monkey may hit the male, pull its skin, or even bite it. A distinctive feature of female red-legged white-rumped langurs is that they take a supine position and look at the male through their shoulders. The male responds by staring at it and then at the right mating place. In the colobus monkeys this technique of pleading is similar, but the smacking sound is different. At mating, female white-rumped langurs and colobus monkeys still use a supine position, while female proboscis monkeys and long-tailed langurs use the four-legged and combined posture of other monkeys. The female proboscis monkey will continue to shake her head, while the spouse will make a pouting expression during mating.
Newborn cubs are about 20 cm long and weigh 0.4 kg. They are born with their eyes open and can cling to the mother monkey, but the cubs of the olive colobus monkey are held in the mouth by the mother monkey. Newborn pups have short, soft body hair, and the coat color is usually different from that of adults. In addition to the slightly darker facial skin, the pups of the proboscis monkey and the red-legged white-rump langur have a lighter color of the body skin and the crusty skin on the sciatic bones than they did as adults. They generally give birth to 1 cub per litter and rarely twins. In addition to the red-green colobus monkey and the Indonesian langur, the female monkey acquiesced to other females to help it with her children. Pups are subjected to frequent touch shortly after birth and may be taken up to 25 meters away from the mother monkey. Mother monkeys can feed themselves and "others" at the same time. Female monkeys who are taking care of their children sometimes suddenly "leave their posts without permission" and ask the mother of the cubs to rediscover the "crying" cubs. For long-tailed langurs, females begin to alienate young monkeys at about 5 weeks, which can promote the independence of the cubs, improve their survival rate in a high-mortality group dominated by only one male, and allow the mother monkeys to have more time to focus on finding food. Young monkeys begin to change into adult fur at 5 to 10 months of age, and reach the size of adult monkeys around 5 years old.
Compared to macaques, colobus monkeys generally show relatively low levels of aggression, sexual relations within social groups, vocal communication, and even gesture communication, and most of their behavior is described as "dull and serious." One of the reasons may be related to their eating behavior. Arboreal animals' food is evenly dispersed throughout the forest, so they need to sit and eat for long periods of time without the need for close group cooperation, so facial expressions are rare. When climbing or walking on fruit trees, they are careful to avoid their companions who are already occupied there. Once they have occupied a feeding ground, they face the periphery of the tree, which allows them to engage in long-term feeding activities with minimal communication with their neighbors. Although the peak period of their diet is in the morning and evening, due to the low nutritional content of the food, this prompts them to eat almost all day, thus further reducing the complexity of social communication behavior between them.
Groups of colobus monkeys range from usually only one male colobus monkey to more than 120 long-tailed langurs (most of which were created temporarily in search of water together). Groups of 200 to 300 (groups of golden snub-nosed monkeys even exceed 600) have also been reported, although these groups may have been formed by smaller family units. Populations of proboscis monkeys, red-legged white-rumped langurs and red-green colobus monkeys are reported to be around 60 in size, while most of the subfamily have a population of 40. Some colobus monkeys have much smaller average population sizes, ranging from a population of 3.4 "monogamous" Mintawi langurs, guinea-tailed langurs and Javan grey langurs with an average of only 3.4 to a population of 37 Hanuman langurs. Populations of guinea-tailed langurs and white-shouldered langurs usually have 5 members; groups of black langurs, purple-faced langurs, hated langurs, and black and white colobus monkeys range from 6 to 9; and the number of other species is generally 10 to 18. The exception is the black-palmed green colobus monkey, which can reach a population of 50.