The golden lion tamarin forms social family groups. Males help to raise their offspring, and often carry their young on their backs in between feedings. Tamarin young are usually twins. Golden lions live primarily in the trees. They sleep in hollows at night and forage by day while traveling from branch to branch. Long fingers help them stay aloft and snare insects, fruit, lizards, and birds. These interesting animals are critically endangered, as are many of the forests in which they live. Brazil's Atlantic coastal rain forests are disappearing due to ever-expanding logging, agriculture, and industry, and unfortunately, the golden lion tamarin is in danger of vanishing with them.
Golden lion tamarins are social and groups typically consist of 2-8 members. These groups usually consist of one breeding adult male and female but may also have 2–3 males and one female or the reverse. Other members include subadults, juveniles and infants of either sex. These individuals are typically the offspring of the adults. When there is more than one breeding adult in a group, one is usually dominant over the other and this is maintained through aggressive behavior. The dominance relationship between males and females depends on longevity in the group. A newly immigrated male is subordinate to the resident adult female who inherited her rank from her mother. Both males and females may leave their natal group at the age of four, however females may replace their mothers as the breeding adult, if they die, which will lead to the dispersal of the breeding male who is likely her father. This does not happen with males and their fathers. Dispersing males join groups with other males and remain in them until they find an opportunity to immigrate to a new group. The vast majority of recruits to groups are males. A male may find an opportunity to enter into a group when the resident male dies or disappears. Males may also aggressively displace resident males from their group; this is usually done by two immigrant males who are likely brothers. When this happens, only one of the new males will be able to breed and will suppress the reproduction of the other. A resident male may also leave a vacancy when his daughter becomes the breeding female and he must disperse to avoid inbreeding. Golden lion tamarins are highly territorial and groups will defend their home range boundaries and resources from other groups.
Tamarins emit "whine" and "peep" calls, which are associated with alarm and alliances respectively. "Clucks" are made during foraging trips or during aggressive encounters, whether directed at conspecifics or predators. "Trills" are used to communicate over long distances to give away an individual's position. "Rasps" or "screeches" are usually associated with playful behavior. Tamarins communicate though chemicals marked throughout their territories. Reproductive males and females scent mark the most and their non-reproductive counterparts rarely do so. Dominant males use scent marking to show their social status and may suppress the reproductive abilities of the other males.
The mating system of the golden lion tamarin is largely monogamous. When there are two adult males in a group only one of them will mate with the female. There are cases of a male mating with two females, usually a mother and daughter. Reproduction is seasonal and depends on rainfall. Mating is at its highest at the end of the rainy season between late March to mid-June and births peak during the September–February rains. Females are sexually mature between of 15–20 months but it isn't until they are 30 months old when they can reproduce. Only dominant females can reproduce and will suppress the reproduction of the other females in the group. Males may reach puberty by 28 months. Tamarins have a four-month gestation period. Golden lion tamarin groups exhibit cooperative rearing of the infants. This is due to the fact that tamarins commonly give birth to twins and, to a lesser extent, triplets and quadruplets. A mother is not able to provide for her litter and needs the help of the other members of the group.The younger members of the groups may lose breeding opportunities but they gain parental experience in helping to rear their younger siblings. In their first 4 weeks, the infants are completely dependent on their mother for nursing and carrying. By week five, the infants spend less time on their mother’s back and begin to explore their surroundings. Young reach their juvenile stage at 17 weeks and will socialize other group members. The subadult phase is reached at 14 months and a tamarin first displays adult behaviors.