The yellow-eared marmoset (Callithrix aurita) is a small marmoset native to the rainforests of southeastern Brazil. Its body length is about 30 cm and it weighs approximately 400–450 g. It frequently uses the lower strata of the forest and rarely descends to the ground. Using its claw-like nails, it clings to trees. It is diurnal and feeds on insects such as ants and on tree sap (gum), although its teeth for obtaining sap are not as well developed as those of the common marmoset. Groups typically number around ten individuals and have social structures of either one breeding male with multiple breeding females or multiple breeding males with multiple breeding females. Breeding can occur year-round; the gestation period is about five months and usually two offspring are born. Parental care is shared by fathers and siblings as well. Population numbers have declined due to forest destruction and fragmentation and capture for the pet trade, and the species is classified as Critically Endangered (CR).