The pygmy marmoset (Cebuella pygmaea) is a small New World monkey native to rainforests of the western Amazon Basin in South America. It is notable for being the smallest monkey and one of the smallest primates in the world at just over 100 grams (3.5 oz) (Madame Berthe's mouse lemur is smaller). It is generally found in evergreen and river edge forests and is a gum-feeding specialist, or a gummivore.
About 83% of the pygmy marmoset population lives in stable troops of two to nine individuals, including a dominant male, a breeding female, and up to four successive litters of offspring. The modal size of a standard stable troop would be 6 individuals. Although most groups consist of family members, some may also include 1-2 additional adult members. Members of the group communicate using a complex system including vocal, chemical, and visual signals. There are three main calling signals that depend on the distance the call needs to travel. These monkeys may also make visual displays when threatened or to show dominance. Chemical signaling using secretions from glands on the chest and genital area allow the female to indicate to the male when she is able to reproduce. The female gives birth to twins twice a year and the parental care is shared between the group.
The pygmy marmoset has been viewed as somewhat different from typical marmosets, most of which are classified in the genera Callithrix and Mico, and thus is accorded its own genus, Cebuella, within the family Callitrichidae. It is listed as Least Concern by the International Union for Conservation of Nature as it is common across its wide range and not at immediate risk of widespread decline. The biggest threats to the species are habitat loss and the pet trade.