Pygmy marmosets are two species of small New World monkeys in the genus Cebuella. They are native to rainforests of the western Amazon Basin in South America. These primates are notable for being the smallest monkeys in the world, at just over . They are generally found in evergreen and river-edge forests and are gum-feeding specialists, or gummivores. 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 six individuals. Although most groups consist of family members, some may also include one or two additional adult members. Members of the group communicate using a complex system including vocal, chemical, and visual signals. Three main calling signals 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 (where they were placed in a subgenus) and Mico, and thus is accorded its own genus, Cebuella, within the family Callitrichidae. The biggest threats are habitat loss and the pet trade. Evolution and taxonomy There has been debate among primatologists concerning the proper genus in which to place the pygmy marmoset. An examination of the interstitial retinol binding protein nuclear gene (IRBP) in three marmoset species showed that Callithrix as constructed in the 1990s also needed to include C. pygmaea to be monophyletic, and that the times of separation of pygmaea and the argentata and jacchus species groups from one another are less than 5 million years ago, as might be expected for species of the same genus. However, subsequent separation of the argentata and jacchus species groups into different genera (the argentata group having been moved to Mico) justifies maintaining a separate genus for the pygmy marmosets, as Callithrix is no longer paraphyletic. There are two species described of the pygmy marmoset: Cebuella pygmaea – Western pygmy marmoset Cebuella niveiventris – Eastern pygmy marmoset There