The western pygmy marmoset (Cebuella pygmaea) is a marmoset species, a very small New World monkey found in the northwestern Amazon rainforest in Brazil, Colombia, Ecuador, and Peru. It was formerly regarded as conspecific with the similar eastern pygmy marmoset, which has whitish underparts. Although the western pygmy marmoset occurs further west than the eastern pygmy marmoset, the primary separators of their ranges are the Amazon River (Solimões River) and Marañón River, with the western occurring to the north of them and the eastern to the south. Physical description The western pygmy marmoset is the worlds smallest monkey. Pygmy marmosets have only a slight degree of sexual dimorphism; in the wild the average male weighs , which is slightly smaller than the average female at . Fully grown western pygmy marmosets have an average body length of , and a tail marked with black rings that is on average longer than their bodies at . They have fluffy "brownish-gold fur with black ticking on their shoulders, backs, and heads, while their ventral fur is light yellow to white". There is very little visible physical differences between the two species in the Cebuella genus, with only a slight distinction in pelage colour. Similar to other New World monkey species, the Western pygmy marmoset has long claw-like nails on all of their digits, which are mainly used for climbing and foraging. Their skull is similar to that of the Callithrix genus, possibly due to their tree-gouging behavior and their appendicular skeleton has been shaped by the long periods of clinging, climbing, and their "upside down and under branch locomotion". While pygmy marmosets leap the farthest relative to the length of their bodies, they are not very efficient in their leaps, and thus rarely do. Distribution Western pygmy marmosets are native to South America, occupying a range which intersects borders with Bolivia, Brazil, Colombia, Ecuador and Peru. In Brazil, they can be found in the upper Amazon basin, north of Solimões River, west of the Japurá River and south of the Río Caquetá in Colombia. There is a small subpopulation which is isolated west of the Rio Huallaga in northern Peru, which are of concern to researchers, as anthropogenic stresses are threatening this small population. They prefer a lowland habitat within tropical forests that are flooded for more than three months per year. Thus, their habitats are in tropical climates within swamps or