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.
The western pygmy marmoset is the world's smallest monkey. Pygmy marmosets have only a slight degree of sexual dimorphism; in the wild the average male weighs 110 g (3.9 oz), which is slightly smaller than the average female at 122 g (4.3 oz). Fully grown western pygmy marmosets have an average body length of 13 cm (5.1 in), and a tail marked with black rings that is on average longer than their bodies at 20 cm (7.9 in). 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 are 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".[6] 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.
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 rainforests.
The western pygmy marmoset is an exudativore-insectivore, thus its diet comes mainly from the exudates (fluids) from trees, such as tree sap, gum, resin, and latex from lianas and trees
. Through evolution of feeding on gum and sap from trees, the western pygmy marmoset developed dental adaptations that its ancestors did not have. These dental adaptations allow it to actively stimulate the fluids from the trees, as it gouges holes in the bark. It can also be found, on occasion, eating arthropods (such as insects, particularly grasshoppers, and spiders), small lizards, fruits, and flowers.
The pygmy marmosets frequently have fraternal twins. When born, their infants are smaller than expected compared to their gestation length. This is probably due to their slow growth rate when in utero.
Studies have shown that Cebuella pygmaea is potentially able to prevent reproduction in their population through hormonal suppression. If a single dominant female marmoset does not wish for her subordinate group members to reproduce, she can produce pheromonal signals which suppress hormones (LH secretion) which leads to ovulatory failure. However, this does not always work. If a subordinate female group member succeeds at breeding, the dominant female might resort to killing the offspring.
