are few morphological differences between these species, as they may only differ slightly in color, and they are only separated by geographical barriers, including large rivers in South America. The evolution of this genus diverged in terms of body-mass from typical primates, with a high rate of body-mass reduction. This involves large decreases in pre-natal and post-natal growth rates, furthering the thought that pro-genesis played a role in the evolution of this animal. Physical description Pygmy marmosets are the world's smallest primates, being the smallest true monkey, with a head-body length ranging from and a tail of . The average adult body weight is just over with the only sexual dimorphism of females being a little heavier. The fur colour is a mixture of brownish-gold, grey, and black on its back and head and yellow, orange, and tawny on its underparts. Its tail has black rings and its face has flecks of white on its cheeks and a white vertical line between its eyes. They have many adaptations for arboreal living including the ability to rotate its head 180 degrees and sharp claw-like nails used to cling to branches and trees. Their dental morphology is adapted to feeding on gum, with specialised incisors that are used to gouge trees and stimulate sap flow. The cecum is larger than usual to allow for the greater period of time gum takes to break down in the stomach. Pygmy marmosets walk on all four limbs and can leap up to between branches. Ecology Geographic range and habitat Pygmy marmosets can be found in much of the western Amazon Basin, in Brazil, Colombia, Ecuador, Peru, and Bolivia. The western pygmy marmoset, Cebuella pygmaea, occurs in the state of Amazonas, Brazil, eastern Peru, southern Colombia, and north-eastern Ecuador. The eastern pygmy marmoset, C. niveiventris, is also found in Amazonas, but also in Acre, Brazil, eastern Peru, and northern Bolivia. The distribution of both species is often limited by rivers. They typically live in the understory of the mature evergreen forests and often near rivers. Population density is correlated with food tree availability. They can be found between ground level and about into the trees but generally does not enter the top of the canopy. They are often found in areas with standing water for more than three months of the year. Diet These monkeys have a specialized diet of tree gum. They gnaw holes in