The Atlantic titi monkey or masked titi (Callicebus personatus) is a species of titi, a type of New World monkey, endemic to Brazil. Distribution and general features The Atlantic titi is part of the genus Callicebus which is composed of thirteen species and sixteen subspecies. Some more recent classifications of titis recognise many more species, and split the genus into three separate genera. The C. personatus group is retained in Callicebus. Three subspecies of Callicebus personatus have been described in scientific literature: C.p. melanochir, C.p. nigrifrons, and C.p. personatus, each of which has been treated as a full species by some authors. Atlantic titis are territorial, middle-sized, cebid monkeys; usually 1–2 kg in size. C. personatus are found primarily in the humid forests east of the Andes Mountains, specifically in the coastal, inland forests of south-eastern Brazil. The states they have been documented to dwell within include Bahia, Espírito Santo, Minas Gerais, Rio de Janeiro, and São Paulo. Of all the Callicebus species, Atlantic titi monkeys have the largest geographic range except for C. torquatus, covering approximately 1,000 kilometers. They customarily share their living area with other monkeys such as the lion tamarin and spider monkey. Subspecies variations The three subspecies of Callicebus personatus are products of clinical differentiation (when certain portions of the population are cut off from the parental stock due to an ecological barrier such as a river and are thus completely genetically isolated from one another). Distinctions between each of the Callicebus subspecies are based on coat color. The evolution of these coat colors is termed metachromism. The pigments are produced by melanocytes which deposit the pigments into the medulla of growing hair so that the hair grows with the color of that distinct pigment. The characteristic pigment of the Callicebus is eumelanin (dark brown or black) found in the iris, skin, and hair (usually on the forehead, crown and tail). Diet Atlantic titis are opportunistic feeders. In other words, they eat based on the abundance of food that is available to them in each season. Their food choices are not usually very selective, but they are frugivorous, usually eating fleshy fruits, seeds, and leaves. Geophagy It has been documented that they can be geophagic meaning that at times they eat soil. Various hypotheses exist for their geophagic nature. These include the following: Dietary mineral supplementation Adjustment of pH balance in the stomach Absorption of plant