We were shown these amazing little primates, the smallest in the world; Western Pygmy Marmosets (Cebuella pygmaea). Each is only about the size of a hand, with the smallest we saw likely comfortable curling up in a donut. They stay in one tree and have a tiny home range, excavating holes in bark to get to the sap they eat. They were skittish but as they didn’t go far, relatively easier to photograph than other primates in my experience. Loreto, Peru.
Youlatos, D. (2009). Locomotion, postures, and habitat use by pygmy marmosets (Cebuella pygmaea). In P. Garber, A. Estrada, J. Bicca-Marques, E. Heymann, & K. Strier (Eds.), South American primates: Comparative perspectives in the study of behavior, ecology, and conservation (pp. 279–296). Springer. https://www.academia.edu/download/45603455/4_Youlatos_2009_Smallest_20Anthropoids.pdf