O'Connor, E., Reinhardt, V. 1994. Caged stumptailed macaques voluntarily work for ordinary food. In Touch 1(1), 10-11.
Ninety-nine of the biscuits were placed in a freely accessible dish attached to the mesh of the front of the cage. Thirty-three biscuits were offered in a food puzzle. Twenty-five percent (4/16) of the subjects...