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 ignored the food puzzle during the 1-hour observation, and 75% (12/16) gathered biscuits from both the dish and the food puzzle. Those foraging from both containers consumed an average of 59 biscuits. They spent 64 seconds collecting 51 of the biscuits from the dish and 182 seconds retrieving the other eight biscuits from the puzzle. Dan spent 286 seconds retrieving 12 biscuits from the food puzzle after leaving 21 freely available dish-biscuits untouched.