Ross, P. W., Everitt, J. I. 1988. A nylon ball device for primate environmental enrichment. Laboratory Animal Science 38(4), 481-483.

We have devised a simple device consisting of a hard nylon ball on a chain which appears to provide an activity stimulus for some individually housed, colony reared, young male rhesus monkeys (Macaca mulatta). In order to avoid boredom with the devices, they are introduced either on the outside or the inside of the cages on a rotational basis and not given to all of the animals at the same time. ... We feel that macaques fare better when they are housed in cages on the upper tier or racks. This may relate to better interactions with the animal care technicians when they are housed in this position. We maintain an active program of cage rotation.