Schapiro, S. J., Suarez, S. A., Porter, L. M. et al. 1996. The effects of different types of feeding enhancements on the behaviour of single-caged, yearling rhesus macaques. Animal Welfare 5, 129-138.

Enrichment techniques designed to encourage captive primates to spend more species-appropriate amounts of time in foraging behaviours have been successful. The present study measured the behavioural effects of four feeding enhancements: two devices (mats and puzzles) and two foods (produce and frozen juice), on four cohorts (n = 63) of single-caged, yearling rhesus macaques (Macaca mulatta). Devices required considerable manipulation to retrieve rewards, whereas enrichment foods required additional processing. Analyses compared periods when one of the enhancements was available to interim periods when no enhancements were available. Planned comparisons revealed that subjects spent more time feeding, and less time inactive, self-grooming, exploring and behaving socially when feeding enrichment was available. Enrichment use in minutes/observation hour was as follows: Turf mats 25.8 minutes; Acrylic puzzles 22.1 minutes; Produce 17.4 minutes; Frozen juice 14.6 minutes. ... We feel that a feeding enrichment program similar to the one that we used [for single-housed subjects], that provides some combination of stimulating devices and foods that are novel and require processing, can have a very positive impact on the behaviour of captive primates. We have used a similar feeding enrichment program for older, pair-housed and group-housed rhesus with less success.