Bayne, K., Hurst, J. K., Dexter, S. L. 1992. Evaluation of the preference to and behavioral effects of an enriched environment on male rhesus monkeys. Laboratory Animal Science 42, 38-45.

With simultaneous exposure, the single-housed subjects spent the greatest portion of the interactive time [30 minute-observation sessions] on the perch [16.8%], the second greatest amount of time spent divided approximately equally between interacting with the Kong [5.0%] and Tug-A-Toy [4.9%], and the least amount of time spent manipulating the grooming board [0.4%]. No statistically significant decline in use of (or habituation to) the enrichment devices was detected. A galvanized steel perch (Lab Products) ... was placed approximately 20 cm [emphasis added] off the floor of the cage, parallel to the side wall (p. 39).