Kaplan, M. L., Jobao, B. J. 1991. A unique housing system for rhesus macaques. Lab Animal 20(6), 48-50.

From each bank of four dog cages, we removed the ceilings of the lower two cages, making a single housing unit for macaques. To each cage we added three resting perches, a climbing ladder, windows on both sides, and a ring for attaching foraging or enrichment devices. The new, spacious primary enclosure for macaques also provided an efficient inter-cage transfer system. ...Our earlier attempts to stimulate the animals' psychological well-being with toys, television, ropes, and mirrors were equivocal, leaving no apparent benefit.