Reinhardt, V. 1989. Evaluation of the long-term effectiveness of two environmental enrichment objects for singly caged rhesus macaques. Lab Animal 18(6), 31-33.

Subjects were exposed to a PVC pipe and to a gnawing stick uninterruptedly for one year. The singly caged monkeys spent on average 28% of the total observation time [120 min] with the PVC pipes but only 3% with the branch segment [gnawing stick]. ... While perching, the monkeys sat in front of the cage for 95% of the time, in the middle or rear of the cage for 5% of the time. ... The proportion of time spent with the pipes was three times greater for animals living in lower-row cages than for animals living in upper-row cages. ... In the elevated position, the light exposure was increased, a fact that made the pipes of particular value for the lower-row cages animals.