Line, S. W., Morgan, K. N., Markowitz, H. 1991. Simple toys do not alter the behavior of aged rhesus monkeys. Zoo Biology 10, 473-484.

By the fourth week of exposure only 36% of the [single-caged] monkeys were seen using them [rubber toys, nylon ball, gnawing stick] even once, and only 17% did so during more than one of three observations.