The use of toys for primate environmental enrichment (1989)

Watson, D. S. B., Houston, B. J., Macallum, G. E.

Abstract

We have investigated a variety of objects and toys, and have discovered that all [single-housed] monkeys do not like all toys, and that what interests one monkey today may hold no interest for him next week. ... Hard nylon balls have been used here with moderate success. Although some of our animals ignore these balls, others have been occupied for hours rolling them down the cage bars, holding them, spinning them in their food bins, or chewing on them.