Renquist, D. M., Judge, F. J. 1985. Use of nylon balls as behavioral modifier for caged primates. Laboratory Primate Newsletter 24(4), 4.

The [single-housed] animals play with the ball [Nylaball?] by rolling it in the cage. Adult females hold it as a surrogate infant. Stumptail macaques often chew the ball. ... The inert nature of the nylon renders it nontoxic, and any chewed particles pass through the intestinal tract.