Can puzzle feeders be used as cognitive screening instruments? Differential performance of young and aged female monkeys on a puzzle feeder task (1999)

Watson, S. L., Shively, C. A., Voytko, M. L.

Abstract

The monkeys' interest seemed directed more toward the task than the reward, as evidenced by the observation that animals would occasionally discard an uneaten pellet in order to work on the task at the next level of difficulty.