Hoffman, H., Green, J., Deutsch, J. et al. 2014. Mirrors and stainless steel balls for macaques: A discussion. Enrichment Record 19, 22-25.

I received the steel balls from Bio-Serve a few days ago; WOW what an awesome toy for the macaques! They absolutely love playing with them. The individual in these photos is Eve, a young adult cyno. She carries the ball with her everywhere and does so many things with it. She pushes it around, tosses it in the air, holds it high up and watches her reflection, shakes it to feel the sand inside swish around. I know that's what she is doing because I did it too before I gave the ball to her and she appears to use it like a bowling ball, and watches how it wobbles back and forth while it's rolling. Then she lays on her back and playfully lifts it up and down with her hands and feet like she's weight lifting. So much fun watching all of them enjoy this new toy!