Monkeys can discriminate quantities of food or inedible items much better when they are not allowed to eat the chosen food reports a paper published in Nature Communications this week. These findings suggest that how the monkeys perceive the presented stimulus mentally is more important than the actual item.

Many animals, including dogs, cats, chicks and some fish can do basic arithmetic to distinguish different quantities. Previous reports have shown that monkeys perform quite badly when discriminating different amounts of foodstuffs. Vanessa Schmitt and Julia Fischer show that olive baboons (Papio Anubis) and long-tailed macaques (Macaca fasciularis) are much more accurate in their decision making in regards to quantity when they are presented with inedible items ― pebbles ― rather than food. They also perform more accurately when they are subsequently rewarded with a food type that is different to the type that they were attempting to quantify. Their findings suggest that the physical appearance of the stimuli is less important that the mental picture of the reward.