Quantity (like size) matters when we are making decisions about all aspects of our life. If something costs the same but we have the choice to have more or less of it, have you ever deliberately chosen less? Even when on a diet? There’s a very good reason for this. It is adaptive to go for more; there is selection pressure to select the larger food patch when all else is equal. But, when does our ability to judge what is more or what is less develop? And, given the importance of this ability for survival, are similar mechanisms that govern the choices in humans observed non-human animals? Do they judge more or less, more or less similarly to us?

At a very early age human babies are able to discriminate between quantities. It has been suggested that this does not require learning but is rather an innate component of human cognition. Interestingly, implicit or unconscious measures of cognition which do not require an explicit choice on the part of the infant have revealed evidence of relative quantity sensitivities as early as one year old. To examine this infants can be presented with an array containing a certain number of stars they keep seeing the same number until they are no longer interested. They are then presented with a novel number and, if their interest is piqued, they may look longer. But they can only do so if they can tell the difference between the numbers. However, in explicit judgment tasks, where they have to choose actively the greater quantity, children are unsuccessful until around 2.5 or 3 years of age.

This is rather surprising and it has been suggested that it may be the result of an inadequate learning experience or a lack of developmental maturity which prevents young children from performing in such a task. To test whether this is something specific to humans or whether it evolved earlier in the primate lineage it is essential to look for the ability to discriminate between quantities in other primate species.

There has been much work investigating the mathematical processing abilities of non-human primates and the data reveals that they are able to learn to use quantity information and appear, to some extent, to use similar mechanisms to those observed in humans. Generally primates are asked these sorts of questions using touchscreen computers as it allows experimenters to control for lots of different extraneous variables. They learn to touch the screen to receive a food treat, once they have learned this, they can then be asked all sorts of questions relating to mathematics. Depending on the task, they can be trained to select the lower or higher number of objects or to order them. (The video at this link is really worth watching.)

Though we have a good understanding of some areas of mathematical processing in animals, it is still unclear whether quantitative judgment develops as early in non-human primates as it does in humans. Further, by testing both infant and adult monkeys we can probe the question of development vs learning in a manner that is impossible to do with humans. This approach has recently been explored by Jessica Cantlon and colleagues (University of Rochester) in a paper published in the journal Psychonomic Bulletin and Review.

In this paper the authors investigate spontaneous quantity judgments in infant (1 year old) and adult olive baboons (Papio anubis). It was important to have this contrast in age as it allowed the authors to examine the role of development and previous learning in quantity discrimination.

One key aspect of the design is that the animals do not need to learn the task but rather are put in a situation in which they can spontaneously choose whichever quantity they want. This allows the authors to directly compare the data between both adult and infant monkeys and with that obtained with human infants.

Thus, baboons were presented with tubes which contained food (peanut halves). The animals simply had to choose the one that they wanted and got to eat the contents of the tube that they selected. Initially they received ratios of 1 vs 3 peanuts in the two containers, and 1 vs 2, before being tested on harder ratios, going up to 7 vs 8. The researchers found that both the infant and adult monkeys spontaneously chose the larger quantity on the majority of trials. Further, as the ratios between quantities got closer together, the animals’ performance decreased. This is a classic finding and fits with previous work on quantity discrimination in many primate species (including humans) and suggests that the monkeys used analog quantitative reasoning to solve the task. This finding provides the first evidence suggesting that this sort of reasoning emerges spontaneously (without specific training in the task) in monkeys; further it reveals evidence of early emergence in monkey development because the infant and adult baboons did not differ in their performance.

So, what does this mean? Well, it gives us important insights into some key questions related to human cognitive development. The early development of quantitative reasoning in monkeys suggests that there may be a common developmental basis to quantity judgment in humans and non-human primates. It also suggests that the ability to make quantitative judgments may be constrained by maturation as quantitative cognition may be constrained by rate of growth. The monkeys develop faster than humans and were able to make these judgments at 1 year old whereas humans are able to do so at around 2.5 or 3 years of age. What we don’t currently know is the age at which the monkeys develop these abilities. Interestingly, there was no difference in performance between the infant and adult monkeys which again supports the idea that this task does tap into spontaneous quantity discrimination.

This work adds to the growing literature suggesting that there is a fundamental link between the quantitative abilities of humans and non-human primates. So next time you see monkeys at the zoo, or watch footage of them in their natural environment. Take a minute to consider how they are making decisions about where to go, which group to stay near and which food patch to exploit and whether you would make similar or different choices.