Monkeys know their plurals

Anybody who reads this blog knows that I am deeply skeptical of claims about animal language. Some of the best work on animal language has come from Marc Hauser‘s lab at Harvard. Recently they reported that rhesus monkeys have the cognitive machinery to understand the singular/plural distinction.

First, a little background. Many if not most scientists who study language are essentially reverse-engineers. They/we are in the business of figuring out what all the parts are and how they work. This turns out to be difficult, because there are many parts and we don’t really have the option of taking apart the brains of random people since they usually object. So the task is something like reverse-engineering a Boeing 747 while it’s in flight.

There are many different ways you could approach the task. Hauser tries to get at language by looking at evolution. Obviously, rhesus monkeys can’t speak English. Just as obviously, they can do some of the tasks that are necessary to speak English (like recognizing objects — you have to recognize something before you can learn its name). Any necessary components of language that non-human animals can successfully perform must not be abilities that evolved for the purpose of language. If you can figure out what they did evolve for, you can better understand their structure and function. So the next step is perhaps to figure out why those particular abilities evolved and what non-human animals use them for. This ultimately leads to a better understanding of these components of language.

That is one reason to study language evolution in this manner, but there are many others (including the fact that it’s just damn cool). If you are interested, I suggest you read one this manifesto on the subject.

Back to the result. Nouns in many languages such as English can either be singular or plural. You couldn’t learn to use “apple” and “apples” correctly correctly if you couldn’t distinguish between “one apple” and “more than one apple”. This may seem trivial to you, but no non-human animals can distinguish between 7 apples and 8 apples — seriously, they can’t. In fact, some human groups seemingly cannot distinguish between 7 apples and 8 apples, either (more on that in a future post).

So can rhesus monkeys? Hauser and his colleagues tested wild rhesus monkeys on the beautiful monkey haven of Cayo Santiago in Puerto Rico. The monkeys were shown two boxes. The experimenters then put some number of apples into each box. The monkeys were then allowed to approach one box to eat the contents. Rhesus monkeys like apples, so presumably they would go to the box that they think has more apples.

If one box had 1 apple and the other had 2 apples, the monkeys went with the two apples. If one box had 1 apple and the other had 5, the monkeys picked the 5 apple box. But they chose at random between 2 and 4 apples or 2 and 5 apples. (For those who are familiar with this type of literature, there are some nuances. The 2, 4 or 5 apples had to be presented to the monkeys in a way that encouraged the monkeys to view them as a set of 2, 4 or 5 apples. Presenting them in a way that encourages the monkeys to think of each apple as an individual leads to different results.)

This suggests that when the monkeys saw one box with “apple” and one with “apples,” they knew which box to choose. But when both boxes had “apples,” they were at a loss. Unlike humans, they couldn’t count the apples and use that as a basis to make their decision.

Full disclosure: I considered applying to his lab as a graduate student. I am currently a student in a different lab at the same school.

Caveat: These results have not been formally published. The paper I link to above is a theory paper that mentions these results, saying that the paper is under review.

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