Why Monkeys Can Only Count To Four

This video is sponsored by Brilliant.  More about them at the end of the video. There’s an island in the Caribbean where I  used to perform magic tricks for monkeys.

Hi, I’m David, and this is MinuteEarth. I was working on a big study  about how animal brains work, and as part of that research, I  carried around a big bag of props. Whenever I found a monkey hanging out by itself,  I’d bring out this stage – basically a platform that I’d then hide from the monkey’s  view with a big piece of posterboard.

I’d take an apple out of the bag  and dramatically lower it behind the posterboard, then I’d take out  a second apple and then a third. Finally, with a flourish, I’d remove the  posterboard, revealing the apples to the monkey. Usually, there would be three apples  there but occasionally – thanks to a hidden chamber where I could stash  or retrieve an apple – the number of apples on the platform wouldn’t match the  number of apples I’d removed from the bag.

When those mismatches happened, the monkeys  would stare as if they were surprised – or had even been tricked – which was super cool because  it suggests that they have the ability to count. And monkeys aren’t alone. Over the past  few decades, using similar sleight-of-hand experiments, magicians – I mean, researchers  – have found that pretty much every animal we can test – from spiders to sea lions –  seems to get surprised when the number of items they expect to see doesn’t match  the number of items that are revealed.

At least that’s what happens when the illusion  is performed with numbers smaller than four. But if I lowered, say, four apples during the  trick and revealed five apples, the monkeys didn't seem surprised at all; nor would any other  species of animal that scientists have tested. This suggests that maybe non-human  animals can’t count any higher than four.

But that’s not exactly right. Because while  monkeys aren’t surprised when they see four apples get lowered and five get revealed,  they are surprised when four apples get lowered and six are revealed, or when nine  apples get lowered and twelve get revealed. What the monkeys actually seem to be doing is  comparing the relative sizes of two sets of stuff.

One set is the amount of stuff they see  go behind the posterboard and the other set is the amount of stuff they see get revealed. So if they expect there to be a pile of apples  that looks like this, and instead they see a pile that looks like this, that relative  difference is big enough for them to notice, but if they expect a pile that looks like this  and instead they see a pile that looks like this, that relative difference seems to be small  enough that they don’t seem to notice. Why do animals do this relative comparison,  though, instead of just, you know, counting stuff?

Well, when there's a very small amount  of something important out there – like, say, yummy apples, or scary predators  – it makes sense for an animal to be able to discern small differences between amounts. But when there are lots of things out  there, knowing the exact number isn’t as important – it’s more about knowing when  the amounts are different enough to matter. And “different enough” to an animal, seems to  be a difference greater than 25% – more or less.

Being able to specifically count numbers  of things may just be a waste of energy and brainpower in most situations. Humans  – with our super-ability to count and differentiate specific numbers  – are actually the weird ones. This skill seems to be unlocked by  language, which allows us to give a name to every single individual number, and  thus seems to move our counting ability out of the parts of the brain that are used  to generally compare set sizes and into a special highly-developed part of the brain  that's devoted to more complicated computation.

That’s why, when I repeated my  magic show with adult humans, they were of course surprised, even when  I did the trick with bigger numbers. But – to show how important language is – when  I did the show for preverbal humans – you know, toddlers – they reacted the exact same way the monkeys did – they weren't surprised by  small differences between big numbers, but they were very impressed when  three apples magically became two! Well, not exactly – they didn’t  seem to care much about apples.

I had to trick them with something they  cared more about: stuffed animals. Language, though isn't everything when it comes  to computation; you also have to develop that part of your brain devoted to math-ing.  And Brilliant is where you learn by doing, with thousands of interactive lessons in  data analysis, math, programming, and AI.

Brilliant helps you build real skills  through hands-on problem-solving based on lessons created by MIT and CalTech professors. And Brilliant just launched  a ton of new data courses, all of which use real-world data to train you to  see trends and make better-informed decisions. I’m currently making my way through their  new course on exploring data visually, where I’m learning how to parse and  visualize not just large quantities, but massive datasets in order to  make them easier to interpret.

To try everything Brilliant has to  offer for free for a full 30 days, visit brilliant. org/MinuteEarth or click  on the link in the description. You’ll also get 20% off an annual premium  subscription.

Thanks Brilliant.