It’s hard to imagine a world without zero, but it’s actually a surprisingly recent idea. It was discovered way later than numbers like two or 14 or even 3.9 or 64/13. And when you think about it, that’s not surprising – after all, if you’re an ancient Babylonian trying to keep track of your sheep, zero is pretty useless. Even if all your sheep died from some sheep plague, you wouldn’t say “I have zero sheep” like some nerd doing a math test. You’d say something like “pass the opium, Esagil-kin-apli, all my sheep are dead, I’ve no sheep left, it’s been a hell of a day.”
So it may come as a surprise that humans are not the only animal who can understand the concept: there’s convincing evidence that honeybees can conceptualize zero as a quantity, and so can the (already impressively numerate) rhesus macaque. And according to new research from the University of Tübingen in Germany, crows can too. In a paper published this month in the Journal of Neuroscience, scientists have shown that the brainy birds can not only understand zero as a numerical quantity – as “something” rather than “nothing” – but they also may well have an idea of it as the smallest number on a mental number line.
“These animals may not need the concept of numerosity zero – unlike countable numerosity – in their day-to-day living,” study co-author Andreas Nieder told IFLS. “But they are smart enough to be able to learn this abstract category.”
The researchers trained the birds to play a kind of computer game, Nieder explained, where they were presented with two displays, one after the other, consisting of between zero and four dots. The crows’ objective was to correctly indicate whether the two displays had the same number of dots as each other. While they played the game, the scientists monitored their brain activity. Previous experiments had shown that the corvid songbird crania contained neurons that would light up for one, two, three, or four dots respectively, but nobody knew if there would be a similar reaction to a display of zero dots.
“[W]e monitored the activity of single neurons in a part of the brain ... known to be involved in cognitive tasks,” Nieder explained to IFLS. “By correlating the neuron’s responses with the crows’ behavior, we could explore how the crows’ numerical perceptions in the task emerged from the activity of such neurons.”
The scientists discovered that the crows’ brains do indeed have specific neurons that light up for “zero” – but that wasn’t their only discovery. By seeing when the crows failed at the task – when they confused three dots for four, for instance – the team were able to unravel just how sophisticated the birds’ understanding was.
“[The crows] confused the empty set more often with numerosity one than numerosity two,” Nieder told IFLS. “This effect [would only be] expected if ... crows can understand the empty set as the smallest numerical value on the number line.”
The most astonishing thing about this discovery, Nieder told IFLS, is just how different crow brains are from apes’. The two species diverged around 320 million years ago, he explained, and birds don’t even have a neocortex like mammals do. And yet, the birds have evolved this ability, completely independently, for abstract numerical conceptualization.
“It was surprising to see that an endbrain that has evolved anatomically distinctly and independently from the endbrain of mammals is capable of grasping the empty set,” Nieder told IFLS. “I think it shows that some phylogenetic preconditions exist … that may constitute an evolutionary basis for what in humans can develop into a full-blown understanding of the number zero.”