Think about all the numbers between 1 and 10, inclusive.

If you’re like the vast majority of people, you just automatically pictured a row of digits, starting with 1 on the left and ascending to 10 on the right. This is the mental number line. Until recently, it seemed to be unique to humans. That’s not to say that other animals can’t count or deal with numbers—they plainly can. But it looked like we were the only species that thought about numbers in this way.

Not anymore. In a really clever experiment, Rosa Rugani from the University of Padova has shown that baby chickens probably have a mental number line too. Once they fixate on a specific number, they associate smaller numbers with the left side of space, and larger numbers with the right—just like us. This supports the idea “that culture is not crucial for the mental number line,” says Rugani.

The seeds of this experiment were planted in 2007. Rugani put domestic chicks and Clark’s nutcrackers in front of a row of sixteen objects, which extended away from them. She trained them to peck at the fourth one. When she rotated the row so that it ran horizontally instead, the birds mostly pecked the fourth object from the left. They were counting from left to right.

They might have had a mental number line. Then again, they might just prefer to pay attention to objects in the left side of space. (Humans have the same bias; it’s called pseudoneglect.)

To clarify the matter, Rugani designed a new and deceptively simple experiment. She placed three-day-old chicks in front of a panel showing 5 squares, and taught them to walk around it to get some food. She then presented the chicks with two identical panels, one on their left and one on their right. If these showed 2 squares, the chicks walked towards the left one 70 percent of the time. But if the panels showed 8 squares, the chicks’ preference flipped: they approached the right one 70 percent of the time.

View Images Credit: Rosa Rugani.

Since both panels were identical, there’s no reason why the chick should prefer one over the other. And if it was showing pseudoneglect, it should always prefer the left panel. That’s not what happened. If the testing panels showed a smaller number than the training one, the chicks walked left; if they showed a bigger number, the chicks headed right. That’s pretty strong evidence for a left-to-right mental number line.

Rugani repeated the experiment with different numbers. This time, the training panel had 20 squares, and the testing ones had either 8 or 32. The chicks behaved in the same way: smaller number, head left; bigger number, head right. Note that the ‘small’ number here—8—is the same as the ‘big’ number from the first experiment. If the chick saw 5 squares first, it headed towards the right 8-square panel. If it saw 20 squares, it approached the left 8-square panel.

This showed that the chicks are comparing the relative size of the numbers. “A number is not either small or large in an absolute sense, but rather it is smaller or larger with respect to another number,” says Rugani. In other words, their number line can slide, just like ours can.

“This finding completely overturns the idea—widely held in the past—that our mental number line is a human invention that depends on culture and on instruction,” says Maria Dolores de Hevia from Paris Descartes University, who studies the same topic in human babies.

But if a chick—an animal with no reading skills, arithmetic, language—also has a mental number line, that changes things. It strongly suggests that human culture only shapes and tweaks something that is innate. “And it adds to the idea that our most sophisticated numerical abilities derive from a core, and evolutionary ancient, sense of number, which evolved in such a way that it exploits our visuo-spatial resources,” says De Hevia. “The question now is: why?”

There might not be a why. The direction of the line might be totally arbitrary. Then again, in many animals including chicks and humans, the right half of the brain takes the lead in visual, spatial, and numerical tasks. And since the right hemisphere directs our attention to the left side of space, perhaps that’s why the number line begins on the left.

Her team are now carrying out similar studies in other species. They’re trying to identify the specific parts of the brain that deal with numbers, and to identify the genes that influence number sense. “This would help us to understand the evolutionary origin of the mental number line,” she says.