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