Newborn chicks associate numbers that have lower numerical values with the space to their left and higher numbers with their right. The findings, published in Science this week, suggests that the cute peeps may be using the same “mental number line” as we do. The tendency may be hard-wired.
Our continuous, left-to-right spatial representation of numbers comes up in our everyday lives, yet the origin of this particular orientation is still being debated. Is it innate . . . or did it develop with cultural experience over time?
A team in Italy led by Rosa Rugani from the University of Padova conducted a series of experiments with 64 three-day-old domestic chicks (Gallus gallus), who learned to circumnavigate a panel to reach a food reward within a diamond-shaped arena. During training, the panel had a card depicting five little squares dots to familiarize the chicks with this target number (pictured below, A). In tests, the researchers presented each chick with two panels: One on its left and one on its right, with both depicting the same number of dots as each other, but a different number than the target.
Each chick underwent two tests (pictured, B and C). In the "small number test," the identical panels depicted two dots, or a number of elements that was smaller than the target. Then, with the "large number test," the panels each contained eight dots, or a number of elements that was was larger than the target. The team recorded the chicks’ behavior and scored the panel that the chicks inspected first.
When the identical panels had only two dots, the majority of the chicks looked for the scrumptious mealworm behind the panel on the left. When the panels had eight dots, they investigated the panel to their right for the grubby treat. The team repeated the experiment with a target of 20 square dots and test cards with 8 and 32 squares. The chicks consistently looked for the smaller numbers to the left, and the larger numbers to the right of the target.
The findings suggest that the tendency to map numerical values left to right across a mental number line emerged naturally regardless of cultural background or mathematical training. While longhorn chicks aren’t necessarily “counting” in the same sense that we are, Science explains, they are distinguishing between smaller and larger numbers of objects. "All we can judge is behavioral responses,” Rugani tells BBC. “Therefore, we don't actually know if it is a real 'number line' but it strongly resembles what is observed in the human number line."
The brain’s asymmetry may have shaped our mental number line a long, long time ago. This tendency to visualize a line with smaller values on the left and larger values on the right likely evolved millions of years ago—before our ancestors split from those of modern birds.
Images: Rosa Rugani, University of Padova (top, bottom), R. Rugani et al., Science 2015 (middle)
