Three-day old chicks use a mental number line that reads left to right, just as humans do. | Rosa Rugani
A new study shows that chicks use the same mental number line, or left-to-right spatial representation of numbers, as humans. The findings suggest that this tendency to map values from left to right may have evolved millions of years ago, in the shared ancestor of humans and modern birds.
In their study, the researchers showed that newborn chicks associate low numerical values with space to their left and higher numerical values with space to their right.
Whether this left-to-right spatial representation of numbers is innate or if it develops with experience has been the subject of long-standing debate. But now, Rosa Rugani from the University of Padova in Italy and colleagues suggest that the role of culture and language is secondary in determining the orientation of the mental number line.
Their report is published in the 30 January issue of Science.
Trained on a target of number five, chicks move to the left when presented with two panels with a lower number value, in agreement with a left-to-right number line. | Rosa Rugani
"We cannot say that this spatial numerical association is innate, but that it's precociously available soon after birth," said Rugani. "Our data constitute strong evidence that numerical mapping onto space originates from pre-linguistic and biologically determined precursors. I would not be surprised at all to find this numerical spatial mapping in other animals and newborn infants."
The researchers first familiarized chicks with the number five by teaching them to walk around a panel with five dots printed on it, in search of a mealworm snack. They then showed the chicks two different panels — one on the left and one on the right of the chick — that both depicted a smaller value of two dots. Notably, the researchers observed that the majority of chicks approached the panels to their left when the smaller numbers appeared.
Rugani and her colleagues then showed the chicks two identical panels that depicted a larger value of eight dots. This time, they discovered that most of the chicks chose to investigate the panels to their right.
Rugani and her team studied two sets of numbers with the chicks — 2 to 8 and 8 to 32 — to make sure that their results were robust. In all cases, the chicks consistently looked for the smaller numbers to the left and the larger numbers to the right of their targets.
When faced with panels containing higher number values, the chicks veered to the right panel, as expected from a left-to-right mental number line. | Rosa Rugani
"The first characteristic of the human mental number line is a left-to-right orientation," explained Rugani. "The second main characteristic of the mental number line is its relativity — the fact that a number is not just large or small, per se, but that it's larger or smaller with respect to another number."
Taken together, the results of their two experiments suggest that newborn chicks visualize a number line in the same way humans do. The researchers also suggest that the brain's asymmetry might have played a role in shaping this mental number line long ago.
"The behavior we observed may be the result of a preferential processing of space and numbers under the control of one [brain] hemisphere — the right one," said Rugani. "Brain asymmetry is a common and ancient trait in vertebrates, selected during evolution as it helped animals to improve processing of different kinds of information within one or the other specialized hemisphere…This is one of the issues we would like to experimentally tackle next."