In the early 1800s, Sir Francis Galton, a British polymath, showed for the first time that humans describe and think of numbers as being represented on a mental number line oriented from their left to right. Such a spatial-numerical association has been explained as being due to writing and reading habits. However, three-day-old domestic chicks, Gallus gallus, show a similar and spontaneous spatial-numerical association.

Three-day-old domestic chicks learned to circumnavigate a screen to receive a food reward. At training, the screen depicted a certain number of elements; namely, 20 images as shown in Figure 1. At test, chicks were shown two screens: one at their left and one at their right side. Both screens depicted the same number of images, which was however a different number from the one they had experienced in training. To choose which screen they would approach first to look for food, birds could not rely on familiar information concerning spatial position or numerical cue. Whenever chicks faced a smaller number of images, such as the 8 versus 8 scenario shown in Figure 2, they chose the left panel — whenever they faced a larger number of images, such as the  32 versus 32 scenario shown in Figure 3, they chose the right panel. This shows that really young and almost naïve animals associate small numbers with their left — and large numbers with their right.

Interestingly, this spatial-numerical association is not absolute but relative. When the number experienced during training was 5, the number 8 was associated with their right side of space. This indicates that, also for young animals, a number is not large or small per se, but it is smaller or larger with respect to a reference value, which is the number experienced during training in this scenario.

Overall, this is strong proof that numerical mapping originates from pre-linguistic and biologically determined precursors.

Related scientific publication
Rugani, R., Vallortigara, G., Priftis, K. & Regolin, L. Number-space mapping in the newborn chick resembles humans’ mental number line. Science 347, 534–536 (2015).