The number 5 sits to the left of the number 10. The number 15 is to the right of 10, but to the left of 20. Most people would have no problem agreeing with these statements, because most people have spatial associations for numbers. This is called the “mental number line”: smaller numbers go on the left, increasing towards the right.

What’s not clear is where this number line comes from. It’s possible that it’s a product of mathematical education, or even that it’s a product of human culture and language, found universally regardless of education levels. On the other hand, it’s possible that it’s an in-built feature of our brains, one that's possibly shared with other species.

A recent study in Science tested three-day-old chicks to see if they have number-space mapping like humans do. Using such young animals allowed them to be sure that the chicks weren’t overtrained in the use of numbers, which could cause them to demonstrate atypical behavior.

Each chick was placed in front of a panel with five dots. Behind the panel was food, and once the chick had figured this out enough to walk around the panel looking for food 20 times in a row, it moved past the training phase onto the testing phase. The testing phase involved two panels equidistant from the chick, both with the same number of dots: both panels either showed two dots, or eight dots. The test was whether the chick would choose to walk around the left-hand panel, or the right hand panel.

When the panels had two dots, the chicks walked around the left-hand panel approximately 70 percent of the time. When the panels had eight dots, the chicks walked around the right-hand panel approximately 70 percent of the time. This suggests that they associated the smaller number with “left” and the larger number with “right” compared to the number they’d been trained on.

The next group of chicks was trained on a panel with 20 dots. This time, in the testing phase, the panels had either 8 or 32 dots. Once again, when the panels had 8 dots, more chicks went around the left-hand panel; when there were 32 dots, they went to the right.

The results “provide further evidence that language and culture are not necessary for the development of mathematical cognition,” says lead researcher Rosa Rugani. They also show that chicks process number size as relative to other numbers, not absolute. When the chicks were trained on five dots, panels with 8 dots were larger and therefore on the right; when they were trained with 20 dots, the eight-dot panels were smaller. Therefore, the processing of a given number on either the left or the right side will vary depending on the number it is compared with, Rugani explains.

It’s important not to think that the chicks are actually counting, she adds. Being able to see the difference between amounts is not the same thing as being able to count up to 32. But, even though animals clearly don’t have the same advanced number cognition as humans, we can still draw important conclusions from studying them. “The investigation of animal cognition could help us to understand the cognitive abilities that can be mastered without language,” she explains.

Previous research on mental number lines has created some contradictions. One study showed that people who grow up reading text and numbers right-to-left in Arabic have a reverse number line, from right to left, suggesting cultural factors affect the spatial mapping. On the other hand, infants as young as seven months seem to prefer being shown dots in a 1-2-3 left-to-right pattern, compared to a 3-2-1 left-to-right pattern, suggesting that the ability emerges before much cultural conditioning or language learning can take place.

Showing other animals had a number line might influence this debate. However, it’s not so clear that the conclusions of this paper are supported by its evidence, says Dr Andrew Wilson, a researcher in psychology and cognitive science at Leeds Beckett University.

There are two principal problems with the study, Wilson argues. “The mental number line effect in people shows itself as a nudge, a tendency,” he says. “But the chicks’ behavior was surprisingly reliable: only two chicks out of 64 in the study produced an effect in the 'wrong' direction. That suggests that something much more stable and reliable was driving their searching.” Although the researchers were careful to control the visual stimuli as carefully as possible, there may have been an uncontrolled factor in the mix, he explains.

More importantly, he adds, it’s not clear that a concept of a mental number line would come into play in how a chick searches for food. Either the chicks saw some kind of similarity between the training card and the card they chose to look behind, or they somehow were compelled to look for food in accordance with their mental number lines. The latter explanation is unconvincing, Wilson argues.

“The authors have worked hard to address some potential confounds in the study,” he says. “But there many reasons why the chicks could be biased to head in one direction, and jumping so quickly to a ‘mental number line’ style explanation is premature at best.”

Science, 2015. DOI: 10.1126/science.aaa1379 (About DOIs).