In the mind’s eye of a bird brain

Print Friendly
In the mind’s eye of a bird brain
A photoshopped chick stands in front of an illustrated array of cards with different numbers of red dots, arranged low to high from left to right. Thought bubble extending from chick reads, "left means less, right means more.
A three-day-old chick is the same age and strain as the birds in the study shows the general idea of the mental number line: Small numbers go on the left, large on the right.
Chick photo: Rosa Rugani, University of Trento; Composite: The Why Files

Imagine two cards side by side. One shows three squares, the other 10. Odds are you’ve placed the card with three squares to the left of the one with 10.

Now imagine cards showing 10 and 20 squares. Odds are the 10-square card is now on the left.

Congratulations! You have just demonstrated an innate human tendency called the mental number line: Smaller to the left, larger to the right.

In a study this week in Science 1, Italian researchers report the same trait in 3-day-old chickens.

By placing a worm behind the cards, they induced the chicks to choose which of two identical cards to visit first. And lo and behold, the chick visits the card that is correctly placed in that small-to-the-left-large-to-the-right number line.

Just like people:

If the training number was five and the test number was two, they visited the two that was on the left in more than 70 percent of tests.

If the training number was five and the test number was eight, they visited the eight on the right, again in more than 70 percent of tests.

Chick flick shows mental number line

Adapted by The Why Files from original from Rosa Rugani, University of Trento

“Chicks chose respectively the right or the left side of their visual space depending on the numbers seen at test being greater or smaller than the number seen at training,” according to first author Rosa Rugani, a fellow with the Center for Mind/Brain Sciences at the University of Trento, Italy.

We can’t intuit what the birds were thinking — or even if they were thinking — but the best way to explain it is that the birds were somehow more comfortable when number sequence matched their mental number line. Or maybe they expected that number in its number-line location, and so they responded faster.

Lining up the numbers

The idea of a mental number line originated in the late 1800s, Rugani said, when Viennese psychologist Francis Galton “first showed that humans describe and think of numbers as being represented on a mental number line oriented from left (small numbers) to right (large numbers).”

The mental number line is everywhere!

Tape measure: hjl, calipers: Morpheu5

Could the left-to-right order reflect the pattern we see in the placement of words on the page? Perhaps. Readers of Arabic, which reads right-to-left, tend to envision small numbers on the right. However, readers of Hebrew (another right-to-left script) who also read English show the small-on-left pattern, suggesting that this is the innate pattern, although it can sometimes be overcome.

Math: It’s an animal thing

If you’re astonished that chicks have such a number sense, Rugani says Clark’s nutcrackers and rhesus macaque monkeys also show some form of “number-space mapping.”

And that’s not the limit, she adds. “The capability to solve numerical tasks is widespread among all tested species (e.g. frogs, honey bees, fish, birds, etc.).” Some animal mathematical performances resembles those of human adults who are prevented from using language.

That, Rugani says, “suggests that animals’ numerical cognition originates from pre-linguistic precursors that are widespread in all the animal kingdom,” and that the mapping of numbers “is widespread among all species.”

From photo by Tina Asami Newton

In a commentary2 in Science, Peter Brugger, of the department of neurology at University Hospital Zurich, wrote that the Rugani results “show that newborn chicks can understand both relative and absolute quantities. They also suggest that the brain may be prewired in how it relates numbers to space.”

Comprehending numbers can help an animal identify a larger food source or survive, Rugani wrote us. “Imagine a situation in which an animal can observe for a safe point of view a predator that enters a den. Later it sees another predator enter the same den. Thereafter a single predator exits. Also imagine that our animal wants to enter the den, but only if the animal can solve simple summations and subtractions will it survive.”

Add it up, she says, and “Numerical competence and the association of numbers with space originate from very ancient precursors.”

So next time you are addled by algebra, don’t blame it again on your ancient ancestors!


On June 26, Science published several comments questioning aspects of the Rugani study.

Read them: 1, 2, 3

– David J. Tenenbaum

3 4 5

Kevin Barrett, project assistant; Terry Devitt, editor; S.V. Medaris, designer/illustrator; David J. Tenenbaum, feature writer


  1. Number-space mapping in the newborn chick resembles humans’ mental number line, Rosa Rugani et al, Science, 30 January 2015
  2. Chicks with a number sense, Peter Brugger, Science, 30 January 2015
  3. Our brains have a map for numbers.
  4. Kids’ mental number lines reveal math memory.
  5. Learned, not innate human intuition: Study finds twist to the story of the number line.