Chicks show left-to-right number bias
Like many humans, newly hatched birds favor small quantities on the left
LITTLE TO THE LEFT Young chicks seem to have their own kind of mental number line with smaller quantities toward the left.
R. Rugani
Chicken Little knows small — and like many humans may have a left-to-right bias for a mental number line of small to big quantities.
In a new kind of test, more chicks appeared to favor smaller quantities on the left and larger ones on the right, says experimental psychologist Rosa Rugani of the University of Padua in Italy. This left-to-right bias for magnitude showed up even when researchers tweaked the test so the same quantity, eight dots, appeared small or large depending on the context, Rugani and her colleagues report in the Jan. 30 Science.
It’s the only “convincing” evidence so far in animals for associating one direction with smaller quantities and the other with larger ones, says Peter Brugger of University Hospital Zurich. It’s a simple version of what many people do —creating a left-to-right mental number line like an invisible ruler. But debate has brewed over how to test for a similar preference in animals.
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Earlier tests looked for a version of mental number lines in animals by showing lines of objects to rhesus macaques, chickens and seed-caching birds called Clark’s nutcrackers. First, the animals were positioned at the end of a line of objects, as if on a street corner looking down the block at the houses. Researchers trained the animals to pick a certain object in the series by numerical order, such as the fourth in the row. For the test, the object row was turned 90 degrees. The animal now faced the row as if standing in the street in the middle of the block looking head-on at the houses. In this case, fourth from the end could mean fourth from the right or fourth from the left. Animals often chose the object by starting from the left, and researchers took that as a suggestion of a left-to-right bias in mental arrangements of quantity.
Rugani, who tested birds this way, wasn’t satisfied. The birds could be doing the task with spatial cues instead of numerical ones, she says. In any case, many birds – like humans – have a bias to pay extra attention to whatever is to their left.
After several years of fiddling with alternatives she calls “not so elegant,” she and her colleagues devised a new method. They trained 15 recently hatched chicks to poke behind a card with five dots on the front to find a mealworm treat. The researchers then offered the chicks pairs of other cards, both with the same number of dots, to see which one the birds checked first for goodies.
If the matching cards had a smaller number of dots compared with the training card, chicks tended to investigate the left card first, the researchers found. If the matching cards had more dots than the trainer, chicks tended to investigate the right-hand card.
When both cards in the pair had only two dots, for instance, birds in 71 percent of the trials first went left, as if they had a mental predisposition to link small stuff with leftward space. When both cards showed eight dots, 71 percent of the time birds went right, as if they assumed that bigger quantities belong on the right.
Then researchers manipulated the set-up so that the same eight dots seemed a relatively small quantity. In this second experiment, researchers trained a different group of 12 chicks to find a treat behind a card with 20 dots. When tested with eight-dot cards, now just a measly speckling, most chicks turned left — toward the small side. And when shown cards with a number larger than 20, these chicks were much more likely to go right.
Calling the birds’ links between direction and quantity a mental number line is just a metaphor, Brugger cautions. But some form of spatial bias for small and large does explain the results of this test, he says. “I can’t think of an alternative.”
Behavioral neuroscientist Randy Gallistel of Rutgers University in New Brunswick, N.J., notes that, evolutionarily, the apparent similarity between a human and chick bias could be “deep and ancient or it could be a fluke of convergence.” As for why the chicks may have evolved a bias for small stuff on the left instead of the right, he suspects there’s no profound story of a benefit. “One thing that doesn’t get emphasized enough,” he says, “is that important parts of the evolutionary story may be just accidents we get stuck with.”
CHICK TEST In a new method of testing for mental number lines, a young chick learns that a card with five dots hides a treat behind it. Then researchers test the chicks on pairs of matching cards with a different number of dots from the training card. If the matching cards have a smaller number of dots compared with the trainer, chicks tend to investigate the card on the left first. If the matching cards have more dots than the trainer, chicks tend to investigate the right-hand card.
Credit: R. Rugani
Editor’s note: This story was updated February 13, 2015, to correct the x-axis label on the Experiment 2 graph. The chicks were trained on 20 dots, not five.
