Like many humans, bees seem to prefer their number ordered from left to right.
Honey bees trained to recognize a specific number tend to fly to the left when given two side-by-side options of a smaller number and to the right when the options represent a larger number, a new study claims. The finding suggests that bees have a “mental number line” and that this association has biological roots, the researchers report Oct. 17 in the Proceedings of the National Academy of Sciences.
While some scientists agree that the study makes a compelling case for a mental number line in bees, others argue that the new work is an oversimplification of complex human behavior.
Many humans have a mental number line that often places smaller numbers on the left and larger numbers on the right; If you are asked to arrange several bunches of grapes by size, you will probably line them up by increasing the number of grapes from left to right. Whether this association is present at birth or learned later in life has long been a matter of debate.
Previous works have shown that bees can countand that they even understand the concept of zero (Serial number: 7/6/18). “When you realize all these facts, an obvious question [is whether honeybees have] the so-called mental number line,” says Martin Giurfa, a biologist at the Université Paul Sabatier in Toulouse, France. Working from home during the COVID-19 closures, Giurfa tested 134 bees (apis mellifera) about their abilities to order numbers using a design developed with researchers who had done similar experiments with chicks Y human babies (Serial number: 01/29/15).
First, Giurfa had to teach her bee students to recognize numbers. Using sugar water, she lured bees into a test chamber built from a repurposed wine crate. For each bee, she hung a panel on the back of the box with a certain number of symbols (one, three or five) and fed them sugar water so they learned to associate the number with food. By varying the appearance of the symbols between visits, she ensured that the bees were learning the number itself and not certain shapes or arrangements.
After 30 trips to the box, it was time for a test: Giurfa removed the training panel and placed two mirror image panels, one on the left wall of the box and one on the right. These new panels had the same number of symbols as the training panel, fewer symbols, or more.
To which panel did the bees fly, to the left or to the right? “It depends on your reference number,” says Giurfa. Of the bees trained on “one,” 72 percent flew to the “three” panel on the right, but of the bees trained on “five,” 73 percent went to the “three” panel on the left. “That’s exactly the concept of the mental number line,” says Giurfa. “You line up the numbers based on your reference.” If the test number was the same as the training number, the bees showed no preference for left or right.
These experiments “make a very compelling case” for a mental number line in bees, says Felicity Muth, a biologist at the University of Texas at Austin who was not involved in the study. “They have a series of checks in place that really rule out any of the alternative explanations that I can think of.”
Giurfa believes that these results show that mental number lines, or at least some component of them, are present throughout the animal kingdom. However, not everyone is convinced.
“Oversimplification of complex human concepts, such as the ‘number line,’ should be avoided, as they severely distort the reality of the phenomena that make them possible,” says Rafael Núñez, a cognitive scientist at the University of California, San Diego.
Nunez, co-author an article critical of the above study on chicksbelieves that animal research should address why bees and chicks would have innate mental number lines, while some human groups, such as those who have studied in Papua New Guinea, No. Giurfa acknowledges that culture plays a role in explaining why not all adults naturally order numbers from left to rightbut feels the proof is there for a biological underpinning (Serial number: 8/23/21).
This study stops short of explaining why the brains of bees, chicks, and babies have converged in the same numerical order from left to right, but it does offer one possible answer: their asymmetric brains. All three have brains that process information differently on the left and right sides. “It could be an inherent property of these lateralized brain systems,” says Giurfa.
A shared system for organizing numbers, if it were really widespread, would highlight how strikingly similar the minds of animals can be to our own. Although some cognitive powers appear to be uniquely human, Giurfa believes there is danger in dismissing animal abilities. “We are different from animals in some ways,” she says, “but we are very similar in others. Denying this similarity is not what will help us understand what we are.