Yeah, one fish, two fish, but can fish actually count?

Studying zebrafish could help us unravel the neural circuitry underlying numerical skills

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Zebrafish

Photo by Lynn Ketchum, courtesy of Oregon State University via Flickr

Understanding and using quantitative information is important for animals to compete for resources, protect themselves from predators, get access to territories and for reproductive benefits. Certain species also exhibit an awareness of numerosity, or being able to differentiate between groups of varying numbers, at fine scales. While humans rely on a symbolic sense of numerosity (we have symbols and names for counts), animals depend on non-symbolic representations of numbers. Although they use this information for their survival, does this mean they are really counting? 

Researchers from the lab of Giorgio Vallortigara have shown that zebrafish do in fact use ordinal counting in order to get to a “reward." Ordinal counting keeps track of a position (as in, the second door out of seven), as opposed to cardinal counting, which tracks an amount ("there are seven doors"). Zebrafish were trained to exit an arena with seven identical exits in a corridor, using only the second exit. This was done by blocking the other exits with a clear sheet of plastic. On the other side of the corridor was a reward of food and other fish. If the fish swam towards other exits, they did not receive any reward. But what happens when all doors are up for ‘swims’? 

Post training, the fish were tested by decreasing the length of the corridor, decreasing the distance between exits, or increasing the number of exits. In the first two experiments, they chose the second exit with a frequency greater than random chance, indicating that they were in fact “counting” to choose which exit to use.

In the last experiment, along with choosing the second exit significantly more than the others, they also chose the second-to-last exit (second from the other end of the corridor) more than chance level. The higher numbers seemed to "confuse" the fish. In such contexts, the fish switched to using spatial feedback, along with ordinal information, to increase their chances of getting lunch with their pals.

Developing zebrafish brain

Confocal micrograph of the developing zebrafish brain

Dr. Steve Wilson. CC BY

Although evidence of numerical skills has been reported in birds, amphibians, mammals and even insects, studying it in a well-established model system like zebrafish opens up avenues to decode the mechanisms underlying this behaviour.