Can Fish Count?

Brian Butterworth



Nature: Books in Brief. Andrew Robinson: Can Fish Count?
(6th April 2022).

Natural History Vol. 130, Issue 5 by Laurence A. Marschall
Can Fish Count? by Brian Butterworth

They can, and so can a host of other species, according to neuropsychologist Brian Butterworth, who recounts the numerous ways that biologists have demonstrated the near-ubiquity of mathematical talent in the animal kingdom. That evolution selects for numerical ability is not that surprising: if three foxes go into a bush and only two come out, a wise bird, seeking a place to alight, needs to be able to do the math if it wants to survive to reproductive age. Neuroscientists have identified localized areas in the brains of humans, apes, and monkeys that deal with calculations of this sort, and they suspect that equivalent structures exist in other vertebrates, such as birds, and in such tiny invertebrates as bees and ants.

Since non-humans don't use words to count, testing their mathematical abilities requires considerable ingenuity. Otto Koehler (1899-1974), a pioneer in studies of avian numerosity, presented cards marked with a certain number of dots to jackdaws (Corvus monedula), and then trained them to receive a treat only if they opened the lid of a box with the same number of dots. To check whether they were actually counting, he devised a second test in which the birds didn't get rewarded until they retrieved a specific total number of tokens distributed among several boxes, some containing one, two, or none. The jackdaws performed splendidly, accumulating numbers as high as seven. In a similar experiment performed by Butterworth's associates, guppies (Poecilia reticulata) were trained to open doors marked with various numbers of marks - but only rewarded with food if they chose the door with four. Despite their diminutive size compared to jackdaws, they quickly learned the trick.

So animals can be trained, but do they actively use these talents in the wild? A variety of investigations indicate that they do. Lions in the Serengeti make decisions on whether to fight or flee based on comparisons between the number of hostile invaders and the number of friendly defenders in their pride. Male Tungara frogs (Engystomops pustulosus) in Central and South America signal their superior fitness by counting the croaks of their competitors and adding an extra one to their own mating calls. Honey bees, who communicate the routes to food sources in a well-known waggle dance, estimate distance by counting landmarks. And ants, in an extraordinary experiment, have demonstrated the ability to measure the path to food by counting steps--which can number in the hundreds.

Though many species can do math, some individual members of a species cannot. In studies of guppies, for instance, there are fish that consistently fail tests that others easily pass. The origin seems to be a genetic inability to perceive numerosity, a "dyscalculia" similar to the difficulty people with dyslexia have in making sense of written language. Readers who have difficulty with numbers, estimated to be about 4-5 percent of the population, may take some consolation in this. But all readers, whatever their level of numeracy, should marvel at how the universe manifests its mathematical substructure to so many earthly creatures.

(June, 2022)