Some mammals, birds, fish, and insects possess a rudimentary sense of numbers. For example, in the face of intruders, lions decide whether or not to attack based on the roars of the intruders in comparison to those of their own group.
However, it is often unclear whether this number sense is actual counting, or is based on non-numerical cues (shape, sound, etc). Christian Agrillo (University of Padova, Italy) and coworkers have shown that mosquitofish can count, in the absence of extraneous non-numerical cues.
Mosquitofish.
Eastern mosquitofish (Gambusia affinis) are a small (less than 3 inches long) fish species. They are also short-lived, less than three years.
They can rapidly reproduce, by as much as a factor of 17 in five months. The scientists collected some mosquitofish from northern Italy for their experiments.
Training and testing the mosquitofish.
For training regarding numerical sense capabilities, a group of fish was taught to discriminate between two doors. This training consisted of associating the correct door with two or three shapes.
The scientists randomly selected the shapes in each experiment out of 100 of their designs (such as stars, circles, and rectangles). The motivation (reward) for the fish choosing the correct door was to rejoin their usual group after being placed in an unfamiliar tank.
Non-numerical properties of the shapes (luminescence, perimeter, surface area, and space occupied by the shapes) were either uncontrolled, controlled one at a time, or controlled all at once. The fish underwent six trails per day for up to ten days, and all successfully completed the training, except those who became ill over the course of training.
Once the training phase had been completed, the fish underwent the next stage, in which their capabilities were put to the test. Here, no reward (no rejoining of their normal group) was offered.
Non-numerical and counting abilities.
After training, when either no non-numerical property of the shapes, or their luminosity only, was controlled, the fish were able to nonrandomly choose the correct door (66% ± 7% of the time in the former, 62 ± 5% in the latter). In contrast, when the other non-numerical properties of the shapes were controlled one at a time, the choice was either statistically random or leaning towards the incorrect choice.
However, when all of the non-numerical properties of the shapes were controlled simultaneously, the fish were able to make the correct choice as frequently as when the numerical cues were not controlled. This strongly suggests that mosquitofish can explicitly count when necessary.
Mosquitofish may sometimes/often make use of non-numerical cues. However, they do not require such cues to make number-based decisions.
These successful experiments, in conjunction with extensive previous and ongoing tests with other critters, suggest that either the mental capacity to count (based on visual stimuli) originated long ago, or that many species have acquired the ability over the course of their evolution. These experiments may also inspire further tests to determine if counting capability is fundamentally built into the vertebrate cognitive center.
for more information:
Agrillo, C.; Dadda, M.; Serena, G.; Bisarra, A.
Use of number by fish.
PLoS ONE 2009, 14, e4786.