Fish can estimate

Humans have at least two distinct ways of evaluating the numbers of objects.  For very small numbers (usually no more than four or five) we can immediately tell how many items there are without counting.  For larger numbers, we can compare relative amounts (a group of twenty is larger than a group of eight). You may wonder if these abilities are among the vanishingly few traits that set us apart from the rest of the animal kingdom.  If so, you would be wrong.  Many other creatures can also compare and contrast numbers, including other primates, dogs, and even guppies.  Yes, fish can estimate numbers.

The two numerical representation systems are referred to as the ‘object-file system’ and ‘analog system’.  Because of the object-file system, we can distinguish between two and three objects with the exact same speed and ease as we can between one and two objects.  The ratio of the objects makes no difference.  However, with larger numbers, the analog system kicks in, and the ratio of objects becomes increasingly critical.  We can tell fifteen from fifty objects at a glance, but not fifteen from eighteen.  Thus, non-human organisms that have both of these systems should also display ratio dependent abilities to discriminate between groups of large numbers but not small numbers.

Christian Agrillo, Laura Piffer and Angelo Bisazza from the University of Padova and Brian Butterworth from University College London compared the numerical acumen of undergraduate students and guppies (insert obligatory joke about both groups being in schools).  The students were asked to compare the numbers of dots flashed on a computer screen, whereas the guppies (an extremely gregarious species) were presented with the choice to associate with a larger or smaller shoal of fellow fish.

Both groups could tell two from three as easily as three from four.  Both groups also distinguished large groups that differed by 50% (four versus eight) more easily than groups that differed by only 25% (six versus eight).  In other words, both guppies and students displayed evidence of having both the object file system and the analog system of estimating numbers.  


There are two possible explanations for this.  Either number sorting ability goes very far back in our evolutionary past, or it has evolved independently multiple times.  It would be interesting to see whether solitary animals also have these abilities.