Ontogeny of the capacity to compare discrete quantities in fish.

Numerical abilities of adult fish equal or even exceed those of many birds and mammals. The ability to estimate shoal size was previously found to be inborn in guppies, although it improves with age and experience. To study the influence of stimulus variables, here 4- to 9-day-old fish were trained to discriminate between groups of two-dimensional geometric figures when they could use both numbers and cues that co-varied with numerosity (e.g., area) or numerical information only. Subjects rapidly learned to discriminate between quantities under the first condition, but failed to discriminate in the number only condition even with very easy discriminations (1 vs. 4 items). Newborns also failed when stimuli differed in area but not in number, suggesting that, unlike adult fish, newborns can solve this quantity task only if they can use multiple cues. Comparison with shoal discrimination experiments indicates that newborns' capacity to use number is specific to social context. Differences in developmental trajectories suggest the presence in fish of multiple quantification mechanisms which are domain-specific and serve to solve a limited set of problems.