Discrimination of the larger shoal in the poeciliid fish Girardinus falcatus.

Two decades of comparative studies have shown that numerical abilities are widespread in the animal kingdom. In particular, the ability to select the larger of two quantities (known as the ability to go for "more") has been found in apes, dolphins, birds and amphibians. This primary quantity discrimination has not been investigated in fish in a systematic way although several studies suggest that fish have the ability to distinguish among different-sized groups of conspecifics. We investigated the spontaneous choice for the larger of two shoals in female goldbelly topminnows (Girardinus falcatus) and compared two different methods to determine an efficient procedure for studying the numerical competence of fish. Female topminnows that were placed in a novel and potentially dangerous environment were found to choose the larger shoal suggesting that the capacity to go for "more" was similar to that observed in other non-verbal animals. The two methods proved not to be sensitive in revealing differences between the numerical contrasts, however, combining the results of the two experiments, we found that when groups differed in size by one individual, choice of the larger shoal was significantly above that predicted by chance up to a maximum of three fish in the larger shoal (i.e. 2 vs 1 and 3 vs 2 but not 4 vs 3 or larger). When presented with two shoals with a 1:2 numerical ratio, choice was significantly above chance in 4 vs 2 but not in 8 vs 4 comparisons.