Fish can use numerical information when discriminating between small discrete quantities

Fish can use numerical information when discriminating between small discrete quantities Christian Agrillo (christian.agrillo@unipd.it) Laura Piffer (laura.piffer@unipd.it) Department of General Psychology, Via Venezia 8 35131 Padova (Italy) Department of General Psychology, Via Venezia 8 35131 Padova (Italy) Marco Dadda (marco.dadda@unipd.it) Angelo Bisazza (angelo.bisazza@unipd.it) Department of General Psychology, Via Venezia 8 35131 Padova (Italy) Department of General Psychology, Via Venezia 8 35131 Padova (Italy) Giovanna Serena (giovanna.serena@unipd.it) Department of General Psychology, Via Venezia 8 35131 Padova (Italy) Abstract Research on human infants and non-human animals has demonstrated that rudimentary numerical abilities pre-date the evolution of human language. Nonetheless animals can base their quantity judgments on continuous perceptual cues that correlate with number and it is unclear whether animals other than mammals and birds can use number. In the present work we adopted two different approaches to investigate whether basal vertebrates, such as fish, can discriminate between small quantities by using numerical information only. In experiment 1 we investigated the non numerical perceptual cues preferentially used by fish to discriminate between two and three elements; in experiment 2 fish were then trained to distinguish between the two quantities when continuous variables were controlled for. Lastly (experiment 3) we observed the spontaneous choice between a group of two and a group of three social companions using a method similar to ‘item by item’ presentation of the stimuli to prevent the access to non-numerical information. On the whole, our results suggest that fish can represent and use numerical information for discriminating small quantities. Keywords: Numerical competence; Continuous variables; Comparative cognition. Introduction The ability to count objects or discriminate among sets containing different number of items can affect survival and reproduction in many natural contexts. Laboratory studies have shown that mammals (Beran, 2008; Kilian et al., 2003; West & Young, 2002), birds (Brannon et al., 2001) amphibians (Uller et al., 2003) and fishes (Agrillo et al., 2007) are able to discriminate between two quantities of objects. However, since stimulus numerousness co-varies with non- numerical extent, such as the total area occupied by objects, the sum of their contour, their density and luminance, organisms can provide quantity judgments without necessarily being capable of abstract numerical representation (Feigenson et al., 2002b; Pisa & Agrillo, In a recent study (Agrillo et al., 2008) we systematically investigated the ability to discriminate between two quantities in a small freshwater fish, the mosquitofish (Gambusia holbrooki). As many other fish, mosquitofish placed in an unknown, potentially dangerous environment, select the larger of two simultaneously presented shoal, probably because larger groups offer better protection from predators (Agrillo & Dadda, 2007; Hager & Helfman, 1991). In choice tests mosquitofish were found to discriminate between two shoals that differed by one element when the paired numbers were 1vs2, 2vs3 and 3vs4, but not 4vs5 or larger group sizes, a limit that coincides with that observed in monkeys and apes when using a spontaneous preference paradigm. Using larger shoals as stimulus, a discrimination was still possible provided the number ratio was 1:2 or smaller (e.g. 4vs8, or 8vs16 fish but not 4vs6 or 8vs12 fish). Within the large number range (> 4 elements), preference for the large shoal significantly increased with decreasing numerosity ratio in accordance with Weber’s Law while such relationship was not observed within the small number range, suggesting the possible existence in fish of two numerical systems as evidenced in primates (Flombaum et al., 2005; Hauser et al., 2000) and human infants (Feigenson et al., 2002a). However we found that, when we controlled for continuous variables (the cumulative area occupied by fish and the overall quantity of movements of the fish within a shoal), fish failed to discriminate, suggesting that they attend more to non- numerical extent than to numerosity itself. The fact that mosquitofish spontaneously compare different quantities by using non numerical perceptual cues that correlate with number does not necessarily imply that fish are unable to discriminate two groups on the basis of the numerosity of the items alone. Overall perceptual cues may simply be the easiest indicators of numerosity in this task, and indeed both adult humans and non-human mammals, in many circumstances, base number estimation primarily on