Large Number Discrimination by Mosquitofish

Abstract

BackgroundRecent studies have demonstrated that fish display rudimentary numerical abilities similar to those observed in mammals and birds. The mechanisms underlying the discrimination of small quantities (<4) were recently investigated while, to date, no study has examined the discrimination of large numerosities in fish.Methodology/Principal FindingsSubjects were trained to discriminate between two sets of small geometric figures using social reinforcement. In the first experiment mosquitofish were required to discriminate 4 from 8 objects with or without experimental control of the continuous variables that co-vary with number (area, space, density, total luminance). Results showed that fish can use the sole numerical information to compare quantities but that they preferentially use cumulative surface area as a proxy of the number when this information is available. A second experiment investigated the influence of the total number of elements to discriminate large quantities. Fish proved to be able to discriminate up to 100 vs. 200 objects, without showing any significant decrease in accuracy compared with the 4 vs. 8 discrimination. The third experiment investigated the influence of the ratio between the numerosities. Performance was found to decrease when decreasing the numerical distance. Fish were able to discriminate numbers when ratios were 1∶2 or 2∶3 but not when the ratio was 3∶4. The performance of a sample of undergraduate students, tested non-verbally using the same sets of stimuli, largely overlapped that of fish.Conclusions/SignificanceFish are able to use pure numerical information when discriminating between quantities larger than 4 units. As observed in human and non-human primates, the numerical system of fish appears to have virtually no upper limit while the numerical ratio has a clear effect on performance. These similarities further reinforce the view of a common origin of non-verbal numerical systems in all vertebrates.

Non-visual numerical discrimination in a blind cavefish (Phreatichthys andruzzii)

“…All subjects proved able to select the reinforced numerosity, representing the first evidence of non-visual numerical abilities in fish. In this experiment, half of the subjects were trained toward the larger number as positive, and half of the subjects were trained toward the smaller one as positive; no difference in accuracy was found as a function of the type of reinforce, as previously reported in other teleost fish Agrillo et al, 2010a;Agrillo et al, 2012a).…”

“…It would currently be challenging to understand why cavefish differ in numerical acuity from other fish species (Gómez-Laplaza and Gerlai, 2011b;Agrillo et al, 2009;Agrillo et al, 2010a;Agrillo et al, 2011). One may argue that the object representation using non-visual modalities might be less precise.…”

“…Spontaneous choice tests Piffer et al, 2012;Gómez-Laplaza and Gerlai, 2011b) and training procedures (Agrillo et al, 2010a;Agrillo et al, 2011) showed that fish are capable of processing both small and large numbers with a performance similar to that described in mammals and birds tested with similar paradigms (Barnard et al, 2013;Hunt et al, 2008;Revkin et al, 2008). In particular, guppies (Poecilia reticulata) and mosquitofish (Gambusia holbrooki) show the ability to discriminate between shoals differing by one up to four items (one versus two, two versus three, and three versus four); larger quantities can also be discriminated provided that the numerical ratio between the smaller and the larger quantity is at least 0.50 [i.e.…”

“…We firstly assessed (Experiment 1) whether cavefish show the ability to discriminate between two quantities by presenting a 0.33 ratio (two versus six objects), a ratio that is known to be easily discriminated by teleost fish in the visual modality (Agrillo et al, 2010a;Agrillo et al, 2012a;Gómez-Laplaza and Gerlai, 2012). At this stage, a control for continuous quantities was not included.…”

Numerosity representation is encoded in human subcortex

“…Mosquitofish can perform various numerical tasks including picking larger shoals (38), choosing more companions (39) or mates (40), or discriminating larger quantities (41), and guppies have been shown to differentiate ratios with levels of precision similar to that of undergraduate college students (42). Moreover, number processing is not restricted to vertebrates as both honeybees (43) and spiders (44) possess some ability to process number, as well.…”