Abstract
The ability to discriminate between sets that differ in the number of elements can be useful in different contexts and may have survival and fitness consequences. As such, numerical/quantity discrimination has been demonstrated in a diversity of animal species. In the laboratory, this ability has been analyzed, for example, using binary choice tests. Furthermore, when the different number of items first presented to the subjects are subsequently obscured, i.e., are not visible at the moment of making a choice, the task requires memory for the size of the sets. In previous work, angelfish (Pterophyllum scalare) have been found to be able to discriminate shoals differing in the number of shoal members both in the small (less than 4) and the large (4 or more) number range, and they were able to perform well even when a short memory retention interval (2-15s) was imposed. In the current study, we increased the retention interval to 30s during which the shoals to choose between were obscured, and investigated whether angelfish could show preference for the larger shoal they saw before this interval. Subjects were faced with a discrimination between numerically small shoals (≤4 fish) and also between numerically large (≥4 fish) shoals of conspecifics. We found angelfish not to be able to remember the location of larger versus smaller shoals in the small number range, but to exhibit significant memory for the larger shoal in the large number range as long as the ratio between these shoals was at least 2:1. These results, together with prior findings, suggest the existence of two separate quantity estimation systems, the object file system for small number of items that does not work with the longer retention interval and the analogue magnitude system for larger number of items that does.
Highlights
A diversity of nonhuman animals, as well as humans, possess nonverbal ability to discriminate between different quantities of items
We investigated whether angelfish could exhibit a preference toward the location at which they previously saw the larger of two small shoals
Angelfish were required to remember previously seen shoals differing in the number of shoal members, a quantity discrimination task
Summary
A diversity of nonhuman animals, as well as humans, possess nonverbal ability to discriminate between different quantities of items (reviewed in Feigenson et al 2004) This ability has evolutionary advantages since it allows individuals to distinguish places with more food (LuconXiccato et al 2015), the size of the rival groups (BensonAmram et al 2011), the number of mates (Lemaıtre et al 2011), the number of shoaling members for protection (Agrillo et al 2007), the brood size for parental investment (Forsaktar et al 2016), the more vulnerable group of prey for hunting (Panteleeva et al 2013) or the larger social group more ideal for protection in potentially dangerous environments (Hager and Helfman 1991). The AMS follows Weber’s law: the larger the ratio of the larger set to the smaller set (the larger the relative difference between the compared sets), the more accurate and reliable discrimination becomes
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