Can all fish count to four?

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Mosquitofish have been shown to count as well as college students, but how do angelfish (Pterophyllum scalare) stack up?

The counting abilities of the angelfish were put to the test in a study by Luis Gómez-Laplaza and Robert Gerlai that has been published in the most recent issue of the journal Animal Cognition. 

The authors tested the abilities of angelfish to discriminate between larger and smaller quantities by letting juvenile angelfish choose from shoals of two differing sizes that were presented to the fish (young angelfish display strong shoaling tendencies). 

This was done by placing the test fish in a tank flanked by two other tanks on each side. Shoals of differing sizes were presented to the test fish in one set of experiments and their preferences for joining one or the other shoal were recorded. 

The authors found that the angelfish were able to discriminate between shoals of differing sizes and preferred to join the larger shoal as long as the ratios between the two quantities provided were 1.8:1 (nine vs. five) or higher.  Once the ratio dropped below this threshold, the fish were no longer able to discriminate between the two quantities and showed no marked preference for either of the two shoals presented.

Subsequent experiments by the authors (which await publication) have indicated that angelfish might even be able be able to distinguish between quantities in a 1.5:1 (nine vs. six; six vs. four) ratio, but only if the quantities provided are very small.  For instance, the results of the authors’ experiments indicated that angelfish can distinguish between two and three, but not if the quantities presented were greater than four. 

According to Gerlai, "This ability does resemble 'counting' individual items as opposed to estimating quantities, but this counting ability does not extend beyond three."

This inability to count to four might reflect the fact that it has never been particularly advantageous for the angelfish to do so, and does not necessarily mean that angelfishes are unable to count higher numbers, added Gerlai.

"There is always a possibility that it is not their ability that is limited, but that there is no evolutionary benefit for that particular individual to distinguish between two shoals that differ after a certain size.  It doesn’t reflect what the brain could do but what the brain is set up to do through evolution."

For more information, see the paper: Gómez-Laplaza, LM and R Gerlai (2011) Can angelfish (Pterophyllum scalare) count? Discrimination between different shoal sizes follows Weber’s law. Animal Cognition 14, pp. 1–9.