Stingrays and zebra mbuna fish can perform simple addition and subtraction involving numbers between 1 and 5.
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"The numerical understanding of cichlids and stingrays was examined regarding addition and subtraction abilities within the number space of one to five. Experiments were conducted as two-alternative forced-choice experiments, using a delayed matching to sample technique", V. Schluessel, N. Kreuter, I. M. Gosemann and E. Schmidt wrote.
"On each trial, fish had to perform either an addition or subtraction, based on the presentation of two-dimensional objects in two distinct colors, with the color signaling a particular arithmetic process.
"Six cichlids and four stingrays successfully completed training and recognized specific colors as symbols for addition and subtraction. Cichlids needed more sessions than stingrays to reach the learning criterion.
"Transfer tests showed that learning was independent of straightforward symbol memorization. Individuals did not just learn to pick the highest or lowest number presented based on the respective color; instead, learning was specific to adding or subtracting ‘one’.
"Although group results were significant for both species in all tests, individual results varied. Addition was learned more easily than subtraction by both species. While cichlids learned faster than stingrays, and more cichlids than stingrays learned the task, individual performance of stingrays exceeded that of cichlids."
Vera Schluessel at the University of Bonn in Germany and her colleagues designed tests to work out whether ocellate river stingrays (Potamotrygon motoro) and zebra mbuna (Pseudotropheus zebra) can add or subtract the numerical value of “one†from numbers between 1 and 5.
The team first trained six zebra mbunas and four stingrays to make choices after seeing an initial stimulus.
The fish started in a section where they were shown an initial picture of two shapes for 5 seconds.
After this period, they could swim through an open door into a test area where there were two more pictures of shapes: one showing a single shape and another showing three shapes.
If the two shapes in the initial picture were yellow, the fish were rewarded in the test area if they swam towards the image showing just one shape – a subtraction operation. If the two shapes in the initial image were blue, the fish were rewarded in the test area for swimming towards the image showing three shapes – an addition operation.
Once the fish had been trained to associate yellow with subtraction and blue with addition, the team tested their ability to count. In the first test, the animals were presented with an initial image of three shapes in either yellow or blue.
Then, in the test area, they could choose from either two shapes or four shapes.
The zebra mbunas correctly chose addition (the four-shape option) 82 per cent of the time when they saw a blue stimulus, and successfully chose subtraction (the two-shape option) 68 per cent of the time following a yellow stimulus. Meanwhile, the stingrays successfully added 96 per cent of the time and correctly subtracted 90 per cent of the time.
The zebra mbunas correctly chose to add one rather than two 70 per cent of the time and subtracted exactly one 66 per cent of the time. The stingrays successfully chose to add one rather than two 90 per cent of the time and subtracted exactly one 87 per cent of the time.
In all tests, the team controlled for the total area of the shapes in each picture, because some people think the fish may simply process the combined surface area of objects rather than truly counting in the sense that we understand it. ■