Freshwater stingrays (Potamotrygon motoro) and zebra mbuna cichlids (Pseudotropheus zebra) both demonstrated this ability independent of just symbol memorization (transfer test)
"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'," Bonn University zoologist Vera Schluessel and colleagues wrote in their paper.
Researchers presented Fishes with two gates displaying cards with different numbers of shapes. For example, they were trained that being shown a card with three blue squares meant the correct door with the reward behind it would be the one with four blue squares – they would have to add one. Or, if the card shapes were yellow, they would have to subtract one from the number of shapes to identify the correct door.
While not all individuals succeeded in learning the task (6 out of 8 cichlids and 3 out of 8 stingrays figured it out), those that did were remarkably adept at it. Stingrays chose the correct door 94 percent of the time for addition and were 89 percent accurate for subtraction.
Both species found addition easier to learn than subtraction – the fishes required fewer training sessions to get the gist of addition and displayed higher performance levels.
Cichlids were faster at understanding their task, and more of them were successful; the team notes this may be because these fish had participated in other cognition experiments before, whereas the stingrays had not.
So these two completely different groups of fishes seem to have the ability to perform basic arithmetic despite not having any known ecological or behavioral need for it, Schluessel and team point out.
Both species are opportunistic feeders – not hunters – and show no requirement for counting in their mating behaviors, nor do they seem to prefer particular-sized social groups.
But this ability could be critical for something we just haven't looked for yet, such as distinguishing individuals by recognition of physical traits.
"A more interesting question is as to why animals such as fish… are still commonly referred to as 'primitive' or 'lower' vertebrates," the team posed. "It seems obvious that fish, their cognitive skills and their status to be considered as sentient animals urgently needs to be revisited, specifically in light of the detrimental anthropogenic threats fish face every day."
These fishes join honeybees on the small list of animals with confirmed capabilities of adding and subtracting outside of birds and mammals. There are likely more such clever creatures that we have yet to test.
"Fish possess many of the same cognitive abilities and to a similar extent as birds and mammals," the researchers concluded.
While we've come a long way from believing animals are thoughtless biological machines driven solely by instinct, we've still only just started working out just how intelligent our fellow living beings really are.
After all, our unprecedented cognitive abilities didn't merely arise miraculously – they developed within species that preceded us from seeds planted in our ancient evolutionary history, shared by many other animals on our planet.
This research was published in Scientific Reports.
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