The image above shows the Ebbinghaus illusion. To our eyes, the red circle on the left looks bigger than the one on the right. In fact they're the same size, but the surrounding circles deceive us. Even when we know it's a trick, we can't help but see the illusion.

Now it's clear that fish see it too.

Researchers at the University of Trento in Italy trained eight mature redtail splitfins to respond to two circles of different sizes. Some were conditioned to prefer a small disc, others a large one. Their reward was an area of their water tank which had better food.

Then the team, led by Valeria Anna Sovrano, presented the fish with an Ebbinghaus illusion. Those that had been conditioned to prefer the small orange circle associated with the one which looked smaller, while those trained to prefer large circles associated with the one that looked bigger. The results have been published in the journal Animal Cognition.

 

It was once believed that the neural processes underlying the Ebbinghaus illusion had only evolved in primates, says co-author Orsola Rosa Salva. She argues that the processes are probably far more common. Over the last few years, studies have shown that birds, dolphins, monkeys, apes and even bees can see a range of illusions.

That's because all animals face the same challenges when trying to perceive the world, such as recognising partially obscured objects or objects that are only partly illuminated. "All animals are affected by these tasks so animals have clearly evolved similar mechanisms," says Salvo.

For example, we instinctively know that a friend who is far away is the same height as when they are standing close by – even though the image they form on our retinas has changed size dramatically. (Admittedly, Father Ted's Dougal never did grasp this).

Understanding this can be a matter of life and death. A large predator in the distance might appear small and therefore harmless, but it is not, and animals benefit from this knowledge.

"What might be happening is what's known as evolutionary convergence, where different visual systems end up using the same perceptual principles because that's what works well," says Olga Lazareva of Drake University in Des Moines, Iowa.

Alternatively, it could be that the process worked well in an early animal and so has been preserved in all the species descended from it, says Laura Kelley of the University of Cambridge in the UK. If that's true, many of the visual illusions we see were first seen by our fishy ancestors.

There is some evidence for that. In September 2014 it emerged that zebrafish can see an even more complex illusion, the "rotating snakes" below.

The patterns seem to be moving, but in reality are completely stationery. Scientists don't quite understand how it works, but evidently the picture confuses our brains' ability to detect motion. The researchers say that motion detection must have appeared early in the evolution of fish, and has been conserved in their descendants, including mammals.

The fact that humans and animals see these illusions is actually a good thing. It indicates that our brains have evolved shortcuts to help us interpret what we see – as opposed to taking in every single line, orientation and colour, which would take far too long.

Illusions are created when these neural rules-of-thumb don't work. As a result, they help us understand how our brains organise the world.

That said, some animals have found ways to harness others' flawed perceptions. For instance, female fiddler crabs prefer to mate with large males. So decent-sized males often position themselves next to small neighbours, in order to look big by comparison – a kind of real-life Ebbinghaus effect.

Bowerbirds can even manipulate each other's sense of perspective. Males build bowers out of twigs to attract females, and decorate them with colourful objects. Males carefully place these objects so that they appear bigger to females looking from a particular angle. This seems to get them more matings.

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