Animals fall for optical illusions too - just not the way humans do

Illusions mess with our brains all the time – not in a harmful way, but creatively. What we think we’re seeing often isn’t what’s actually there.

Take the Ebbinghaus illusion, for example: you look at two identical circles, but one looks bigger. Why? Because it’s surrounded by smaller circles. The other sits next to bigger ones, so it seems smaller. Same size, different context – totally different experience.

That illusion isn’t just a trick for a psychology textbook. It shows something deeper: our brains don’t just receive information from the world – they interpret it. And that got some researchers wondering.

Do humans see illusions this way because of how our brains are wired… or could other animals be fooled the same way?

Shortcuts in the mind’s eye

Our brains take shortcuts to keep up with the world. We don’t process every detail – we zoom out and look at the big picture.

Psychologists call this “global processing.” It’s fast, efficient, and mostly reliable. But it also means we’re open to being tricked by illusions.

Not every animal needs this kind of processing. Some need precision. Others need speed. And depending on the environment they live in, their brains may evolve differently to handle what matters most to their survival.

When instincts meet illusions

To explore this, scientists compared two animals that couldn’t be more different: guppies and ring doves.

Guppies live in wild, fast-moving freshwater streams. Their world is full of shifting light, dense plants, and lurking predators. To survive, they have to make fast calls – spot a threat, find a mate, or stick with a group.

Ring doves, on the other hand, live on land and spend much of their time pecking at tiny seeds on the ground. That means spotting small objects precisely is more important than making quick judgments based on the bigger picture.

The team running the experiment came from a collaboration of researchers in animal cognition and evolutionary biology. They set up visual tasks to test if these animals fall for the Ebbinghaus illusion.

The animal illusion experiment

For guppies, the setup involved fish food flakes surrounded by either smaller or larger circles.Doves pecked at millet seeds arranged the same way.

The question was simple: would the animals treat identical food pieces differently depending on what surrounded them?

The results weren’t subtle. Guppies went for the food surrounded by smaller circles more often. Their perception closely tracked what’s seen in people. The doves, though, didn’t play along.

“Guppies consistently fell for the illusion. When food was surrounded by smaller circles, the guppies chose it more often, as if it really was larger,” the researchers wrote.

“At the group level, [doves] showed no clear susceptibility to the illusion. Some individuals behaved as humans did, others in the opposite way, and many seemed unaffected altogether.”

Perception built for survival

The goal of perception isn’t truth – it’s function. For guppies, it pays to read the big picture fast. Being able to judge relative size at a glance might help them stick with a safer group or avoid a predator.

For doves, precision matters more. Picking out a seed from the dirt requires tuning in to the details and ignoring background clutter. So it makes sense they wouldn’t be as affected by surrounding shapes.

The doves’ mixed responses are also important. Just like humans, not all doves see the world the same way. Some get tricked. Some don’t. That variation gives scientists clues about how both biology and experience shape perception.

Why illusions matter

This research doesn’t just explain a cute fish trick. It gets at a bigger question: how do brains – human or not – make sense of the world?

“Perception is not about accuracy for its own sake, it is about what works in a given environment,” researchers wrote.

By looking at different animals, scientists can trace which parts of perception are shared across species and which are custom-built for a particular lifestyle. This helps them understand how brains evolved – not just to see the world, but to handle it in ways that help a species survive.

The full study was published in the journal Frontiers in Psychology.

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