By Emma Young

It’s been known for centuries that we experience all kinds of optical illusions, and in the past few decades, researchers have shown that some animals, including monkeys, pigeons, and dogs, do too. Now the first ever study of this kind in reptiles has found that even the bearded dragon falls for an optical illusion that we humans succumb to.

Perceptual illusions — subjective interpretations of physical information — are interesting to psychologists because they reveal important insights into how we construct our representations of the world. This new work, published in the Journal of Comparative Psychology, provides evidence that at least one reptile can be counted among the animals don’t simply passively process retinal signals, but actively interpret visual data, too.

Maria Santacà, at the University of Padova and colleagues used the Delboeuf illusion in their new study. Look at the image below: both black circles are identical. But most people will under-estimate the size of the one with a bigger white background and over-estimate the size of the other, leading them to report that former looks smaller.

Recent studies of capuchin monkeys showed that they make the same mistake. And in tests in which the black circles were replaced with identical portions of food, so too did chimpanzees: when given a choice, they generally opted for the circular food portion with less space around it. (In fact, people also over-estimate the size of food portions presented on small plates.)

Reptiles, as the authors write, “were long considered to be sluggish and unintelligent; however, when tested under appropriate experimental conditions, they exhibit an impressive array of cognitive abilities.” For example, recent work has shown that both bearded dragons and red-footed tortoises can perceive similarities between 2D pictures and the objects that they represent.

For the new study, Santacà and her colleagues tested a total of 12 bearded dragons and eight red-footed tortoises, all housed at the University of Lincoln. In place of black circles, they used circles of jelly food beloved by the two species: mango jelly for the tortoises and a kale, cucumber and mint jelly for the bearded dragons. The researchers used two different sizes of white circle, in place of plates. One bigger “plate” was 4.92 cm across, the other 1.82cm.

First they tested whether, when given a choice between a bigger jelly circle and a smaller one, the animals would actually go for the larger one. The bearded dragons did; the tortoises were a lot more inconsistent. Then they repeatedly presented the animals with a 1.5 cm diameter circle of jelly centred on a big or a small circle. The bearded dragons consistently went for the jelly placed on the smaller “plate”. This certainly implies that they mistakenly perceived this jelly to be bigger than the other.

The tortoises, on the other hand, didn’t show any preference for the smaller plate. But in the first stage of testing, they’d been just as likely to make a beeline for the smaller food portion as the larger one, so it’s impossible to draw any conclusions as to whether or not they might perceive the illusion too, the researchers write. (Why would the tortoises not go for the bigger portion? Perhaps because both portion sizes were already quite big, the authors suggest, also noting that none of the animals were food-deprived during the study.)

However, this study does provide the first evidence that a reptile species perceives a visual illusion. “This indicates that, like some mammals, birds and fish, some reptiles can interpret and alter visual input related to object size,” the researchers conclude.

In a commentary on the work, Todd Freeberg at the University of Tennessee writes: “This exciting comparative research raises the possibility of visual perceptual mechanisms that may be fairly widespread in animals.”

– Can reptiles perceive visual illusions? Delboeuf illusion in red-footed tortoise (Chelonoidis carbonaria) and bearded dragon (Pogona vitticeps)

Emma Young (@EmmaELYoung) is a staff writer at BPS Research Digest