# Lizards Succumb To Optical Illusions Too



## David Baxter PhD (Dec 9, 2019)

*Here’s The First Evidence That Even Lizards Succumb To Optical Illusions* 
By Emma Young, _BPS Research Digest_
Dec 9, 2019







 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.






The Delboeuf illusion, via Santacà et al (2019) 

 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.”

*Source*

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


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