Man is not the only animal to perceive and in turn create optical illusions. Other species also warn them, and exploit them to their advantage: but the type of illusion and the accuracy of deception depend on the visual system and on the cerebral complexity of the animal itself. It cannot be generalized, and studies in this field are still in its infancy.

Strategic goals. In general, animals exploit optical illusions for practical survival tasks, such as disorienting predators or conquering partners. For the first purpose all the imaginative examples of animal mimicry apply, but not only: some spiders, for example, set up on the canvas the optical illusion of a spider made of twigs and dry leaves: the artifact of a giant spider, which make sway to ward off enemies.

the ultimate weapon. But it is in courtship that the tricks are refined. The males of bird gardener, a relative of crows widespread in Australia and New Guinea, use a perspective trick to make their most beautiful nests appear in the eyes of their partners.

Image The paved driveway set up by the males of a great gardener (Ptilonorhynchus nuchalis). | Science / AAAS; (inset) JJ Harrison

Let's see what you can do. Before choosing a mate, the females review the nests set up by the suitors to test their architectural skills.

The males of the great gardener (Ptilonorhynchus nuchalis) prepare a path similar to a pergola, made of dry twigs, which ends in a small "garden" embellished with stones, shells and bones.

nothing random. To deceive the eye of the females, used to a three-dimensional vision, the birds place the smaller decorative elements at the beginning of the paved open space, and the larger ones gradually further away. So when the partner admires the work, she seems to see a path covered with stones of uniform and uniform size.

Order wanted. Some researchers have tried to reverse the order of the elements; immediately the gardeners put them back in place, confirming that it is a carefully studied provision: it is the illusion of forced perspective, the same that we use when we pretend, in photos, to support the Tower of Pisa, or to hold a pyramid between the fingers.

Image The Ebbinghaus illusion: which of the two circles is bigger? | Wikimedia Commons

Win "easy". To impress the partner, some violinist crabs (Uca mjoebergi) seem instead to exploit another optical illusion, that of Ebbinghaus, for which the same element seems larger or smaller depending on what surrounds it (see figure).

The females of this crustacean are attracted to partners with larger claws. So to conquer them, the males come close to rivals with the smaller claws of their own, so as to appear more attractive.