Animals, like humans, can fall victim to the Mona Lisa effect

The Mona Lisa effect is a controversial phenomenon in the scientific community. Researchers from Bielefeld University in Germany have discovered that while this optical effect is real, the woman in the painting who lent her name to it does not, in fact, follow her admirers with her eyes.

The reason? This phenomenon generally occurs when the subject of an image is looking straight ahead out of the picture, at an angle between 0 and 5 degrees, they explain in their research, published in 2019 in the i-Perception journal. Beyond 5 degrees, the viewer will no longer have the impression of being followed by the gaze. However, researchers have shown that the enigmatic gaze of Leonardo da Vinci's Mona Lisa is oriented to the right of the observer, with an average angle of 15.4°. Well beyond, therefore, the limit of the famous Mona Lisa effect.

In any case, this optical phenomenon seems to be particularly widespread in the animal kingdom. It even seems to play an important role in keeping potential predators away, as John Skelhorn and Hannah M. Rowland state in a study recently published in Frontiers in Ecology and Evolution.

Indeed, many species of fish, butterflies, beetles and praying mantises have circular markings on their bodies that look a lot like eyes. Scientists even call them "eyespots."

Keeping an eye on predators

These markings have many advantages: they divert the attention of predators to non-vital parts of the animal's body, while also intimidating predators, and even completely dissuading them from attempting an attack. In 2016, this phenomenon even led Australian researcher Neil Jordan to paint eyes on the hindquarters of cows in Botswana, in order to scare away lions that might hope to attack them.

However, some questions remain: do predators confuse the eyespots with the eyes of their prey? Or are they just frightened by any brightly colored circular pattern? John Skelhorn and Hannah M. Rowland attempted to answer these questions by conducting a series of experiments with a hundred newly hatched chicks.

The researchers taught them to attack three types of artificial moths -- the first with ocelli (a round spot with a center that's a different color to the circumference) whose middle circles looked to the left, one with “eyes” that looked to the right, and one with perfectly concentric circles that recreate the Mona Lisa effect.

They also built three small walkways so that the chicks could approach their prey either in a straight line or from the left or right. The chicks were then split into three groups, each using a different walkway. They had to attack the same artificial prey while the scientists timed them.

The results speak for themselves. The chicks approached cautiously from the left and the right, when the artificial moths' eyes appeared to be looking in those directions. This phenomenon was even more pronounced when the chicks had to attack moths whose eyespots recreated the Mona Lisa effect.