Visual Illusions in Frogs and Toads

Abstract

There are situations where frogs and toads radically misjudge the size of a moving object, taking an enemy for prey or vice versa. In the present work, the conditions where this occurs were investigated. Frogs ( Rana temporaria) were shown black balls of diameters 0.25 – 20 cm moving at different distances and velocities, either in the presence or absence of a structured background. When the targets moved at 10 – 15 cm s−1 (at a distance where this corresponded to ∼10 deg s−1) in the presence of the background, frogs turned, approached and tried to catch those with small diameters (0.25 – 2 cm) only. Larger targets mainly evoked escape reactions. Without the background, balls of any real size could be perceived as prey: all targets of angular size about 5 deg or less triggered a ‘food’ turning reaction, implying that the critical distance for this reaction depends linearly on object size. ‘Food’ turning towards big targets (10 or 20 cm) indicates incorrect distance estimation. Moreover, if a big target was moved at high speed (100 deg s−1), the frog not only turned but also flicked its tongue at it without approaching it, even when the distance was up to 1 m. Observations on the toad Bufo bufo were qualitatively similar. Thus it is possible to create an illusion of closeness in frogs and toads by increasing the speed of the moving object. Conversely, it is possible to create an illusion of distance by moving a small object at slow speed close to the animal. The 0.25 cm target moving at a 12 cm distance with a velocity of 0.15 cm s−1 (0.7 deg s−1) in the absence of the structured background evoked the ‘food’ reaction on only 4% of the trials and escape reactions on 80% of the trials. When the background was present, the same stimulus evoked ‘food’ turning only. The escape reaction to a small object can be explained by a drastic overestimation of its distance in the absence of other cues.