Vision in Toads - Natural Toad Behavior

Natural Toad Behavior

The common toad responds to a moving insect or worm with a series of prey-catching reactions: (1) orienting towards prey, (2) stalking up to prey, (3) binocular fixation, (4) snapping, (5) swallowing and (6) mouth-wiping with forelimb (Ewert 1974). This series of movement constitutes a stimulus-response chain, where each reaction of the toad so-to-speak provides the stimulus constellation for the next response. First, if an object is recognized as prey and thus catches the toad’s attention, the toad will orient towards the stimulus by turning its body to face it. Then it approaches the prey, binocularly focusing intently on it. During the attack, it snaps at the object with its tongue or jaws and swallows it. Finally, it wipes its mouth with a forelimb. These actions constitute a series of well-defined behavioral patterns.

One reason for this type of stimulus-response chain is that, unlike humans, toads do not have involuntary saccadic eye movements and they also cannot perform “tracking eye movements” (Ewert 1980). They must, therefore, depend on recognizing the stimulus before they respond. As a result they have developed a specific detection system that, for example, allows them to discriminate between edible prey and dangerous predators.

The lack of saccadic eye movements forces the toad to hold its eyes in rigid positions. Therefore, it must decide whether the object is “prey” or “non-prey” before moving itself. If it orients towards an object, it must already have decided “prey” and then commits itself to snapping by reducing the thresholds for subsequent prey-catching responses. Even when the prey stimulus quickly disappeared after orienting, the aroused toad may sometimes complete the subsequent responses.