Vision in Toads - Prey Vs. Predator Response

Prey Vs. Predator Response

When a toad is presented with a moving stimulus, it generally may react with one of two responses. Depending on the size and the configuration of the stimulus, it will either engage in orienting (prey-catching) behavior or avoidance (escape) behavior, which consists of “planting-down” defensive postures or a crouching avoidance response.

In determining the size of a stimulus, a toad will consider both the angular size, which is measured in degrees of visual angle, and the absolute size, which takes into consideration the distance between the toad and the object. This second ability, to judge absolute size by estimating distance, is known as size constancy.

To study behavioral responses of toads to varying types of stimuli, Ewert conducted experiments by placing the toad in the center of a small cylindrical glass vessel. He then rotated a small stripe (bar) of contrasting cardboard (acting as a visual ‘dummy’) around the vessel to mimic either prey-like or threat-like stimuli; see Video. The rate of turning was recorded as a measure of orienting behavior (prey-catching activity). By changing characteristics of the visual stimulus in a methodical manner, Ewert was able to comprehensively study the key features that determine behavior.

Up to a certain size, squares rotated around the toad successfully elicited prey-catching responses. Toads avoided large squares. Vertical bars nearly never elicited prey-catching behavior and they were increasingly ineffective with increasing height. Horizontal bars, in contrast, were very successful at eliciting prey-catching behavior and their effectiveness increased with increasing length, to a certain degree. Additional vertical segments on top of horizontal bars significantly decreased prey-catching responses. In general, movement of a rectangle in the direction of its long axis is perceived by the toad to be wormlike, whereas movement along the short axis is interpreted as anti-wormlike. By means of a different experimental setup it was shown that the worm vs. anti-worm discrimination is independent (invariant) of the direction the object moves in the toad’s visual field.

It is important to note that stationary objects usually elicit no prey-catching or avoidance responses. However, toads in an untextured surrounding cannot distinguish between the retinal image of a small moving object and the retinal image of a small stationary object induced by the toad’s own motion. Both images elicit prey-capture. In a textured surrounding, however, the induced moving retinal image is ignored. — In addition, the contrast between stimuli and background can significantly affect the type of behavior. In response to a wormlike stripe, common toads orient and snap towards the edge leading in the direction of motion, given that the stripe is black and the background white. If the stimulus/background contrast is reversed, the toad prefers the trailing edge of the white stripe and often snaps behind it. Obviously, “off”-effects (rapid change in luminance from bright to dark) by the moving contrast borders play a guiding role. — Generally, white square objects moving against a black background are more attractive as prey than black objects on white. However, this tendency is plastic and reverses seasonally, where black objects against a white background are much more effective at eliciting prey-catching behavior in the fall and winter (Ewert 1980).