A toad's devious approach to its prey: A study of some complex uses of depth vision

Summary

• 1. The path taken by a toad to reach its prey has been examined both when a chasm or barrier impedes its approach and when nothing is in its way. In both situations the toad plans its route before starting out, and consequently its path must reveal something of its perception of the three dimensional arrangement of objects in its environment.

• 2. In the absence of obstacles the toad directs its approach to the position of the prey just before the toad starts to move. Prey velocity has no influence on approach direction (Fig. 1) and the toad does not correct its course to allow for movements of the prey that occur while the toad is walking (Fig. 2). The distance the toad walks in a single bout depends on the initial separation between toad and prey and it does not alter should the prey vanish or move during the toad's approach (Figs. 3 and 4). Both the distance and the direction of an approach are thus preprogrammed and are not corrected by visual feedback until the toad pauses at the end of a movement.

• 3. If a chasm is placed between the toad and its prey, the toad either leaps across the chasm, steps down into it, or turns away. Its choice of behaviour pattern is dictated by both the depth and the width of the chasm, indicating that it measures both parameters (Figs. 6 and 7). When the chasm is deep but not too wide, the toad leaps across, when the chasm is shallow it steps down into it, and when the chasm is both wide and deep it turns away.

• 4. If there is a paling fence between toad and prey, the toad either detours round the fence or attempts to reach the prey directly (Fig. 8). The toad's choice depends on the distance between fence and prey (Fig. 11). If this is more than 10 to 15 cm the toad tends to detour; if it is less the toad approaches directly. The switch-over point is unaffected by the distance between the starting point of the toad and the fence (over the range 10 to 30 cm), implying that the toad can measure the distance between two objects regardless of its distance from them. The toad thus displays depth constancy.

• 5. Detours are aimed accurately at gaps in the fence (Figs. 8 and 9) and if there is no gap through which the toad can pass, it attempts to reach the worm directly. Toads will make for gaps which are formed by two overlapping barriers placed at different depths (Fig. 12). Such gaps can only be detected if toads can measure the distance between two barriers.

• 6. A toad's decision to make a detour thus depends on its appreciation of the relative positions of several objects in its environment. This suggests that the toad constructs an internal representation of its three dimensional world and that its depth vision is not only used in the direct control of motor programmes.