Do toads plan routes? A study of the detour behaviour ofBufo viridis

Summary

• 1. Given the right circumstances, toads will detour round a paling fence to reach their prey on the other side. In programming this manoeuvre, toads take into account both the position of the fence and the distance of the prey (Fig. 1). Should there be a gap in the fence, which offers a more direct approach, toads will aim for that instead (Fig. 2).

• 2. The argument developed in this paper is that, when a toad decides upon a particular approach, it is guided by the sum of its reactions to several individual features of the situation, such as the length of the fence, the presence or absence of gaps, the gaps' width (Fig. 7) and their proximity to the prey (Fig. 11) and to the toad's long axis (Fig. 10). When there are several possible approaches, toads will select the gap (or edge) which has the most ‘attractive’ combination of features.

• 3. The relative attraction of gaps can be manipulated and toads will then shift their preference. Normally, toads head for the gap lying closest to the prey and to their long axis (Figs. 9a and 12b). However, if the relative salience of a more peripheral gap is increased the bias towards the closer gap is reduced (Fig. 9b).

• 4. Toads tend to choose the closest gap even when it is inappropriate to do so. They seem unable to use the spatial information potentially available to them to pick out the shortest, unobstructed path to their prey. The major support for this view comes from the way they treat double fences composed of two rows of palings. With both fences unbroken, toads usually detour around them (Fig. 2d). However, when a gap is inserted in the front fence, they will often aim for that, regardless that the rear fence blocks their subsequent approach (Figs. 2c and 4). If palings are added to join the ends of the two fences, toads continue to aim for the gap, though once they have entered the space between the two fences, all they can do is to retrace their steps.

• 5. It is not that toads are blind to the rear fence. They can detect gaps in it (Fig. 8a) and their behaviour is influenced by the distance between the rear fence and their prey (Fig. 6). Nonetheless, a gap restricted to the front fence is still treated as a gap, but as less attractive than one extending through both fences (Fig. 8b). And, if such a gap is close to the toad's midline and the prey, then toads are drawn to it, rather than to the ends of the fence.