Animals are equipped with sophisticated guidance systems that enable them to navigate efficiently. Previous studies suggest that foraging animals such as desert ants tend to combine guidance systems in a weighted manner, where the weight given to each cue gradually changes during the approach to the goal. However, when subjected to rapid enforced decision making (e.g. under predation risk), the animal may use alternative mechanisms, as recently suggested for ecologically relevant decisions involving time constraints in humans. We show here that fiddler crabs, Uca perplexa, scuttling to their burrows when threatened, responded to visual cues only if their path integration (PI) systems indicated nearness to their burrows. When homing errors were imposed by placing fake entrances (visual cues) along their homing paths and masking their true burrows (the goal of PI), the threatened crabs altered their behaviour towards visual cues according to the remaining PI vector length; if it was long, they continued running until they arrived at the masked true burrow, ignoring the visual cue, but if it was short, they suddenly stopped at the fake entrance. Our results suggest that, for fiddler crabs, PI and the view are mutually exclusive cues and that they instantly shift from one to the other if they approach the end of the home vector, instead of combining systems in a weighted manner. This could enable them to avoid entering the wrong burrow, where they would be ejected by the resident crab and be subject to predation.
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