While on foraging excursions, fiddler crabs track their burrow location despite having no visual contact with it . They do this by path integration, a common navigational process in which motion vectors (the direction and distance of animals' movements) are summed to form a single "home vector" linking the current location with the point of origin. Here, we identify the mechanism by which the integrator measures distance, by decoupling motor output from both inertial and visual feedback. Fiddler crabs were passively translated to a position such that the home vector lay across an acetate sheet on the ground. After being frightened, crabs tried to escape but slipped as they did so. Detailed high-speed video analysis reveals that crabs measure distance by integrating strides, rather than linear acceleration or optic flow: the number of steps they took depended on both the length of the home vector and how large their steps were, whether they slipped and fell short or not. This is the most direct evidence to date of a stride integrator that is flexible enough to account for significant variation in stride length and frequency.

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SUMMARYForaging fiddler crabs (Uca spp.) monitor the location of, and are able to return to, their burrows by employing path integration. This requires them to accurately measure both the directions and distances of their locomotory movements. Even though most fiddler crabs inhabit relatively flat terrain, they must cope with vertical features of their environment, such as sloping beaches, mounds and shells, which may represent significant obstacles. To determine whether fiddler crabs can successfully perform path integration among such three-dimensional obstacles, we tested their ability to measure distance while we imposed a vertical detour. By inserting a large hill in the homeward path of foraging crabs we show that fiddler crabs can cope with vertical detours: they accurately travel the correct horizontal distance, despite the fact that the shape of the hill forces them to change their gait from what would be used on flat ground. Our results demonstrate a flexible path integrator capable of measuring, and either integrating or discarding, the vertical dimension.