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During foraging trips, desert ants Cataglyphis fortis do not rely only on their well-studied path integration system, they also use olfactory cues when approaching a familiar food source. When a wind is blowing from a constant direction, as is characteristic of their desert habitat, the ants do not approach the feeder directly. They rather steer some distance downwind of the food source to pick up odour filaments emanating from the food. They follow this odour trail upwind, and find the source quickly and reliably.

This approach behaviour was examined in more detail in order to identify the underlying orientation strategy. First, the ants may employ a ’goal expansion strategy’, using odour spread as a spatially limited indicator for the presence of food. In that case, the distance steered downwind of the feeder should be determined by the range of the odour plume (and, for instance, wind speed). It should be independent of the distance between nest and feeder. Second, the ants may apply an ’error compensation strategy’, using odour filaments as a guideline towards the food source. Steering downwind by a margin just exceeding their maximum navigation error will lead the ants safely across the odour guide. In that case, the distance steered downwind of the feeder should increase more or less linearly with the nest-feeder distance.

Our results unambiguously support the second strategy. When feeders were established at distances of 5-75 m from the nest, the distances steered downwind of the food increased from 0.7 m to 3.4 m in a linear fashion. This result was independent of wind speed or wind direction. It translates into an ant’s estimate of its navigation error within a range of 3 degrees to 8 degrees.