Homing in wood ants, Formica japonica: use of the skyline panorama.

Homeward orientation was studied in wood ants Formica japonica that, while foraging, shuttled back and forth along a 7.9m route between the nest and a feeding site located on a wide terrace platform surrounded by a conspicuous landmark panorama. The return runs of the ants were amazingly straight, not only in the controls (starting at the feeding site) but also in ants displaced for various distances to the left and right of the feeding site. These courses, however, were oriented neither parallel to the predisplacement courses nor directly towards the nest. This result excludes the use of chemical cues and celestial compass cues. Furthermore, the nest itself, or some object close to it, could not have served as a beacon. The extensions of the homeward paths taken by ants that had been displaced to various release sites up to 11m sideways from the training route intersected at a point far (approximately 13m) behind the nest. This result suggests that the ants used distant landmarks seen by them in their frontal fields of view. To test this hypothesis, the distant landmark panorama was concealed by an opaque sheet mounted at right angles to the normal return route of the ants and extending up to different elevations. Shielding the lower part of the landmark panorama had no effect on the homeward paths. However, when the screen was mounted in such a way that the ants could only see the top skyline, represented by the upper edges of a line of trees, for part of their return run, it was only during these parts that their path was oriented in the homeward direction. When, during the course of displacement experiments, the ants were deprived of their familiar skyline panorama, they moved in their home direction only for an extremely short distance (0.1-0.4m rather than the usual 7.9m) and then started a systematic search programme. Hence, in the present context, skylight information is not used, at least not extensively. Instead, ants use the distant skyline as a navigational guidemark.