The guidance of desert ants by extended landmarks

Desert ants (Cataglyphis fortis) were trained to follow a fixed route aroun d a barrier to a feeder. Their homeward trajectories were recorded on a tes t field containing a similar barrier, oriented either as in training or rot ated through 22 or 45 degrees. Under one set of experimental conditions, th e homeward trajectories rotated with the orientation of the barrier, implyi ng that the visual features of this extended landmark can determine the rou te independently of compass cues: the barrier provided a 'visual scene' tha t controlled the trajectories of the ants, Under other conditions, the traj ectories after rotation were a compromise between the habitual compass dire ction and the direction with respect to the rotated barrier. Trajectories w ere determined primarily by the visual scene when ants were allowed to retu rn close to the nest before being caught and tested. The compromise traject ories were observed when ants were taken from the feeder. It seems that ant s exhibit at least two separate learnt responses to the barrier: (i) a habi tual compass direction triggered by the sight of the barrier and (ii) a vis ual scene direction that is compass-independent. We suggest that the weight ing accorded to these different learnt responses changes with the state of the path integration system.