Do desert ants smell the scenery in stereo?

Desert ants, Cataglyphis fortis, navigate individually in the inhospitable saltpans of Tunisia using path integration for long-distance navigation, and visual and olfactory landmarks for fine-scale orientation in the vicinity of the nest entrance. Here, we show in a field experiment that the ants are able to locate the nest entrance within a two-dimensional olfactory array. Ants were trained to forage in an open channel and to memorize the nest entrance relative to four odours that were applied at the corners of an invisible quadratic array. In a test situation, the ants pinpointed the fictive nest only when the odours were present at their learned positions. Our results suggest that the ants had learned the olfactory scenery around their nest. Furthermore, unilaterally antennectomized ants could not pinpoint the nest within a two-dimensional array. Hence, this kind of orientation depends on the simultaneous input of both antennae, that is, on a stereo sense of smell. Until now, insects and mammals, including humans, have only been known to use bilateral sensory input to follow a concentration gradient of an odour. Our evidence suggests that desert ants require a stereo sense of smell to make use of the olfactory scenery around their nest for homing.