Which portion of the natural panorama is used for view-based navigation in the Australian desert ant?

Ants that forage in visually rich environments often develop idiosyncratic routes between their nest and a profitable foraging ground. Such route knowledge is underpinned by an ability to use visual landmarks for guidance and place recognition. Here we ask which portions of natural visual scenes are essential for visually guided navigation in the Australian desert ant Melophorus bagoti whose foragers navigate through a habitat containing grass tussocks, shrubs and trees. We captured M. bagoti foragers after they had returned to their nest from a feeder, but before they had entered their nest, and tested their ability to home accurately from a series of release locations. We used this simple release paradigm to investigate visually guided navigation by monitoring the accuracy of nestwards orientation when parts of the ants' visual field were obscured. Results show that the lower portion of the visual panorama is more important for visually guided homing than upper portions. Analysis of panoramic images captured from the release and nest locations support the hypothesis that the important visual information is provided by the panoramic contour, where terrestrial objects contrast against sky, rather than by a limited number of salient landmarks such as tall trees.