Individually different foraging methods in the desert ant Cataglyphis bicolor (Hymenoptera, Formicidae)

Summary

Observations and field experiments on the foraging behaviour of individual workers of Cataglyphis bicolor in a Southern Tunisian shrub desert are reported. The workers search singly for their food (mostly animal carcasses) and are singleprey loaders. The individuals differ to a great extent in their persistence to re-search the place of a find on a previous foraging excursion. The differences range continuously from thoroughly researching a place to just walking by. If, in an experiment, the same reward is offered farther from the nest, each ant persists more in re-searching the place than if food is offered close to the nest. In a further experiment, some individuals persisted less in searching near the former finding site if they had collected a fly than after collecting a piece of cheese. There is, however, evidence that individuals do not differ in their food preference. Persistent individuals, which re-search the place of a former find, are faster than non-persistent ones in retrieving food that is experimentally arranged in an aggregated manner. The experiment failed to demonstrate the (reverse) superiority of non-persistent individuals foraging on homogeneously distributed food. The observations of unmanipulated foraging excursions in the field suggest such an advantage for non-persistent foragers under natural conditions where food in general occurs widely dispersed. The colony as a whole retrieves more food within the same time from an experimental lay-out that is homogeneous than from an aggregated one. The behavioural differences between individuals could be caused by a training bias of the short-lived foragers, leading to a different assessment of the profitability of a searching method which implies returning to a formerly rewarding place. Thus, each worker uses the most promising behaviour according to its individual experience. Alternatively, the individually different searching methods could mainly contribute to the welfare of the colony as a whole rather than leading to a maximal short-term efficiency of each individual. In particular, the colony, disposing of only a few highly persistent foragers, could quickly exploit occasional short-lived, but unpredictible, clumps of food within its foraging range.