Orientation in corpse-carrying ants: memory or chemical cues?

In social insects, rejection of dead nestmates is a common hygienic behaviour that allows colonies to reduce pathogen transmission within the nest. We investigated which orientation processes, chemical cues or individual memory, are used by the common red ant, Myrmica rubra, when it removes dead nestmates far from the nest. Myrmica rubra colonies varied in how efficiently they removed corpses, owing to differences in the number of short-term specialists involved in the transport of several corpses over successive trips. Corpse-carrying ants had to choose between two possible paths leading to areas remote from the nest. Their choices were not influenced by chemical compounds such as a necrophoric trail or any other chemical cues emitted by corpses. On the other hand, corpse-carrying ants that made several successive trips relied on their own spatial memory by choosing the same side as they did before. The strength of this memory was time dependent, an ant being more likely to choose the same side if the time elapsed between two choices was short. This suggests that necrophoresis in field colonies with a low death rate will be likely to lead to dispersion rather than to a piling up of corpses.