Cues or meaningless objects? Differential responses of the ant Formica cinerea to corpses of competitors and enslavers

Group living involves major health risks, since it facilitates the spread of pathogens and parasites among members. To counteract this negative effect, social insects, such as ants, adopt several social defensive strategies, one of the most widespread being the collective disposal of corpses often outside the nest, which represent potential sources of infection. Corpses might even be used during interspecific conflicts to inhibit the activity of the attacked nest or they could be consumed as food. As intra- and interspecific relationships are manifold and often vary in terms of fitness consequences for the interacting species, it can be hypothesized that responses to corpses of different origin can vary. Scattered or piled up corpses of different ant species could act as cues for foragers, signalling the presence of other species, and triggering appropriate responses: alarm, retreat or foraging. We examined the reactions of the ant Formica cinerea under natural conditions to corpses of co-occurring Formica species. Formica cinerea responded differentially to corpses of different origin. Those of the territorial Formica polyctena and the slave-maker Formica sanguinea elicited more aggression and their corpses were removed sooner than other dead bodies. The majority of corpses were carried inside the nest, with the exception of Formica fusca corpses, which may signify the lack of specific interest in this species. The removal of nestmate and foreign conspecific corpses could be explained through social prophylaxis, and that of superior competitors additionally by conflict avoidance and/or by food supplementation. Based on our results, for ants corpses are not meaningless objects scattered around ant nests, but cues that carry information that trigger different behavioural reactions.