Effects of division of labour on immunity in workers of the ant Cataglyphis cursor

Workers in social insects perform different roles, and the environment they experience differs markedly according to the tasks performed. Life history theory predicts that individuals should adapt investment in immune defences according to their cost relative to the risk of pathogen infection. Workers, which forage outside the nest, face harsher environmental conditions with increased risk of injury and infection. We then might expect higher immune defences in foragers than in intra-nidal workers which remain in the relative sanctity of the nest. However, task partitioning in social insects is often age-based, and foragers are usually the oldest individuals. Hence if immune defences degenerate with age, foragers could have lower immune defences than intra-nidal workers. In this study, we examined the difference in immune defences as assessed by the level of phenoloxidase activity (PO) between intra-nidal workers and foragers in the ant Cataglyphis cursor. In three out of four colonies tested, foragers and intra-nidal workers did not differ in their level of PO. In the final colony, the level of workers PO overall was higher than in the other three colonies, and foragers displayed a lower level of PO than intra-nidal workers. These results may suggest that, when a sustained colonial PO activity is necessary, foragers may be less able than intra-nidal workers to activate their PO. Our results are discussed in the light of previous studies that underline the diversity of PO activity patterns in social insects. The variation observed among studies and even colonies clearly emphasizes the plasticity of immune parameters.