Because immune defences are costly, life-history theories predict a modulation of immune investment according to its potential benefits. Social insects provide interesting models since infection risk may vary among individuals within a colony. In particular, the foraging workers, that have to leave the nest, suffer a higher infection risk and can contaminate their nest, which may favour high immune investments. However, evolutionary theories of aging predict that foragers should reduce their immune investment when they suffer high extrinsic mortality. To test these two predictions, we investigated the levels of phenoloxidase (PO) and prophenoloxidase, two important enzymes of the insect immune system, in workers of the ant Cataglyphis velox. We found a higher PO activity in foragers than in intra-nidal workers. This could result from an adaptive upregulation of the harmful PO (an enzyme potentially leading to autoimmune reactions) only when the risk of infection and wounding is high.
