The cost of an immune response: vaccination reduces parental effort

A fundamental assumption of theories of the ecology and evolution of induci ble defences is that protective responses to attacks by parasites or predat ors should not only have benefits, but also costs. The vertebrate immune sy stem is by far the best studied example of an inducible defence, yet little is known about the costs of an immune response, especially in natural popu lations. To test: if an immune response per se is costly, we induced an ant ibody response in female blue tits, Parus caeruleus, by immunising them wit h human diphtheria-tetanus vaccine, and compared their nestling-feeding rat e with that of saline-injected controls. We found that vaccinated females r educed their nestling feeding rate, thus demonstrating a cost of the immune response in the currency of parental effort.