Coevolution of host immune defence and parasite‐induced mortality: relative spleen size and mortality in altricial birds

Abstract

Animal hosts defend themselves against parasites by the antibodies produced by an immune system. It is an inherent assumption that parasites constitute the selection pressure that has given rise to and maintains the immune system. Thus, greater impact by parasites on host fitness should result in greater investment in immune function across species. We tested this prediction by using field estimates of parasite‐induced nestling mortality in altricial birds as an estimate of the fitness cost of parasitism and the relative size of the spleen as an estimate of investment in immune function. The spleen is a peripheral lymphoid tissue that acts as the main site of lymphocyte differentiation and proliferation, and these B‐ and T‐cells are involved in the production of humoral and cell‐mediated immune responses. In a comparative study of 21 species of altricial birds we found a significant positive relationship between relative spleen size and parasite‐induced mortality, accounting for a third of the variance, even when controlling for potentially confounding variables. This finding provides evidence for the level of investment in immune function being related to the natural selection pressure imposed by parasites.