Evidence of Larger Impact of Parasites on Hosts in the Tropics: Investment in Immune Function within and outside the Tropics

Abstract

Parasites may impose stronger selection pressures on their hosts in the tropics compared to non-tropical climatic zones because parasite abundance is not reduced by the adverse environmental conditions of winter as at high latitudes. Furthermore, tropical parasites tend to be more host-specific than temperate zone parasites, and their effects on juvenile mortality of hosts is particularly strong. I tested indirectly whether tropical parasites have had stronger impact on their hosts by making a pairwise comparative study of host investment in anti-parasite defence. Closely related pairs of birds were used to test the prediction that investment in immune function should be greater in the tropics as compared to non-tropical zones. The circulating concentration of leukocytes in the blood was consistently higher in tropical bird species as compared to non-tropical ones. The relative size of the spleen for a given body size was significantly larger in tropical as compared to that of closely related non-tropical species. Interspecific differences in immune function between hosts in the tropics and non-tropical Climate zones should affect divergence in host population dynamics. host population density and diversity, host life-history evolution, and the evolution of host sexuality and sexual selection.