EVIDENCE OF LARGER IMPACT OF PARASITES ON HOSTS IN THE TROPICS - INVESTMENT IN IMMUNE FUNCTION WITHIN AND OUTSIDE THE TROPICS

Parasites may impose stronger selection pressures on their hosts in th e tropics compared to non-tropical climatic zones because parasite abu ndance is not reduced by the adverse environmental conditions of winte r as at high latitudes. Furthermore, tropical parasites tend to be mor e host-specific than temperate zone parasites, and their effects on ju venile 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-parasit e defence. Closely related pairs of birds were used to test the predic tion that investment in immune function should be greater in the tropi cs as compared to non-tropical zones. The circulating concentration of leukocytes in the blood was consistently higher in tropical bird spec ies 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 differ ences in immune function between hosts in the tropics and non-tropical climate zones should affect divergence in host population dynamics, h ost population density and diversity, host life-history evolution, and the evolution of host sexuality and sexual selection.