SELECTION BY PARASITES ON COMPONENTS OF FITNESS IN TRIBOLIUM BEETLES - THE EFFECT OF INTRASPECIFIC COMPETITION

In the past decade, parasites have been widely recognized by populatio n biologists as important selective agents on host populations. Little is known, however, about the mechanisms of parasite-imposed selective processes. We examine the effects of the tapeworm, Hymenolepis diminu ta, on the fitness of the flour beetle host, Tribolium castaneum, unde r high and low intraspecific competition environments. We further iden tify the components of fitness that contribute to the overall parasite -induced reduction of fitness. The fitness components studied include female fecundity, adult cannibalism of eggs, egg-to-adult viability, a nd male sperm precedence. Under high intraspecific competition, parasi te infection significantly reduced the fitness of both male and female beetles; infected beetles showed increased adult cannibalism rate of eggs. Such a parasite-induced behavioral alteration may contribute to the observed reduction in net fecundity. In contrast, under low intras pecific competition, male fitness was not significantly changed, reduc tions in fecundity were not significant, and second-male sperm precede nce was significantly decreased by the tapeworm infection. The egg-to- adult viability of eggs from infected beetles was not significantly ch anged in either environment. These results provide evidence that paras itism can affect host fitness through both fecundity and sexual select ion, and they suggest that the consequences of parasite infection on h ost fitness and the components of fitness can be mediated by host dens ity.