Selection by Parasites on Components of Fitness in Tribolium Beetles: The Effect of Intraspecific Competition

Abstract

In the past decade, parasites have been widely recognized by population 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 diminuta, on the fitness of the flour beetle host, Tribolium castaneum, under high and low intraspecific competition environments. We further identify 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, and male sperm precedence. Under high intraspecific competition, parasite 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 intraspecific competition, male fitness was not significantly changed, reductions in fecundity were not significant, and second-male sperm precedence was significantly decreased by the tapeworm infection. The egg-to-adult viability of eggs from infected beetles was not significantly changed in either environment. These results provide evidence that parasitism can affect host fitness through both fecundity and sexual selection, and they suggest that the consequences of parasite infection on host fitness and the components of fitness can be mediated by host density.