Résumé
Organisms can protect themselves against parasite-induced fitness costs
through resistance or tolerance. Resistance includes mechanisms that
prevent infection or limit parasite growth while tolerance alleviates the
fitness costs from parasitism without limiting infection. Although
tolerance and resistance affect host-parasite coevolution in fundamentally
different ways, tolerance has often been ignored in animal-parasite
systems. Where it has been studied, tolerance has been assumed to be a
genetic mechanism, unaffected by the host environment. Here we studied the
effects of host ecology on tolerance and resistance to infection by
rearing monarch butterflies on twelve different species of milkweed food
plants and infecting them with a naturally occurring protozoan parasite.
Our results show that monarch butterflies experience different levels of
tolerance to parasitism depending on the species of milkweed that they
feed on, with some species providing over two-fold greater tolerance than
other milkweed species. Resistance was also affected by milkweed species,
but there was no relationship between milkweed-conferred resistance and
tolerance. Chemical analysis suggests that infected monarchs obtain
highest fitness when reared on milkweeds with an intermediate
concentration, diversity and polarity of toxic secondary plant chemicals
known as cardenolides. Our results demonstrate that environmental factors
– such as interacting species in ecological food webs – are important
drivers of disease tolerance.