Monarchs medicate their offspring
Because parasites cause disease and death in their hosts, there should be strong selection for hosts to evolve ways to protect themselves against parasites. Monarch butterfly larvae are specialist feeders on milkweeds, but they can use up to 30 milkweed species. Milkweeds vary in their concentrations of toxic secondary chemicals known as cardenolides. Our collaborative work with Mark Hunter (University of Michigan) has shown that milkweeds with high concentrations of cardenolides provide protection against protozoan parasites, thus acting as medicinal plants. This finding led us to investigate whether monarchs can actively use milkweeds as a form of medication. In a series of experiments, we found that monarch caterpillars cannot actively choose medicinal plants when infected. Intriguingly, however, infected female butterflies preferentially lay their eggs on milkweed species that make their offspring less sick. This suggests that monarch butterflies have evolved the ability to medicate their offspring, and also demonstrates that wild animals can use medication against parasitic infections.