Why do male monarchs feed on plants containing pyrrolizidine alkaloids?
The North American monarch (Danaus plexippus) is a well-studied butterfly species that feeds on plants for both nutritional and non-nutritional purposes. Monarchs are known to feed on two types of secondary plant metabolites: cardiac glycosides which are obtained from larval host plants such as milkweed, and pyrrolizidine alkaloids (PAs) which are typically gathered by adult males. Much is known about the chemical defense properties of CGs and their relationship with monarchs, but the relationship between monarchs and PAs is less understood. There is evidence that some other butterfly species feed on PAs as a form of self-medication when they experience a parasite infection. The purpose of this research was to examine whether PA-plant consumption had any self-medicating effects in male monarchs with and without an immune challenge. To test the self-medicating properties of PA-plants, male monarchs were either immune challenged with an LPS injection or left as uninjected in controls, and raised in cages with access to PA or water. We measured the immune capacity of a subset of males across treatment groups on day 7. The remaining individuals are currently being used to measure lifespan. Although this project is ongoing, preliminary data suggests that monarchs who had access to PAs did not have a significant difference in hemocyte counts compared to the control, regardless of the immune treatment. We intend to also quantify phenoloxidase from hemolymph samples to test other aspects of immunity. Many of the experimental individuals are still living, but of those that are deceased, we did not see a significant difference in lifespan between the monarchs fed a PA diet and the control. This study will help us to understand if there is a connection between PA-plant consumption and immune capacity or lifespan and will further our knowledge of monarchs and their unique natural history. Results from this experiment will provide a foundation for further studies on PA-plants, immune-challenges, immune response, monarch self-medicative behaviors, and monarch sex differences.