Parasites are common in nature and by definition cause disease or death to their hosts. As a consequence, hosts have evolved defenses to protect themselves against parasites. The use of medication is one such defense, and is well-known in humans. However, whether animals also use medication is still poorly understood. The caterpillars of monarch butterflies eat milkweed plants, and some milkweeds are toxic to monarch butterfly parasites. This project will study if monarchs can use milkweeds as a form of medication by preferentially laying their eggs on toxic milkweeds that reduce parasite growth and disease in the monarchs' offspring. The researchers will use experiments to test for the medicinal effects of different types of milkweed, including different species and flowering versus non-flowering plants. These experiments will also be used to identify the chemicals that are responsible for the milkweeds' medicinal properties. The project will further test whether monarchs from four populations in North America and Australia have evolved the ability to use milkweeds as medication. Parasite risk varies greatly between these populations, and the project will test whether parasite risk affects the type of medication behavior that monarchs have evolved. In particular, it is expected that monarchs from high-risk populations always lay their eggs on medicinal plants (a form of prophylactic medication), whereas monarchs from low-risk populations only lay their eggs on medicinal plants when infected (a form of therapeutic medication).
This project will enhance our understanding of how animals fight disease in nature, and will emphasize the role of parasites in the evolution of animal behavior. The project will also integrate research with education. Students of all levels (high school, undergraduate and graduate) and from underrepresented groups will participate in the design, execution and presentation of this work. The researchers will also host high school teachers in the lab to develop research projects for use in their classes to strengthen their ecology and evolution curriculum. Finally, the researchers will visit monarch butterfly festivals to teach the general public about science.
Parasites are common in nature and by definition cause disease or death to their hosts. As a consequence, hosts have evolved defenses to protect themselves against parasites. The use of medication is one such defense, and is well-known in humans. However, whether animals also use medication is still poorly understood. The caterpillars of monarch butterflies eat milkweed plants, and some milkweeds are toxic to monarch butterfly parasites. This project studied if monarchs can use milkweeds as a form of medication by preferentially laying their eggs on toxic milkweeds that reduce parasite growth and disease in the monarchs’ offspring. By providing infected and uninfected monarchs from the eastern North American population (which is famous for monarch migration) with a choice between medicinal and non-medicinal milkweeds, the researchers found that infected monarchs preferred to lay their eggs on medicinal milkweed, which reduced infection and disease symptoms in their offspring; in contrast, uninfected monarchs did not have such a preference, demonstrating they did not use medication. The researchers also used experiments to test for the medicinal effects of different types of milkweed, trying to identify the chemicals that are responsible for the milkweeds’ medicinal properties. Milkweeds are well known to contain toxins known as cardenolides, which make monarchs toxic to their predators. This project showed that these same chemicals are also likely responsible for the medicinal properties of milkweeds. The project further tested whether monarchs from multiple populations in North America have evolved the ability to use milkweeds as medication. Parasite risk varies greatly between these populations, and the project tested whether parasite risk affects the type of medication behavior that monarchs have evolved. In particular, it was expected that monarchs from high-risk populations always lay their eggs on medicinal plants (a form of prophylactic medication), whereas monarchs from low-risk populations only lay their eggs on medicinal plants when infected (a form of therapeutic medication). In support of this hypothesis the researchers found that migratory monarchs from eastern and western North America (which have a low disease prevalence) medicated their offspring only when infected, whereas monarchs from South Florida (where parasite prevalence if very high) always preferred to lay their eggs on medicinal milkweed. Self-medication has traditionally been studied in animals with strong cognitive skills and great learning ability, such as apes and elephants. By showing that monarch butterflies are able to use medication, this project has demonstrated that animals without such abilities can medicate as well, and that learning is not necessary to develop medication behaviors. As such, this project suggests that animal medication is much more common than previously assumed. By studying such medication more widely, new disease control strategies could be found. Indeed, by studying a wide variety of animals, humans may copy medicine from animals to develop drugs for themselves. For example, the parasites that infect monarchs are closely related to parasites infecting humans (such as those causing malaria), and it will be interesting to test whether the chemicals in milkweed could be used against human parasites. With respect to broader impacts, this project integrated research with education in several ways. One postdoc, two graduate students, and twelve undergraduate students were trained. The project also trained a high school teacher, and resulted in outreach to the community through butterfly festivals, school visits, science fairs and the building of a pollinator garden at the Carter Center in Atlanta. The research also resulted in popular science publications and contributed to a full-feature documentary movie on self-medication.
