Wing morphology, flight ability, and immune measures in monarch butterflies
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Monarch butterflies (Danaus plexippus) are well-known for their spectacular migrations along the northeastern coast of North America to over-wintering sites in Central America. In response to wounds and infections, they have a complex immune system that includes the production of hemocytes (insect immune cells) and phenoloxidase (PO) activity, which releases melanin, an important compound in immune defense. Migratory populations of monarchs undergo a high energetic cost associated with flight as well as resistance to infection, but potential trade-offs among flight ability, immunity, and wing morphology are unknown. Hemocyte samples were obtained from the same individuals during larval fifth instar and adult stages, and measured for PO activity. Prior to flight trials, adult monarchs were scanned on a flat-bed scanner to obtain wing morphology data such as wing area, loading, aspect ratio, as well as color characteristics. Monarchs were tested for flight ability using a nearly friction-less flight mill; total flight time, distance, initial and final velocity were measured. Results showed no relationships between flight performance and immunity. There were significant relationships between average flight speed and wing coloration, and between adult hemocyte concentrations and almost all measures of wing color. This suggests that wing coloration may be an overall indicator of monarch health in terms of flight performance and immunity.