Summer Eastern North American Monarchs Do Not Use Magnetic Field for Directionality but Extreme Heat Alters Righting Behavior

Summer Eastern North American monarch butterflies (Danaus plexippus) represent one leg of the annual multi-generational migratory-range expansion cycle of monarchs in North America, as they spread north and repopulate the breeding habitat. Migratory monarchs use compass orientation to fly southwest in the fall to their overwintering sites. Though summer monarchs do not have a specific migratory endpoint like fall monarchs, it is possible that summer monarchs migrate north using untested magnetic compass orientation mechanisms. We examined the role of magnetic cues in the righting orientation behavior of summer monarchs to test the hypothesis that summer monarchs are indeed migratory and display northward orientation. Response to magnetic cues can be obtained from a righting response assay, where orientation can be observed in the righting behavior of monarchs on a vertical plane. Because magnetic orientation can be affected by temperature, we hypothesized that if summer monarchs did not show a northward orientation, then exposure to high temperatures (with or without prior experience to high temperature) would induce a northward orientation. We found that summer monarchs would right themselves, but did not use a magnetic compass to orient northward. In addition, summer monarchs exposed to high temperatures did not right themselves consistently. Thus, our results reinforce that summer monarchs are non-migratory in the strict sense and do not use a magnetic compass for orientation, nor is magnetic orientation induced by high temperature; however, high temperatures can induce other behavioral changes. As average temperatures increase and extreme climate events are predicted to increase in frequency with climate change, it is important to identify the behavioral responses to these changes.