| Monarch butterflies, Danaus plexippus, showed appropriate migratory orientation when tethered beneath an artificial sky of polarized light. Wild-caught fall migrant monarchs from Pennsylvania, wild-caught spring migrants from Mexico, and wild-caught spring migrants from California all showed significant responses to rotations of the overhead polarizer. Furthermore, the orientation of the tethered monarchs was consistent with the natural migratory headings of these populations. The monarchs varied their orientation to the overhead polarization in such a way that they would have headed along the appropriate migratory axes had they shown similar orientation to natural zenith polarization at those times of day. Therefore, monarchs can apparently utilize a time-compensated sky polarization compass for migratory orientation. Experiments were also conducted on monarchs reared from eggs in the laboratory under a developmental photoperiod that produced migratory adults. After brief exposure to natural sky, these adults also oriented appropriately to overhead polarization. Some monarchs were reared, exposed to natural sky, and tested under a clock-shifted photoperiod. These monarchs did not show the shift in orientation that would be expected had they been reared and exposed normally, but tested with a clock-shift, suggesting that the association of sky polarization patterns and time is learned rather than innate.; This thesis is the first report of behavioral polarization sensitivity in adult Lepidoptera, and it is the clearest demonstration thus far of an insect using a time-compensated celestial compass for long-distance migratory orientation. The use of sky polarization for simple compass information is probably one of many components needed for the complex navigational task performed by migrating monarchs. |
