Gravisensation and modulation of gravitactic responses by other sensory cues in the monarch butterfly (Danaus plexippus)

Using the monarch butterfly (Danaus plexippus), we studied how animals can use cues from multiple sensory modalities for deriving directional information from their environment to display oriented movement. Our work focused on determining how monarchs use gravity as a cue for oriented movement and determined how cues from other sensory modalities, cues that by themselves also produce oriented movement (visual and magnetic directional cues), might modulate gravisensation. In two tests of gravisensation (movement in a vertical tube; righting behavior), we found that monarchs display negative gravitaxis only (movement opposite to the direction of gravity). Negative gravitaxis can be modulated by either visual (light) or magnetic field cues (inclination angle) that provide directional information. The modulation of gravity-mediated responses, however, depends on the relationship between cues when presented during trials, such as when cues are in accord or in conflict. For example, when light cues that elicit positive phototaxis conflicted with negative gravitaxis (light from below the monarch), monarch gravisensation was unaffected by directional light cues. We also found that the antennae play a role in gravity-mediated movement (righting), as, with antennae removed, monarch movement behavior was no longer the same as when the antennae were intact. Our results demonstrate that monarchs can use and integrate multiple, multimodal cues for oriented movement, but that the use of such cues can be hierarchical (that is, one cue dominant for movement), and the hierarchy of cues, and the responses towards them when found together, depends on the physical relationships between cues during movement.