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.

The cost of movement: assessing energy expenditure in a long-distant ectothermic migrant under climate change.

Migration is an energetically taxing phenomenon as animals move across vast, heterogeneous landscapes where the cost of transport is impacted by permissible ambient conditions. In this study, we assessed the energetic demands of long-distance migration in a multigenerational ectothermic migrant, the monarch butterfly (Danaus plexippus). We tested the hypotheses that temperature-dependent physiological processes reduce energy reserves faster during migration than previously estimated, and that increasing climatic temperatures resulting from the climate crisis will intensify baseline daily energy expenditure. First, we reared monarchs under laboratory conditions to assess energy and mass conversion from fifth instar to adult stages, as a baseline for migratory adult mass and ontogenetic shifts in metabolic rate from larvae to adult. Then, using historical tag-recapture data, we estimated the movement propensity and migratory pace of autumn migrants using computer simulations and subsequently calculated energy expenditure. Finally, we estimated the energy use of monarchs based on these tag-recapture data and used this information to estimate daily energy expenditure over a 57 year period. We found support for our two hypotheses, noting that incorporating standard metabolic rate into estimates of migratory energy expenditure shows higher energy demand and that daily energy expenditure has been gradually increasing over time since 1961. Our study shows the deleterious energetic consequences under current climate change trajectories and highlights the importance of incorporating energetic estimates for understanding migration by small, ectothermic migrants.

Interactions between parasitism and migration in monarch butterflies.

In many species, migration can increase parasite burdens or diversity as hosts move between diverse habitats with different parasite assemblages. On the other hand, migration can reduce parasite prevalence by letting animals escape infested habitats, or by exacerbating the costs of parasitism, leading to culling or dropout. How the balance between these negative and positive interactions is maintained or how they will change under anthropogenic pressure remains poorly understood. Here, we summarize the relationship between migration and infectious disease in monarch butterflies, finding that migration can reduce parasite prevalence through a combination of migratory culling and dropout. Because parasite prevalence has risen in recent decades, these processes are now resulting in the loss of tens of millions of monarchs. We highlight the remaining questions, asking how migration influences population genetics and virulence, how the establishment of resident populations interferes with migration, and whether infection can interfere with migratory cognition.