Experimental Manipulation of Dispersal Ability in A Neotropical Butterfly Anartia fatima (Lepidoptera: Nymphalidae)

Research on endangered British butterflies has found that butterfly populations in small refuges evolve to allocate more mass to the thorax (flight muscle) and less to the abdomen than populations in large refuges. The observed change in mass allocation affects two morphological features relevant to flight: the flight muscle ratio (FMR) and the position of center of body mass (cm_body). The author tested whether a decrease in FMR or a change in cm_body reduced the ability to disperse by experimentally weight-loading Neotropical Anartia fatima butterflies. In one treatment group, FMR was decreased but cm_body was not altered, whereas in the second group FMR was decreased and cm_body was repositioned further posterior. In one mark⁻release⁻recapture (MRR) experiment, butterflies dispersed relatively slowly, and treatment groups did not differ significantly. In a replicate experiment, butterflies dispersed more quickly, and control butterflies dispersed more rapidly than either treatment group. Differences in dispersal were consistent with a causal relationship between FMR and movement. A more posterior cm_body had little effect on dispersal beyond that due to the change in FMR. These results support the hypothesis that an increase in mass allocation to the thorax in small, dispersed refugia is due to selection on the ability to disperse.