In the eastern United States, the mimetic viceroy butterfly (Limenitis archippus) exhibits clinal variation in wing color, ranging from a tawny orange phenotype (L. a. archippus) in the N to a dark mahogany phenotype (L. a. floridensis) in Florida. Geographic distributions of these two subspecies are roughly coincident with the ranges of the viceroy's two eastern mimicry models: the monarch (Danaus plexippus) in the N, and the queen (D. gilippus) in the S. This coincidence has historically been attributed to “model-switching”: presumably, southern viceroys have switched from mimicking the monarch to mimicking the locally predominant queen, due to selective pressure exerted by visually foraging predators. As an initial test of this hypothesis, I sought evidence of selective predation on light and dark viceroys by captive red-winged blackbirds previously exposed to either monarchs or queens. Results were consistent with the model-switching hypothesis: queen-conditioned birds preferentially avoided the dark, queen-like L. a. floridensis, whereas birds exposed to monarchs avoided (to a lesser degree) the light L. a. archippus phenotype. I propose that this differential predation, while perhaps asymmetrical, demonstrates the selective mechanism responsible for the evolution of regional viceroy races, and that geographic model-switching explains the large-scale modern-day correlation between Danaus biogeography and viceroy wing color.
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Vol. 140 • No. 1