Monarch butterfly wing morphology varies substantially throughout their global range, both between resident and migratory populations and also within the migratory North American population. Here, we use a dataset comprising more than 1800 North American individuals collected between 1878-2017 to characterize the factors shaping continent-wide patterns of wing morphological variation. North American overwintering butterflies have forewings that are approximately 4.4% larger than those collected in summer breeding areas. Monarchs overwintering in Mexico have forewings that are approximately 1.8% larger than monarchs overwintering in California, conducive to the idea that migration distance is positively correlated with wing area. We find evidence for a latitudinal cline within North America, such that butterflies collected at higher latitudes have significantly larger and more elongated forewings. We also find a significant increase of approximately 4.9% in forewing area between 1878-2017, but no difference through time in wing elongation. This result is corroborated by a reanalysis of a recently published dataset of more than 600 butterflies from Mexican overwintering sites. We discuss possible reasons for this increase in wing size through time, including northward shifts in the monarch’s breeding range and changes in relative abundance of milkweed hosts, and present experimental data addressing the influence of larval host plant on adult wing morphology. Our analysis suggests that (1) migration is indeed an important selective force for monarch wing morphology; (2) wing size has increased through time in North America; (3) factors such as host plant identity must be considered to fully understand monarch wing morphological variation.
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