A deep dive into the museum collections of one of North America’s most cherished butterflies, the Monarch, has revealed that as non-native populations found themselves in food-rich areas and their need to migrate shrunk, so too did their wingspan. Monarchs are famous for their ambitious, annual, multi-generation migrations that see individuals fly thousands of miles, but as they are increasingly settling in places where their favored botanical species are available all year, they have adapted to a less long-haul approach to life.

The new research, published in the journal Proceedings of the National Academy of Sciences, looked at Monarch specimens, who are native to North America, and compared them against non-native populations in the Caribbean, Central and South America. Whether the spread of these animals was a purposeful adventure or they were simply blown there by accident isn’t clear, but archived specimens revealed that while bigger wings aided their migration, those populations that landed in bountiful pastures soon saw their wings get smaller.

The researchers hypothesized that wing size could be influenced by an individual's genetic material as a migrating or non-migrating Monarch, or as the result of environmental factors present where they first hatched. To find out which it was, Micah Freedman, a graduate student at the Center for Population Biology at UC Davis, took the eggs of non-migrating populations in Hawaii, Guam, Australia, and Puerto Rico and hatched them alongside native migrating Monarchs in California. Despite being transported to migrating Monarch territory, the non-migrating butterflies retained their comparatively little wings, proving that the driving force for this shrinking wingspan was genetic and not environmental.

“Early monarch founders have large and elongated forewings,” the authors wrote in the study. “Loss of migration repeatedly leads to smaller wings, a pattern detectable in both time series with historical specimens and experimentally reared monarchs.

"Our findings provide a compelling example of how migration-associated traits may be favored during the early stages of range expansion, and also the rate of reductions in those same traits upon loss of migration."