Variation in Forewing Size Linked to Migratory Status in Monarch Butterflies

Abstract

Abstract Long-distance migration can be seen throughout the animal kingdom and can have large impacts on population dynamics and species distributions. The act of migration itself also affects the evolution of a species, as evolutionary forces select for certain characteristics in animals conducting long-distance migration. Monarch butterflies are best known for their annual migration from Canada and the northern United States to central Mexico, but some populations of monarchs have lost the ability to migrate. Previous research found that migratory monarchs had larger, more elongated wings than their non-migratory counterparts and it was hypothesized that these traits were beneficial for migration. However, Bergmann’s rule - which predicts larger body sizes with increasing latitude - could also explain this pattern as migratory populations are found at higher latitudes. To understand the role of wing dimensions in migration, we examined forewing size and shape of migratory and non-migratory monarchs from seven worldwide populations varying in latitude. Results showed that larger forewing size was indeed correlated with migratory status rather than latitude. However, migratory monarchs did not have more elongated forewing shape than non-migratory monarchs across the globe. Our study indicates that size may play a larger role than shape in long-distance migratory capability.