Female Monarch Butterflies Are More Successful than Males During the Fall Migration: A Literature Review

The migration of monarch butterflies in North America is one that has been shaped by thousands of years of adaptation. Over the last century, studies of monarch migration have often reported subtle differences in migration-related traits between male and female monarchs. This literature review aimed to organize these disparate reports, which together provide evidence that female monarchs are superior to males in their migration success. Females have higher tag-recovery rates from Mexico, they have greater lipid stores during the migratory journey, and they arrive in Mexico with more fat than males. They also show lower instances of wing damage, both during migration and at arrival to the winter colonies, indicating their migration is less physically taxing. The mechanism for this is not physiological, as there are no sex differences in metabolism during flight, nor are there navigational differences. It is likely a matter of aerodynamics and flight efficiency; females have lower whole-body wing loading indices and their greater wing melanism levels may also be a factor. Female wings are ~58% black-pigmented, compared to ~48% on males. This basic biological difference has never been explained. Studies from other winged animals show how black pigment absorbs solar energy, which can heat the upper wing surface and reduce drag. Since monarchs have a pronounced dimorphism in melanism, but yet other non-migratory Danaid butterflies do not, it can be argued that the monarchs’ unique migration and overwintering phase has driven this greater female melanism. Females would need to retain more lipids over the winter months to have enough left over to develop eggs in the spring. From an engineering standpoint, this sex difference in flight ability could be an avenue for research into the mechanics of flight, i.e., bioinspiration.