New insights on the migration of monarch butterflies in North America: a focused collection of studies

Abstract

America. By using digital image analyses to examine fine-scale features of monarch wing morphology, they determined that monarchs in North America have gradually increased in size (and are continuing to increase) over the last 100 years. They present evidence that this could be due to a shift in the type of milkweed available on the landscape, but it is also possible that this size increase reflects the increasingly perilous migration; the migration acts to remove small-winged monarchs each year. In support of this, the authors show that overwintering monarchs tend to be larger than are nonoverwintering ones, and even that eastern overwintering monarchs are larger than those at western overwintering sites, reflecting the much greater distance (and selective force) of the eastern journey. Long-term declines of monarchs at the eastern overwintering sites, but the absence of similar declines in counts of breeding or migrating monarchs [3-5], has led to much speculation about what is happening to monarchs during the journey to Mexico. One possibility that has been little-explored is the idea that monarchs may be gradually shifting their migratory course away from Mexico, travelling elsewhere to overwinter. The study by Vander Zanden et al. is suggestive of this possibility. These authors examined wing tissue of monarchs collected during winter in south Florida, using a stableisotope approach to elucidate where they originated from. Surprisingly, half of the monarchs tested appeared to come from the Midwestern United States! This implies that Mexico is not the final destination of ALL migrating monarchs in the eastern population. While these results are based on a small sample size, the evidence leads one to question old assumptions about the migratory journey. Migrating while infected with parasites would be challenging for any species, and monarchs are no exception. Monarchs are prone to a debilitating protozoan parasite, Ophryocystis elektroscirrha (OE), which is known to cause reductions in migration success [6, 7]. With this in mind, Davis and de Roode investigated one potential, but https://doi.org/10.1515/ami-2018-0009