Do Healthy Monarchs Migrate Farther? Tracking Natal Origins of Parasitized vs. Uninfected Monarch Butterflies Overwintering in Mexico

Abstract

Long-distance migration can lower parasite prevalence if strenuous journeys remove infected animals from wild populations. We examined wild monarch butterflies (Danaus plexippus) to investigate the potential costs of the protozoan Ophryocystis elektroscirrha on migratory success. We collected monarchs from two wintering sites in central Mexico to compare infection status with hydrogen isotope (δ 2H) measurements as an indicator of latitude of origin at the start of fall migration. On average, uninfected monarchs had lower δ 2H values than parasitized butterflies, indicating that uninfected butterflies originated from more northerly latitudes and travelled farther distances to reach Mexico. Within the infected class, monarchs with higher quantitative spore loads originated from more southerly latitudes, indicating that heavily infected monarchs originating from farther north are less likely to reach Mexico. We ruled out the alternative explanation that lower latitudes give rise to more infected monarchs prior to the onset of migration using citizen science data to examine regional differences in parasite prevalence during the summer breeding season. We also found a positive association between monarch wing area and estimated distance flown. Collectively, these results emphasize that seasonal migrations can help lower infection levels in wild animal populations. Our findings, combined with recent declines in the numbers of migratory monarchs wintering in Mexico and observations of sedentary (winter breeding) monarch populations in the southern U.S., suggest that shifts from migratory to sedentary behavior will likely lead to greater infection prevalence for North American monarchs.

Migration distance as a selective episode for wing morphology in a migratory insect

“…Consistent with previous studies [26, 28, 30, 31], our results showed that monarchs that migrated the farthest distance on average from natal grounds to wintering sites had the longest and largest wings. This observation suggests that migration acted as a selective episode on monarch butterfly wing morphology, specifically on wing length and area, and this relationship did not arise from temperature-dependent effects on wing morphology.…”

“…Moreover, Dockx [27] showed that among monarchs captured in Cuba, migrants originating from northern latitudes had more angular wings than those identified as year-round residents. In more recent work, however, Li et al [28] found no difference in wing shape among monarchs from migratory versus non-migratory populations, and Altizer et al [30] also found no relationship between wing shape and estimated migration distance among eastern North American migrants. Thus, evidence for selection operating on wing shape in migratory monarchs appears to be mixed.…”

“…Individuals with more elongated wings tended to be seen earlier in migration than those with short wings, suggesting that elongated wings increase the speed of migration [29]. At overwintering grounds in Mexico [30] and the west coast of the U.S. [31], there is a negative correlation between hydrogen isotope values ( δ 2 H) and wing size, suggesting that individuals with larger wings derived from more northern locations and so migrate longer distances. These studies were based on single-year sampling, and recent work showed that patterns of monarch natal origin can vary annually [32].…”

Within-wing isotopic (δ2H, δ13C, δ15N) variation of monarch butterflies: implications for studies of migratory origins and diet

“…Care was taken to remove only wing membrane and to avoid wing veins in the sample. Thus, our sample was designed to test for isotopic measurement the effects of natal origin on parasite loads (Altizer et al 2015), the role of wing coloration in flight distance (Hanley et al 2013) and general conservation concerns related to where most individuals are being produced (Flockhart et al 2017). Such tracking studies have used continental patterns, or isoscapes, of both precipitation δ 2 H and milkweed δ 13 C. Despite the demonstrated value of stable isotope measurements of butterfly wings to investigate migration, to our knowledge, there has been no previous investigation into the isotopic variance among different regions of wing material (primarily chitin) despite the fact that monarch wings, as with those of many lepidopterans, are highly variable in pigmentation.…”

“…The measurement of δ 2 H in complex organic materials is challenging since a significant proportion of weakly bonded hydrogen can exchange with hydrogen from ambient water vapor. This effect can be controlled for by using the comparative equilibration approach employed here or use of modified carousel systems Meier-Augenstein et al 2013, Wassenaar et al 2015. Orange coloration is caused by the differential deposition of color pigments such as pterins and ommochromes on to wing scales Janssen et al 2001).…”