How do the demands of reproduction and long distance migration alter immunity in monarch butterflies?

Abstract

Migratory species, in addition to accomplishing some of the most impressive physical feats in the animal kingdom, can be exposed to a large number and diversity of parasites or pathogens and have the potential to transport those natural enemies long distances. Thus, migratory animals are important to consider in the context of infectious disease ecology. An essential step in understanding the role migratory animals play in disease transmission is to characterize how their susceptibility to pathogens—by way of changes in immune defenses—varies across the annual migratory cycle. Both strenuous movement and energetically-costly reproduction vary seasonally in migratory species and are each predicted to come at the cost of immunological parasite resistance, especially when food resources are limited during migration. My dissertation explored how an iconic migratory insect, the monarch butterfly (Danaus plexippus), balances the competing demands of immune defense, movement, and reproduction. I used the combination of a correlational field study and three laboratory experiments to understand how monarch immune measures varied with (i) food limitation across life stages; (ii) migratory distances travelled and the extent of lipid storage during migration; (iii) multiple aspects of reproductive activity; and (iv) short-duration forced flight activity. Although trade-offs between immunity and several physical or life history traits were predicted, in many cases I found immune defenses to be fairly resilient to the potential costs of flight-related energy expenditure. This echoes recent findings by other researchers—with both monarchs and other migratory species—that migrants are physiologically adapted to minimize costs of movement. My results also showed that immune defenses were limited by food availability and reduced by several aspects of reproduction, highlighting the potentially increased susceptibility of wild monarchs during the summer breeding season when parasite pressure is high and food is often limited. This work contributes to the disciplines of animal behavior, disease ecology, and ecological immunology and the results inform predictions about when during their annual cycle migratory species may be most vulnerable to infection.