Monarch Butterfly (Danaus Plexippus) Thermoregulatory Behavior and Adaptations for Overwintering in Mexico

Monarch butterflies from eastern North America migrate each fall to high altitude habitats in central Mexico and must avoid freezing, desiccation, heat stress, and predation while drawing on unrenewable lipid reserves for at least 90 d. Survival involves a heat—gaining morphology and behaviors in which thermoregulation plays a central role. Heat production by shivering permits raising of thoracic temperatures sufficiently for the butterflies to crawl up vegetation to sun—bask where they warm rapidly to flight threshold (°12.7°—16.0°C). Shivering and basking can also achieve or maintain flight temperature during partly cloudly weather. Both behaviors delay the decline of thoracic temperatures below flight threshold and consequent thermal trapping either on the ground beneath clusters of other monarchs, or while the butterflies are at water or nectar sources. The monarch thus is the first butterfly in which shivering has been shown to be of major ecological importance. At the other end of the thermal spectrum, basking in the intense insolation at this lattitude causes rapid warming at ambient temperatures as low as 9°. To avoid overheating and rapid depletion, the butterflies adopt sum—minimizing postures or fly out of insolated clusters and glide above the colony, losing heat to the cold ambient. Calculations suggest that a large proportion of the butterfies dies from depleting lipid stores. Maintenance of low body temperatures to conseve lipids is one of the major reasons why migrant monarchs select these high mountain forest as their overwinterin habitat.