Every year, the monarch populations of eastern North America spend the winter in central Mexico. And if the monarch travels more than 4,000 kilometers to get there, it’s because it finds the conditions in that part of the world ideal for overwintering: temperatures not too hot or too cold. Nevertheless, even at these latitudes, monarchs aren’t sheltered from the elements. Rain, strong winds and sometimes even frost are challenges they have to face. Which leads us to reflect on the role played by the forest in the monarch’s overwintering sites.
The forest as shelter
Under the canopy of mountain forests, monarchs are sheltered from cold, rain and wind. In contrast to open spaces – such as plains, where daytime and nighttime temperatures vary considerably – dense tree cover has a leveling effect on temperature fluctuations. In addition, the trees act as windbreaks, protecting the butterflies from drying and from strong winds. Thus, the oyamel firs and other tree species present in this region help the monarch cope with intemperate weather while benefiting from cool, relatively stable temperatures.
Trees as a source of heat
Trees also provide the monarch with an interesting source of heat. Studies have shown that trunks store energy during the day and radiate warmth during the night. The more significant the diameter of the trunk, the more heat it will diffuse1. Hence, a tree with an average diameter of 72 centimeters will offer an extra 1.1 to 2.2 °C during the night, while a tree with an average diameter of 39 centimeters will offer an additional 0.8 to 1.5°C.2 These few degrees can make all the difference! Trees therefore play an essential role, acting a bit like a hot-water bottle that keeps monarchs’ bodies warm on cool nights.
Getting together to stay warm
It’s not just the forest that protects monarchs from the severity of winter. In its overwintering sites, the butterfly forms dense colonies on the branches and trunks of trees. That behavior is widespread in the animal kingdom, because it brings about two major benefits: protection against predation, and added warmth. Studies have observed that the formation of dense clusters on branches promotes conservation of butterflies’ heat on cool nights. The branches on oyamel firs, which are abundant in the monarchs’ overwintering sites, provide an ideal architecture for the creation of dense colonies.3 The oyamel’s short needles make it possible for monarchs to gather in denser clusters than they could on pines or cedars.3
Spring is just around the corner, and the monarchs will be joining us again soon. In Canada, the breeding grounds visited by the monarchs are important for this threatened species. By taking part in the Mission Monarch program, you’ll contribute to documenting those habitats and to saving this magnificent butterfly!
- Williams, E.H. and Brower, L.P. (2015). “Microclimatic protection of overwintering monarchs provided by Mexico’s high-elevation oyamel fir forests: a review,” in Monarchs in a Changing World: Biology and Conservation of an Iconic Butterfly, 109-116 (Ithaca, NY: Cornell University Press).
- Brower, L.P. et al. (2009). “Oyamel Fir Forest Trunks Provide Thermal Advantages for Overwintering Monarch Butterflies in Mexico,” Insect Conservation and Diversity 2, 3.
- Brower, L.P. et al. (2008). “Monarch butterfly clusters provide microclimatic advantages during the overwintering season in Mexico,” Journal of the Lepidopterists’ Society 62: 177–188.