I just arrived back in New England from the tropics a bit melancholy after chasing around so many butterflies. But today I was jolted out of this funk when I found a Mourning Cloak chilled to its core in an old woodpile. In April it will arise from its long winter rest and fly again.
This butterfly spent the fall feeding and storing fat in its abdomen. Before winter arrives it found a space to hide in the woodpile, but they can use hollow trees or logs, cracks in rocks, or inside old buildings as well. In these protected and somewhat insulated hideouts they enter diapause, a state of dormancy.
They become sluggish as the temperature drops. The freezing point of their cell tissue is lowered by an increased content of sugars, which act as an antifreeze. Mourning Cloaks produce sorbitol, a sugar alcohol obtained by the reduction of glucose. Sorbitol is also a sugar substitute that is often used in diet foods and it can also be found in plants in the genus Sorbus, represented by Mountain Ash in New England. Using electrical conductivity, biologists in Alaska found that Mourning Cloaks do not freeze until the temperature reaches -22o F.
Because the butterfly is unable to move, it is vulnerable to predation by birds, mice and other predators. To help combat detection, they have evolved a cryptic coloration pattern on the underside of their wings. When they close their wings their silhouette is minimized as they blend into the background.
To break diapause in the spring an individual must pass through a long period of cold weather and into a longer daylight period. The cold period must be months long to trigger the end of diapause. If it were shorter the individual might end diapause during a short warm spell only to be clobbered by the next arctic front.
Those that come out during warm late-winter or early-spring days between cold snaps are often short on fat and must seek running sap in an attempt to replenish their stores. Finally, longer and warmer days of spring bring them out to mate and lay eggs for the next generation. These early spring adults often have wings that are very tattered from their relatively long life of over 10 months. And I’ll be watching for them as a harbinger of summer to come.