Winter 2002

Mourning Cloak: the winter butterfly

Photo: Rob Curtis

Thoughts of butterflies during the winter tend toward fond memories of the beauty and warmth of the summer months. As surprising as it may seem, adult butterflies can be found throughout the year in Chicago Wilderness, even in the dead of winter. One butterfly that overwinters as an adult is the mourning cloak (Nymphalis antiopa), a velvety black butterfly with a two-inch wingspan. A bright yellow band lines the entire edge of the butterfly’s jagged wings, just outside of a row of iridescent blue spots.

Mourning cloak caterpillars feed on willows, elms, and hackberries. They are common in many habitats, including savannas, wetlands, and cities. The frigid temperatures that characterize winters in the Chicago Wilderness represent a challenge to all the organisms that inhabit this part of the world. Each butterfly species has a mechanism for coping with winter’s harshness. Some species, like the painted lady, don’t winter here at all. As freezing temperatures arrive, all eggs, larvae, chrysalides, and adults die. Only in the far south do temperatures remain mild enough that these butterflies survive the winter. As warmer temperatures advance northward in spring, so do the butterflies, breeding as they disperse into the interior of the continent.

Unlike painted ladies, most of this region’s butterfly species tough it out through the winter. Each species has one characteristic life stage in which it overwinters. A tiger or black swallowtail, for example, hibernates as a chrysalis. There are at least a few species that overwinter in each of the four life stages (egg, caterpillar, chrysalis, and adult). The mourning cloak is one of about a half-dozen of the area’s species that overwinter as adults.

Spending the winter as an adult does not imply a highly active state. Mourning cloaks may hibernate under loose bark, in a pile of debris, or in a crevice beneath a roof overhang. They frequently stay in the same place for weeks or even months while waiting out the cold temperatures.

The fact that mourning cloaks remain in a state of suspended animation for many months allows them to have greatly extended adult life spans. While a period of about two weeks is a typical life span for most adult butterflies, mourning cloak adults live for 10-11 months – the longest life span of any North American butterfly.

The cold itself is not a direct hazard to the butterflies. However, the formation of ice crystals in body tissue is quickly lethal. Mourning cloaks cope with this problem by secreting large amounts of natural antifreezes – such as sorbitol – into their bodies. These chemicals are produced only as the weather begins to cool.

If you put a mourning cloak collected during the summer into a freezer, it will promptly die due to a lack of these anti-freeze compounds.

The mourning cloak’s hibernation is sometimes interrupted on warm, sunny days. Some accounts speak of butterflies flying over snow during thaws. My own experience suggests that this is actually a very rare occurrence.

More typically, mourning cloaks emerge during the very first warm days of spring. My earliest observation of them in the Chicago area was on the 3rd of March in Harms Woods in Glenview. They were flying with eastern commas, another local species that overwinters as an adult. Although the temperature was around 70°F, no flowers were blooming that early in the season. Both butterfly species were taking nutrients at the only available source – horse droppings scattered along a bridle trail.

–Doug Taron