Naturally Curious: The Mourning Cloak Butterfly
Not only does the Mourning Cloak have the distinction of being one of the earliest butterflies to emerge in the spring, but it is also our longest-lived butterfly. (Mary Holland photograph)
Mourning Cloak butterflies, with wings opened, at top, and closed. (Mary Holland photograph)
If you spend some time in the woods this month, you may well be rewarded with the sight of one of the earliest butterflies to emerge in the northeast — the Mourning Cloak butterfly, (Nymphalis antiopa). Its name comes from the similarity of the velvet-like scales on its wings to the clothing people used to wear at funerals.
The Mourning Cloak butterfly can be out and about this early in the year because it overwinters as an adult, which is quite unusual. Butterflies and all other insects that remain in the Upper Valley year round overwinter in one of four stages — egg, larva, pupa or adult. Typically, this far north, all members of a given species (or family) of butterfly overwinter in the same stage. Thus, most swallowtails pass the winter as pupae, whereas most members of the family Nymphalidae, or “brush-footed” butterflies, (e.g. Mourning Cloak, Question Mark, Eastern Comma and Compton Tortoiseshell) overwinter as adults. Further south, they may overwinter in one of two or three stages.
Most butterflies pass the winter as larvae and pupae, though a few also overwinter as eggs. When spring comes, they have one or more stages of metamorphosis to go through before becoming adults and mating. Not so the Mourning Cloak. As soon as temperatures start to rise significantly (and even during mid-winter thaws) the Mourning Cloak butterfly slips out from behind the loose bark or crevice where it spent the winter and takes to the air. Males typically perch in sunny openings during the afternoon and wait for receptive females.
After mating, the female encircles a branch of a host tree with her eggs. The adult Mourning Cloak female lays her eggs on one of several species of trees whose leaves her larvae feed on, including willows, poplars, elm, hackberry and birch. The red-spotted black larvae, or caterpillars, live in a communal web and feed together on young leaves, and then pupate and emerge as adult butterflies in June or July. After feeding briefly, the adults estivate, or become dormant, until fall, when they re-emerge to feed and store energy for hibernation.
Overwintering butterflies (in any stage) enter into a physiological state called hibernal diapause, characterized by a lowered metabolic rate and biochemical changes. The Mourning Cloak cannot tolerate being frozen, so it reduces the water content of its body and builds up glycerol which acts as an anti-freeze. This process is called super cooling – lowering body temperature, without ice formation, to levels below that at which freezing normally occurs.
As you might expect, emerging very early in the spring has its disadvantages as well as its advantages. On the downside, there are not too many flowers in bloom at this time of year. For the most part the emerging adult butterflies have to make do feeding on tree sap, especially that of oaks. Mourning cloaks walk down the tree trunk to a spot where the tree has been injured, or a branch has broken, and feed head downward.
When their wings are closed, they are extremely well camouflaged. Mourning cloaks occasionally will also feed on rotting fruit and nectar, if they can find any, and they obtain salts and other minerals from puddles as well as animal scat. On the plus side of being early risers, hatching Mourning Cloak larvae have first choice of the emerging green leaves of the forest, as these caterpillars are present long before most species of butterfly larvae.
Not only does the Mourning Cloak have the distinction of being one of the earliest butterflies to emerge in the spring, but it is also our longest lived butterfly. Their average lifespan is an impressive 10 months, but some individuals survive for nearly a year. It’s not surprising that the Mourning Cloaks we see in March look a bit worse for wear, with wings often frayed. Even so, they are a beautiful sight to behold as the snow disappears in our woodlands.
Mary Holland is the author of “Naturally Curious: A Photographic Field Guide and Month-by-Month Journey Through the Fields, Woods, and Marshes of New England.” She has a natural history blog which can be found at www.naturallycuriouswithmaryholland.wordpress.com.