Observations of mourningcloak butterflies, Nymphalis.antiopa, flying in winter are enjoyable because most other insects are absent. Overnight temperatures may drop below ten degrees, but when temperatures rise into the fifties only a few days later, these large butterflies can be seen flying in sunny glades.
Unlike most insects, this mourningcloak does not hibernate deeply in the winter. As temperatures drop, it seeks a sheltered spot underneath tree bark, in a woodpile, or beneath a rock and becomes torpid. But it can revive after only a few hours of warmth and fly out to look for water and nourishment.
Reduced temperatures and shortened day length trigger the butterflies' biological clocks to produce alternate forms of enzymes and antifreezes essential for winter survival.
They do Not Freeze When Temperatures Plummet
To prevent freezing solid, mourningcloaks produce antifreezes: sugars and sugar-derived alcohols. Glucose serves the mourningcloak two purposes: one as fuel, the second as antifreeze. The two functions are opposed because as the glucose is metabolized to produce energy, its antifreeze effectiveness is destroyed. To counter this loss of antifreeze protection, the butterflies produce sorbitol and propylene glycol: sugar-derived alcohols. These alcohols are metabolized slowly and are not degraded to provide energy. Thus, sorbitol and propylene glycol provide reliable protection against overnight freezing and the chilling of the sudden cold snaps frequently experienced during northeastern winters.
Glucose must be replenished from lipids (fats) to restore antifreeze protection and provide the energy to fly on warm days. As temperatures drop at the close of day, the efficiency of the butterfly's enzymes also drop and replenishing the sugars slows to low levels. As long as their hemolymph remains liquid, the butterfly's enzymes will continue to convert fats into sugars, but the rate slows as the temperature drops even though the winter forms of these enzymes operate at lower temperatures than their summer counterparts.
Mourningcloaks Switch From Immobile Torpor to Active Flying in a Few Short Hours
Insects cannot fly unless their muscle temperatures are over 95 degrees Fahrenheit, relying primarily on external sources of energy to raise their muscle temperatures above air temperature. The butterflies orient their bodies parallel to the sun's rays and hold their wings at about a 45 angle to the body. This behavior reflects light onto the body allowing a large amount of heat to be absorbed by the dark surfaces of the wings and body. This heat is transported to the wing muscles in the thorax.
A second method of increasing temperature is by slowly pumping or vibrating their wings. As autumn nights get longer, the butterflies' thoracic muscles produce brown fat and alternative forms of enzymes that function at lower temperatures. By first vibrating, then slowly pumping their wings, these butterflies can increase their thoracic temperature twenty to thirty degrees above that possible by absorption of heat alone.
The combination of absorbed heat and the contraction of muscles raises the temperature of the muscles enough to allow flight. Although the ability of a mourningcloak to raise its body temperatures by forty degrees seems like an amazing feat, white Apollo butterflies, Parnassius apollo, fly in the Colorado Rockies after summer snowstorms when temperatures are just above freezing. These montaine butterflies warm their muscles almost sixty degrees Fahrenheit ( Celsius) above ambient air temperatures.
What Are They Doing the Rest of the Year?
The mourningcloak lives longer than any other local butterfly. Although late summer monarchs will live nine months from August until May, migrating to Mexico and partially back; a mourningcloak emerging in June or July will live through the winter until the next May or June when it reproduces and dies: a total of eleven months as an adult butterfly. Adults feed on tree sap, rotting fruit and fungi, manure liquids, dead animal fluids, and only rarely drink flower nectar. If the summer is hot, the butterflies crawl beneath shelters and estivate (summer sleep). Although estivation is behaviorally similar to hibernation, the two resting periods are biochemically quite different as antifreezes are not produced prior to estivation. If an estivating summer butterfly is placed in a freezer, it will freeze to death because it has no antifreezes.
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