Saturday, December 22, 2012

Monarchs of Winter

I have been reading Winter World, by Bernd Heinrich.  It is a broad discussion, both scientific and poetic, of the ways creatures survive winter.  He covers not just hibernation and migration, but explores torpor, supercooling and animals' use of endogenous antifreeze.  It is a complex subject made clear and entertaining.

Monarch Watch
Typical of the new ideas he introduced me to was the nature of the migration of monarch butterflies.  Their wondrous sojourn for thousands of miles to a patch of mountain forest in Mexico is familiar to most of us.  How their great-grandparents, who left the area the previous spring transmit the location to them three generations later remains a mystery.  (Hint: it isn't Apple Maps or even Google.)

I had always assumed that they headed south to seek warmth, but this is only partially true.   Heinrich explains that migration, like hibernation, is more a matter of preserving energy while food resources aren't available.  They mass on the Mexican trees, hanging out for three months without eating, burning their fat much like a hibernating mammal.  Their stored energy as fat must last until food is again available.

Here is where temperature comes in.  They actually require cool temperatures rather than warmth. The area where most of our monarch population overwinters is above 10,000 feet where the temperature is close to but above freezing.  As described by Monarch Watch, they cluster together in a very specific ecosystem, on specific tree species on steep southwest-facing slopes.  In warm spells they may fly off to get a drink but otherwise they rest, conserving their precious energy.

Clustered Monarchs- Wikimedia
Temperature is critical, for reasons similar to the cooling we use in some heart surgery. Cool tissue uses less energy.  At 15 degrees centigrade (59 Fahrenheit) their energy stores will last 3 months, at 30 degrees centigrade, less than 10 days!  Activity such as flight further depletes their stores.

Knowledge of the details of migration is relatively recent.  Their lifecycle was proven by chemical analysis of their cardenolides, the toxic substance the larvae acquire from feeding on milkweed.  There are differences in the "chemical fingerprint" of the cardenolides.  Studies showed that the chemical found in wintering butterflies and those first heading north from Mexico was distinctive, produced only in milkweed plants from the Northern US and Canada.  Their wintering grounds weren't even discovered until 1975. 

Their spring migration northward is timed to the growth of milkweed food plants which their larvae require to develop.  Within a short time the eggs hatch, the caterpillars go through their instars and enclose (form a chrysalis).  When the adult emerges, it heads north to the next range of emerging milkweed, a procession not unlike the giant combines that follow the wheat harvest in the summer.

Come to think of it, we aren't so different as we adjust our thermostats and throw another log on the fire, seeking not warmth, but the perfect temperature to conserve our energy for the start of spring.

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