Monarchs and temperature

[Woody Woods]

In the exchange between Chip Taylor and Paul Cherubini, the possible impacts
of global warming on Monarch populations have been brought up mainly in
connection with the effects on host plant range. It might also be worth asking
how a long-term warming trend might affect the Monarch overwintering strategy
in Mexico. Ron Gatrelle's note about chilling butterflies for transportation
goes right to the core of that strategy-- the Monarchs essentially put
themselves "into the refrigerator" to conserve lipid reserves, which have to
last about three months. The suite of observed behaviors at the overwintering
sites, as well as the conditions offered by the sites themselves,are all part
of balancing a fragile energy budget, and success depends upon staying cold,
but not TOO cold (see Masters, Malcolm and Brower, 1988, Ecology 69:458). The
energetic margin of error is small enough that many run out of lipid reserves
anyway-- those that began their migration a little too early or late (Gibo and
McCurdy 1995, I think-- haven't got it handy) arrive with less fuel in the
tank. The point is that this is a complex and energetically delicate game
plan, and that gradual increases in overwintering site temperatures night
increase the average "idling speed" and therefore fuel consumption of the
butterflies. Maybe they could compensate behaviorally, maybe not. I think it's
worth asking, though, whether a long-term warming trend is a greater threat
down the road than the occasional exceptional winters Brower has reported. 

Woody Woods
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William A. Woods Jr.
Department of Biology
University of Massachusetts Boston
100 Morrissey Blvd                      Lab: 617-287-6642
Boston, MA 02125                        Fax: 617-287-6650
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