Sunday, May 10, 2009

It's too darn hot!

This past week we made it over 100 F in Tucson for the first time this year. The local meteorologists affectionately refer to the event as “breaking the ice.” The average first one hundred degree day is about May 28, so we were twenty-five degrees above average for at least a day or two. No matter, it doesn’t have to be even that hot before cold-blooded animals (scientists call them “ectotherms”) start feeling uncomfortable. You can often tell when an animal is heat-stressed by the body posture it adopts. It is all done in the name of “thermoregulation,” these behaviors designed for cooling off.

Lizards enjoy basking on large rocks in the morning, but before long the surface gets too warm for their liking. This eastern collared lizard is literally “back on her heals,” pushing her body as far away from the hot rock as she can, and lifting her hind toes to keep from singeing them. Male lizards will do “push-ups” as part of a territorial display, but will not stay propped up for as long as a lizard under heat stress.

Even insects will hot-foot it when need be. I spied this grasshopper, Psoloessa texana, on the sidewalk outside my apartment building. Notice how it has lifted its hind feet off the pavement entirely. I have seen other grasshopper species lift their feet alternately to cool them off briefly while enduring the heat with the opposite appendage. I just about scorched myself while capturing this image, lying on my belly on the hot concrete!

Dragonflies perform an even more contortionistic maneuver called “obelisking,” whereby they orient their abdomen to expose as little body surface to the direct sun as possible.
This gray sanddragon, Progomphus borealis, is pointing itself straight up in the air, at around noon on a hot day in Sabino Canyon. Quite the dramatic angle, literally.

There are endless ways in which reptiles and insects cope with both heat and cold. An excellent popular summary of insect behaviors can be found in the book The Thermal Warriors: Strategies of Insects Survival, by Bernd Heinrich (Harvard University Press, 1996). A slightly more technical reference is Biology of Desert Invertebrates, by Clifford S. Crawford (Springer-Verlag, Berlin, Heidelberg, and New York, 1981). Children might enjoy the article I did for Ranger Rick nature magazine, entitled “Dune ‘Buggies’ and Other Desert Survivors,” in the September, 2000 issue (vol. 34, no. 9, pp. 14-21).

All this talk about heat making you hot? Let’s go grab a cold one at an air-conditioned watering hole, whadda ya say?

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