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Desert Diary
Biology/Surface:Volume Relationships


There's a story sometimes used to illustrate the relationship between surface area and volume--a relationship of importance to organisms. The tale goes like this: drop a mouse from a 10-story building, and it'll hit the ground and scamper off. Drop a cat, and it'll lay there stunned and finally stagger off. Drop a horse, and you have one huge splatter. Rather macabre, but makes the point that as size increases, the volume, or weight, of a body increases much more rapidly than the area of its surface. Air friction efficiently slows down a mouse with its relatively large surface area compared to its weight. By the time you get to the horse, air friction can hardly affect it, and the horse accelerates almost as if in a vacuum.

The relationship is of crucial importance to desert mammals. Activity during the day requires a large surface area to eliminate excess body heat. It's not an accident that humans, with their relatively large volume and small surface area, quickly adopted the siesta during the heat of the day.
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Contributor: Arthur H. Harris, Laboratory for Environmental Biology, Centennial Museum, University of Texas at El Paso.

Desert Diary is a joint production of the Centennial Museum and KTEP National Public Radio at the University of Texas at El Paso.