Speedup

Physiologists S. W. Britton and R. F. Kline used to sit in their laboratory at the University of Virginia and wonder why the sloth is so slothful. As good Darwinians they realized that the basic reason for the slothfulness of the sloth is that he is beautifully adapted to his environment. He hides from his enemies instead of fleeing; being a vegetarian, he does not have to chase his food. But other animals have been known to alter their innate behavior because of outside influences.* Why not the sloth?

Physiologists Britton and Kline went down to Panama, collected a few sloths (which are fairly tame and amenable) and got to work. First, they clocked the animals' normal progress along the underside of a horizontal pole. Speed of a two-toed sloth: a third of a mile an hour. Speed of a three-toed sloth: two-ninths of a mile an hour.

Sloths are not only undermuscled for their weight but also have an uncommonly low temperature. So Scientists Britton and Kline left their sloths out in the tropical sunshine long enough to raise their temperatures by five or six degrees, and the change was miraculous: they moved 50% faster. Similar speedups were also obtained by injections of adrenalin and prostigmin (an intestinal stimulant), and by scaring them. Subjected to such speedup techniques as this, the Virginia physiologists were pleased to report in Science last week that one thoroughly stimulated sloth hustled along the pole at the relatively dizzy pace of one mile an hour.

*Harvard's famed Astronomer Harlow Shapley, during a brief excursion into entomology many years ago, discovered that the hotter an ant was, the faster it ran--in fact, that you could tell the temperature of an ant by its rate of movement.

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