The University of Chicago Magazine

October-December 1996

The Strange Laboratory of Dr. LaBarbera


The other end of the size spectrum--the commonplace become gigantic--is much more the norm in monster movies. The archetype is, of course, King Kong. Of the many Kong movies, the best are the original (1933) with Fay Wray, the 1976 remake with Jeff Bridges and Jessica Lange, and a 1949 clone entitled Mighty Joe Young (whose special effects, by Ray Harryhausen, are breathtaking). Yet all underestimate the vulnerability of large animals.

Take the climactic scene from the 1976 remake, where Kong lies in the streets of Manhattan after his fall from the World Trade Center, and Jessica Lange comes to bid him a tearful goodbye. Remember, the tiny humans in Dr. Cyclops were invulnerable in a fall because they were small; larger animals will have higher terminal velocities and greater kinetic energies on impact. British geneticist J.B.S. Haldane describes it in his classic essay, "On Being the Right Size": "You can drop a mouse down a thousand-yard mine shaft; and, on arriving on the bottom, it gets a slight shock and walks away....A rat is killed, a man broken, a horse splashes."

Haldane was being quite literal. In fact, our ancestors used this aspect of scaling to gruesome effect--a common strategy during medieval sieges was to take a carcass of a horse, let it ripen for a few days in the sun, and then catapult it over the walls of the besieged town. On impact, the carcass would indeed splash, spreading contagion throughout. The same would have been true (but more so) of Kong after his fall from the heights: Pink mush would have covered the streets of Manhattan.

A second, more subtle, problem pervades the Kong movies. There is a maximum mechanical stress, or force-per-unit area, that a bone--or any other material object--can withstand. The load the bone must bear should be proportional to the animal's mass. Now, with an increase in size but no change in shape, the load on the bone will increase in proportion to the increase in mass (length cubed), but the bone's cross-sectional area will only increase as length squared. Eventually, the animal's bones will break under its own weight.

One way around this problem is to change the bones' shape as size increases, so that the cross-sectional area better follows the increase in the animal's mass. This is a widespread trend in biology--larger animals have proportionally stouter, thicker bones. Compare the skeletons of a cat and lion or those of a deer and moose. This observation is not exactly hot news. Both the trend and a fundamentally correct explanation for it were given by Galileo in 1638.

Andrew Biewener, a professor in Chicago's organismal biology & anatomy department, has revisited this question, with surprising results. At least for the long bones in the limbs of mammals, the changes in shape that accompany evolutionary changes in size are not sufficient to compensate for the increased loads. Since all bone has virtually the same breaking stress, this implies that larger animals increasingly push the limits of their skeletons' strength.

However, Biewener's direct measurements of bone deformations as an animal walks or runs show that the safety factor (the ratio of breaking stress to working stress) only ranges from three to five. This is remarkably risky design--most things that humans build have safety factors from ten to several hundred. Biewener has looked at animals from chipmunks to elephants and finds that the safety factor is constant across this 25,000-fold size range. That constancy is achieved by a combination of the shape changes described by Galileo and by adaptations in the animals' behavior, particularly adjustments in posture to insure that the loads the bone must bear are directed to minimize bending (mechanically the worst loading mode).

Back to King Kong. At the start of the movie, Kong is about 22 feet tall (I took measurements off stills), but by the time he climbs the Empire State Building, he appears to be 50 percent bigger, presumably because he was allowed bananas ad libitum.

Even at 22 feet tall, Kong is four to five times the size of your garden-variety lowland gorilla. A fivefold increase in height implies a 25-fold increase in bone cross-sectional area and a 125-fold increase in body mass; the stress on Kong's bones thus should be about five times greater than the stress on a normal gorilla's bones. But, according to Andy Biewener's data, a safety factor of five is extreme for mammals; Kong's excessive body size should have exhausted the safety factor. True, Kong stands a bit straighter than the average gorilla so he may gain a bit back, but it's clear that he's pushing the envelope. Is that why he has such a short fuse and is always rushing around, roaring and bashing things? Not only does he continually run the risk of breaking his legs, undoubtedly his feet hurt.

Continue reading, "The Strange Laboratory of Dr. LaBarbera"

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