Edward Willett

Movie monster biology

Not long ago I wrote an article emphasizing that science fiction is, first and foremost, fiction, and that a little fudging of the science for the sake of the story is expected and accepted.

Having said that, however, I must also admit that nothing warms the cockles of my heart (what exactly is a cockle, anyway?) like a big juicy article pointing out silly science in movies, and that’s just what Michael C. LaBarbera, a biology professor at the University of Chicago, has provided.

His article (much too long to go through in detail: you can read the whole thing here) starts off with a discussion of size.

Giant critters have been a mainstay of monster movies since the original King Kong: giant ants, giant monkeys, giant reptiles, giant octopuses, you name it. But all of these giant creatures face a big (sorry) problem, based in simple geometry: areas are all proportional to some measure of length squared, but volumes are proportional to some measure of length cubed.

What that means is that if you make something twice as tall as it normally is, its surface area doesn’t double, it quadruples—and its volume doesn’t just quadruple, it octuples.

Make a monkey the size of the original King Kong, and you don’t get an animal that can bounce around and smash things and climb the Empire State Building with a screaming woman in its paw: you get an animal that can’t take a step without risking a broken bone. And talk about fallen arches…!

Shrinking things is no more practical. Consider The Incredible Shrinking Man. He stops shrinking when he is about an inch tall, so he’s been reduced by a factor of about 70. That means the surface area of his skin has been reduced 70 X 70, or 4,900 times, while his mass has been reduced 70 X 70 X 70, or 343,000, times.

We lose heat through our skin; body mass helps us generate and keep it. The Incredible Shrunken Man is going to have a hard time keeping his temperature up with his body mass having been reduced so much more, proportionally, than his surface area. In fact, he’ll probably have to eat 24 hours a day, and if he falls asleep, he may well starve to death before morning.

He’ll also need to drink a lot to make up for moisture lost through his relatively large surface area—but he’s going to face another problem: he can’t just scoop out a handful of water from a drop because surface tension is as strong as gravity at his size. He’ll have to lift an entire drop to his lips, and he’d better judge the size carefully, because surface tension will force it down his throat whether he likes it or not in the same way it draws water up the stems of plants.

On the plus side, he’ll be much, much stronger proportionally. In the movie, he fights a spider using a needle as a sword. He struggles to lift the needle, but in fact he should be able to wield it with ease–is relative strength would have increased 70 fold because the amount of force a muscle can produce is proportional to its cross-sectional area (length squared) while body mass is proportional to volume (length cubed).

So much for giant monkeys and tiny people. What about giant insects?

Alas, they don’t fare any better. They have to worry about their legs buckling, because their legs are essentially just thin-walled tubes. Lengthen the tubes, and the length, diameter and thickness of the exoskeleton go up by the square of the extra length—but again, body mass goes up by the cube of that additional length. A good hard blow to the side of one leg should cause it to buckle like a drinking straw.

Just to be fair, LaBarbera also writes about what movies got right, praising in particular the dinosaurs in Jurassic Park (although he does have one or two nits to pick there, too).

He notes he went to see the film with a paleontologist colleague: “As the rest of the audience cringed and shrieked, my colleague and I excitedly whispered comments to each other—‘All right! Classic large predator behavior patterns!’, ‘Look at that! They got the bipedal kinematics just right!’ Folks in adjacent seats were not amused.”

Perhaps not. But I wish I’d been there to hear what they had to say.

With so much bad science in movies, it’s always a thrill to see something gotten right.

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