Sizing Things Up

By Eric M. Jones

MY FATHER ONCE REMARKED that if domestic cats were merely 20 percent bigger, they could—and occasionally certainly would—kill small children. After a long pause he mused, “Maybe that's why humans won’t let them get any bigger.”

I would guess he is right. House cats aren’t going to get bigger.

But many people cling to the belief that humans themselves are getting bigger and bigger, and that this is due to some nearly-real-time-observable evolutionary force at work. They look at their humongous cheeseburger-fed “Bubba” and compare him to their tiny great-grandparents who grew up in war torn Eastern Europe or wherever and conclude, “Kids are getting bigger!”

But this is a common misinterpretation of the facts. Any creature’s genetics determine its mature size when environmental conditions are optimum. If the creatures are able to breed, they pass on their genetic blueprints, not the consequences of their environmental fortunes—or misfortunes.

“Lamarckian Inheritance” claims that Bubba’s “soft” genetics will pass his bulk onto to his kids. Of course, the great-grandparents had 18 kids and Bubba can’t get a date—so even if it did work that way it wouldn’t work that way.

It turns out that any genetic strain of humans has maintained nearly the same height over millennia except when starvation, wars, disease, pestilence, famines, ice ages, etc., limit their size. Then they tend to expand to their original design-size as soon as conditions allow. But humans just aren’t changing size otherwise.

“But ...” they respond, “Old cabins and houses were built very small.”

A false clue, Watson! When you had to build your own housing ... and quickly, a few square meters would do the job. You’d do the same today. And when the main concern was heating it and keeping your stuff secure from ravaging bears and other humans, size mattered—and smaller was better. Besides, the National Building Codes weren’t around.

Many unseen forces determine the size of things and sometimes mysteriously cause sizes to change: Most medieval knights were hulking brutes of over 1.8 meters tall and powerfully built. Their suits of armor were extra-extra-large. But the suits of armor that we see today in museums are quite small because the smaller ones were convenient to save and easy to ship compared to the larger sizes. The ones that got here in someone’s luggage are considerably smaller, and were more likely to be decorative from the start. This effect can be seen by comparing the sizes of existing suits of armor in European museums with those in the U.S. Little knights weren’t likely to be combatants; they just stood around and looked cool.

This is hardly a surprise. Antique dealers note that the farther west an antique travels, the smaller it seems to become. The really big cabinets and tables rarely get very far from where they started. Years ago there were places in Montana, Wyoming, Colorado and New Mexico where large tables, pianos, stoves and furniture of all sorts could be found half-buried in the ground because they had been abandoned by settlers heading west in over-loaded Conestoga wagons. The smaller stuff continued on to the west. So they started out with big stuff and arrived in California with small stuff. It was like magic.

I have heard that Pekingese dogs once commonly weighed a hundred kilograms and were used to guard Chinese temples—and they just got smaller over time through selective breeding. But this would have required real genetic changes. Your little “Foo-Foo” can’t grow big enough to guard your house, and in fact its forbearers were the same size in the time of the ancient Chinese emperors. Curiously though, Pekingese dogs, despite their small size seem to be the domesticated dogs that are the most closely connected by their DNA to wolves.

In 1926, J. B. S. Haldane wrote a marvelous essay entitled “On Being the Right Size.” He discussed the critical principle that doubling the linear size squares the surface area and cubes the volume. This explains everything from why elephants can’t jump over fences (because as an animal’s dimensions increase, the mass is cubed while the strength of its bones—being proportional to the cross-sectional area of the bones—is only squared) to why a flea can’t be the size of a man, and myriad other observable phenomena, such as why Olympic gymnasts are petite and why babies are in danger from hypothermia. If a baby is 1/3 your height, it has 1/9 your surface area and 1/27 your mass. So the baby has 1/3 the mass-to-surface-area ratio of an adult. Since mass (or volume) is where the body's heat is generated and surface area—the skin—is where it is lost, a baby needs to be fed lots of fat and kept warm or it will easily die of hypothermia.

The applications of these principles are vast and far-reaching. They explain why there aren't any animals smaller than penguins in the Antarctic: Small animals have too small a volume per surface area. (By the way, if there were penguins in the Arctic, polar bears would have eaten them, or vice versa.)

Humans are quite restricted in size: Yeah well, there are legends and myths of “giants” three meters tall. But the functional fall-off for humans occurs very rapidly over two meters. The tallest NBA player was 2.33 meters but the tallest good NBA player is only 2.25. The truth is—no human is more than a cripple at 2.5 meters tall. As size increases, the mass increases faster than the strength of the bones needed to carry it.

Goliath (1 Samuel 17:4: “Then a champion came out from the camp of the Philistines. His name was Goliath ...”) was most probably about 2.1 meters tall (and one would guess 125 kilograms). To be fair, David was just a young boy, probably 1.5 meters tall and 45 kg soaking wet. Even today Goliath would be considered a very big guy.

So there are forces at work that affect the sizes of creatures big and small. But this too is often misrepresented. One often hears the pronunciamento that high-gravity planets would be populated only by lumbering turtle-like life forms. Yet on planet Earth, land creatures have adapted from springboks which can run 100 km/h and can leap 4 m straight up and jump a horizontal distance of 15 m, to huge tortoises to elephants to spider monkeys. In the distant past, land animals were the size of ... well, dinosaurs. (And the reason for their size might have been forage of low quality, requiring giant guts, requiring giant predators. But it is more likely that since the oxygen level was 9 percent higher than it is today and the insects, too, were giants, that the oxygen level greatly determines the size of creatures.)

In the oceans, the ratio of creatures’ sizes is even more extreme. Let’s not forget that flying or swimming is the most common form of locomotion for Earth creatures, and while elephants don’t fly, blue whales sort of do.

Atmospheric density and gravitational forces have some reasonable correlation (despite Venus being the exception that proves the rule), but the size constraints for flying creatures imposed by gravity still seem small. In the air, hummingbirds the size of peanuts migrate thousands of kilometers. Other birds have evolved to be bigger than small aircraft. It turns out that atmospheric density only influences the lower-speed limit for flight. In extremely thin air, creatures of any size can fly if they fly fast. Creatures also employ a dozen different techniques to fly. We might someday discover alien flying creatures of almost any size, regardless of a planet’s gravity.

So if intelligent extraterrestrials could fly naturally (and why not?), would they want to build some starship (in which they couldn’t fly) to travel to some other world where they couldn’t—for whatever reason—fly? 

Eric M. Jones is the Contributing Editor of “Perihelion.” He is an engineer, designer, consultant, and entrepreneur, currently working in his Internet business PerihelionDesign, designing, building and selling unique products, parts and materials for people in the home-built experimental aircraft community.