How Big Can a Land Animal Get?

Imagine taking a helicopter to an uncharted island, only to be ambushed by a massive ape-like creature standing more than 100 feet tall and weighing 158 tons. With shocking strength, this simian foe sends a tree trunk right through your chopper, before going on to crush, stomp and bellow his way through your friends for the next two hours. This is the plot of the movie Kong: Skull Island, a new take on the 80-year-old franchise based around the infamous King Kong.

Now, Skull Island never claims to hew to scientific accuracy. But we had to ask: Could a creature as large as this living skyscraper ever exist on our planet? Would it have the strength to crush helicopters in its hands, or would it merely collapse under its own weight?

To start, let's be clear that there's no way King Kong or any other gigantic ape is living somewhere undetected on Earth. "King Kong as shown in the movie probably isn't a physically viable organism," says Jonathan Payne, a paleobiologist at Stanford University who has done extensive research on how body size has evolved over the history of life. The main reasons: Gravity and biomechanics.

If you took an animal and blew it up in size, mathematics dictates that the creature's mass would increase cubically, or by a power of three. However, by the same ratio of size increase, the width of the creature's body, and thus its bones and muscles, would increase only by a power of two, says Payne. "As you get bigger you need to dedicate more and more of your body mass to your bones to support yourself," he says. 

That’s why you don't see creatures like daddy longlegs—those spider-like arachnids that appear in your bathroom and are usually no bigger than an inch long—clocking in at much larger sizes. "Their legs would shatter under their bodyweight," Payne says. (Shudder.)

Because of these laws, taking your typical 350-pound Western gorilla and simply scaling it up by a factor of 20 would be physically impossible; the resulting creature's skeleton and muscles wouldn't be able to support its mass. Larger animals need bigger and thicker limbs to hold themselves up, says University of New Mexico paleoecologist Felisa Smith, which makes it unlikely that any creature on land has ever exceeded 100 tons.

"Poor King Kong couldn't even roll over," says Smith—much less attack people and helicopters.

So it's no surprise that Earth's biggest terrestrial animals—elephants—today fall far short of King Kong size. African elephants, for instance, can reach about 13 feet tall and weigh up to 7.5 tons. In the past, however, life got far larger: Dinosaurs like the Titanosaur weighed in at nearly 80 tons—10 times larger than the African elephants of today, but still nowhere near as big as the fictional King Kong.

The reason has to do with the fact that dinosaurs were reptiles, and today we live in an age dominated by mammals. To maintain their higher body temperatures, warm-blooded mammals spend about 10 times more energy than cold-blooded reptiles do on their metabolisms. This is energy that a mammal can't devote to increasing its body size. So it makes sense that the largest mammals we know of are roughly one-tenth as large as the largest reptiles ever found, Smith says.

What about the blue whale, which is believed to be the largest animal to ever exist on Earth, weighing in at more than 200 tons? In water, the rules are different. Water's buoyancy helps support the bodies of sea creatures, taking some of the strain off their muscles and skeletons. Smith says blue whales could theoretically get even bigger than they are presently, but biologists believe that the relatively short gestation period of blue whales for their body size—just 11 months—limits their size.

(Similarly, it’s possible that on a planet with lower gravity than Earth’s, such as Mars, terrestrial creatures less encumbered by their loads could grow much larger.) 

But there's another major factor that limits an animal's size: food. A 158-ton ape is going to need a lot of food to support itself, and it is not likely to find that amount of food on Skull Island, unless helicopters full of tasty humans crash there regularly.

Usually, getting one’s hands on more food means having access to proportionally more territory, Smith says. Blue whales swim across ranges of thousands of miles to find krill to eat, and African elephants can cover up to 80 miles in a day looking for vegetation. Large animals tend to get smaller on islands to compensate for the fact that there are usually fewer potential food sources, Payne says, such as the extinct dwarf elephant species that once lived on islands in the Mediterranean Sea. So if anything, King Kong would more likely be a dwarf gorilla than a massive one.

What evolutionary pressures would make it more appealing to be a larger animal, given the obvious drawbacks? "There has to be a selective advantage for being bigger," Smith says. For example: not getting eaten. Since smaller animals are more easily picked off by predators, natural selection can drive a species to get bigger to help defend itself better. This can be a tradeoff, however, since larger animals tend to move a lot slower than smaller ones (see the above lesson on biomechanics).

Being a lot bigger also means you can get a lot more food, Payne says. The classic example is the giraffe, whose massive height allows it to reach vegetation that no other animal can. Similarly, blue whales can filter large amounts of water with their baleen teeth, which allows them to capture up to 8,000 pounds of finger-sized krill per day.

Let's face it: Scientifically speaking, King Kong may be as much a leap of imagination as Hollywood itself. But Payne isn't willing to fully rule out the possibility of life ever getting that large. "I don't like to ever say never on these things," he says. "Every time you think that life can't do something, it often figures out ways to do it … Life surprises us in all kinds of ways."

Editor's Note, March 22, 2017: This article initially misstated that increasing a creature's mass cubically would increase it by a factor of three. It has been corrected.

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Ben Panko is a staff writer for Smithsonian.com