Science Behind the Fiction: How big could a gorilla grow before exploding?

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Apr 10, 2018

If you were a child in the decades leading up to the millennium, it's likely you played at least one version of Rampage growing up. It wasn't the most impressive game in terms of story. It didn't break new ground or have a lasting impact on the legacy of video games as a medium, but there was something so satisfying about taking on the role of a giant monster and laying waste to a city.

The first incarnation of Rampage was released in 1986 by Midway Games and was exclusive to arcades. In it, you could play as one of three characters: George the gorilla, Lizzie the lizard, and Ralph the werewolf, all of whom began life as ordinary human beings until a variety of circumstances changed them into giant animal monsters.

Over 30 years and many sequels since its initial release, director Brad Peyton (San Andreas) was tapped to bring a big screen version of the game to life, with Dwayne "The Rock" Johnson starring. And why not, in a world where Battleship, one of the most story-void board games of all time, gets a movie, why not this?

Johnson plays primatologist Davis Okoye, the head of an anti-poaching group who shares a special bond with an albino silverback gorilla named George. Missing from this adaptation is any transformation from human to non-human. All three of the video game characters, George, Ralph, and Lizzie, are present in mutant form.

It's obvious, after all, that transforming human beings into mutated anthropomorphic animals is ridiculous (nobody tell Bebop and Rocksteady). How far afield is the rest of the premise of Rampage? Let's break down what we know about the film and find out.

What's happening to my friend?

A genetic editing company called Engyne (never trust a company with any variation of that name to do any sort of genetics work. Jurassic Park, anyone?) has developed a serum promising to offer incredible speed and strength to any organism that takes it, undoubtedly to be used for military applications. According to the film's trailer, George grows from seven feet and 500 pounds, to "nine feet and pushing 1000" overnight.

Here's where we hit our first real stumbling block. While genetic manipulation could hypothetically provide the necessary template for a larger body, the movie expects us to accept that George has doubled in total weight overnight. That mass can't just come from nowhere. Changes in DNA alone can't explain that sort of rapid growth.

The diet of the silverback gorilla is primarily plant-based. While they are capable of eating meat, usually insects or small animals, the majority of their diet consists of plants, like bamboo, that are easily attainable in their environment. Owing to their large size and the relatively low caloric value of plants like bamboo, the average gorilla must consume 60 pounds of vegetation each day, and that's just to maintain a healthy weight. While the specifics of weight gain and weight loss aren't entirely understood, the common rule of thumb is that a pound equates to 3,500 calories, which is good enough for our purposes.

In order to gain 500 pounds, not taking into account all of the biological processes required for transforming those raw calories into all of the various body tissues, George would have to consume 1,750,000 calories.

One pound of raw bamboo contains approximately 122.5 calories. In order to achieve the one-and-three-quarter-million calories needed for a 500-pound weight gain, George would have to consume more than 14,000 pounds of bamboo. And this doesn't take into account the increased metabolic cost of George's growing body. As he grows, the calories needed to sustain his size increases in kind. Which is to say the 14,000-pound estimate is incredibly conservative. It's more likely, in a mad dash to placate an insatiable hunger, Davis Okoye would discover his best friend burst at the middle, partially digested vegetation spilling out all around him. That sounds like just about the least fun movie-going experience ever.

Let's give George the benefit of the doubt and assume he was able to find and eat the requisite bamboo, ants, or small animals to sustain his runaway growth. What impact would this rapid expansion have on his body and mobility?

Gorillileo Gorillilei and the Square-cube Law

In 1638, Gallileo published his final book, Discourses and Mathematical Demonstrations Relating to Two New Sciences. In it, he described what's come to be known as the square-cube law. Simply put, it states that whenever an object increases in size, it's surface area increases proportional to the square of the multiplier while the volume increases proportional to the cube of the multiplier.

This is best illustrated by visualizing a cube. Supposing a cube with sides measuring one centimeter, we would measure surface area of one face of the cube at one centimeter square, while the volume is one cubic centimeter. Doubling that cube in size results in a face with a surface area of four square centimeters and a volume of eight cubic centimeters. Tripling the original cube results in a face with a surface area of nine square centimeters and a volume of twenty-seven cubic centimeters. In short, the relationship between surface area and volume becomes more strained, the larger an object becomes.

Tripling the size results in an increase in surface area of nine, but an increase in volume of twenty-seven. The relationship between surface area and volume becomes more strained, the larger an object becomes. This means that as an object grows, the pressure put on a cross section of that object increases exponentially.

The square-cube law has many real-world applications. It's used in engineering calculations to determine the upper limit of structures before they collapse under their own weight and it impacts biomechanics in the same way. There is a reason that the build of small animals like insects is drastically different from large animals like elephants. An ant, for instance, scaled to the size of an elephant not only wouldn't be able to take a single step, it's body would shatter under its own pressure, leaving it an easy meal for the ever-growing George. The good news is, we have little to fear of an attack from fifty-foot spiders. It seems there is some justice in the universe.

There aren't any official numbers on George's final size but one scene present in the trailer shows George in the city, standing at roughly the same size as a three-story building. A conservative estimate puts him at 30 feet, on par with the "30-foot wolf" mentioned by Jeffrey Dean Morgan's character. Let's do some math.

Knowing that George was originally seven feet tall and 500 pounds, and assuming a consistent body ratio, a thirty foot height gives us a multiplier of 4.285. But we can't just multiply the weight by that number. Volume is cubed, remember. So we multiply by that number three times, giving George an estimated weight of 39,339 pounds, roughly 20 tons. That's one seriously massive gorilla.

When it comes to load bearing, the majority of George's weight will be supported by his femur's. A comparison of human and gorilla skeletal structures reveals moderately similar femur bones in terms of length and width.

It's estimated that a cubic centimeter of bone can withstand 19,000 pounds of compressive force. Well beneath George's estimated weight. Back of the napkin math suggests that the largest George could possibly get before the standing weight of his own body would crush his femur bones is 23 and a half feet. At that height, his estimated weight would come in at 18,916 pounds. And that's just standing, to say nothing of walking, jumping, or leaping from a tall building.

Not to mention that's the upper limit of strength for compressive force. A sidelong attack to the leg from a charging opponent would snap the bone with much less force. The truth of the matter is that a battle between animals of even half the movie's suggested size would likely result in a blubbering puddle of fleshy weeping as soon as the first aggressor made contact.

Of course the Wolf Flies

In the second trailer for Rampage, we see that the wolf, Ralph, flies. It's a fun visual and adds an interesting element to an already bombastic movie premise.

Of course, flying mammals aren't unheard of. While bats are the only known mammal with the ability to fly, they make up wholly one-fifth of all mammal species. So, while they may seem a rarity, in terms of pure numbers, the ability to fly is a relatively common mammalian trait.

The largest flying mammals are the Australian flying foxes, with wingspans of approximately five feet. Such massive wingspans are necessary to account for the flying foxes weight, coming in at roughly two and a half pounds.

Wing load is a measurement that calculates total weight against the surface area of the wing. The larger the body mass to wing ratio, the higher the wing load and vise versa. The estimated upper limit of wing load for effective flight is 25 kg/m2.

The average male wolf comes in at between 3.4 and 5.2 feet and 66 to 180 pounds. Assuming an average for Ralph, we'll put him at 4.3 feet and 123 pounds. Expanding him to 30 feet gives us a multiplier of 6.976 (don't ask why the serum has a greater impact on wolves than gorillas). This gives us an estimated weight of 41,756 pounds or 18,940 kilograms.

In order to achieve flight, Ralph would need to have wings of at least 757.6 square meters or a wingspan or roughly 90 feet, roughly three times his length, minimum. The wings as portrayed in the film would do little more than perforate the first time they were deployed.

Suffice it to say, if you're seeing Rampage in theaters you'll want to turn off your brain and let the charm of Dwayne Johnson carry you away to the land of impossible adventure.

Rampage hits theaters on April 13.