How Strong Is a Chimpanzee?

The bone-crushing power of the apes has been greatly exaggerated.

Travis the Chimp in an Old Navy commercial

After last week’s chimpanzee attack in Connecticut, in which an animal named Travis tore off the face of a middle-aged woman, primate experts interviewed by the media repeated an old statistic: Chimpanzees are five to eight times stronger than people. The literature—or at least 19th-century literature—concurs: Edgar Allan Poe’s fictional orangutan was able to hurl bodies and pull off scalps. Edgar Rice Burroughs’ fictional anthropoid apes were likewise possessed of remarkable strength. Even Jules Verne’s gentle ape, Jupiter, had the muscle to drag a stuck wagon from the mire.

Pulled scalps? Unstuck wagons?  No doubt, chimpanzees are different from us. Their climbing lifestyle accentuates the need for arm strength. A chimp on four legs can easily outrun a world-class human sprinter. But it sounds extreme to suggest that humans are only an eighth as strong as chimpanzees. Consider that a large human can bench-press 250 pounds. If the “five to eight times” figure were true, that would make a large chimpanzee capable of bench-pressing 1 ton. It’s just the sort of factoid the zoo staff might tell you to keep you from knocking on the glass.

The suspicious claim seems to have originated in a flapper-era study conducted by a biologist named John Bauman. Poe’s story of the scalp-pulling orangutan struck Bauman as being “grotesquely impossible.” In 1923, he noted that every expert in the field believed apes were vastly stronger than humans—yet none had ever tried to prove it. So he packed up a device used to measure pull strength, called a dynamometer, and set out for the Bronx Zoo.

The apes were less-than-willing participants in the study. They were more apt to tear apart the shiny dynamometer than pull on it, and, unless the ape had a “distinctly vicious disposition,” she was unlikely to approach the experimental task with much vigor. Bauman managed to rig his device outside the cage, feeding in a rope for the apes to work on. Then, amazingly, one of the Bronx chimpanzees—a former circus ape named Suzette—managed to pull 1,260 pounds.

Bauman took his study on the road, attempting tests at the Philadelphia Zoo and making inquiries as far afield as Chicago and Cincinnati. In 1926, he returned to the Bronx Zoo, successfully testing the largest chimpanzee then in captivity. That animal, named Boma, pulled 847 pounds one-handed.

How did that compare with humans? As a college teacher in South Dakota, Bauman did what any good scientist would do: He recruited the football team as research subjects. He found that not one of his “husky lads” could pull more than 500 pounds with both hands, and only one had a one-handed pull above 200. What’s more, the football players were free to use the dynamometer as they wished, while the chimpanzees had been forced to pull the apparatus from a clumsy posture in their cages. It appeared that chimpanzees really could be more than five times stronger than humans.

Thus the number entered the anthropology textbooks and made its way into the talking points of recent primatologists like Jane Goodall and Sue Savage-Rumbaugh.

But the “five times” figure was refuted 20 years after Bauman’s experiments. In 1943, Glen Finch of the Yale primate laboratory rigged an apparatus to test the arm strength of eight captive chimpanzees. An adult male chimp, he found, pulled about the same weight as an adult man. Once he’d corrected the measurement for their smaller body sizes, chimpanzees did turn out to be stronger than humans—but not by a factor of five or anything close to it.

Repeated tests in the 1960s confirmed this basic picture. A chimpanzee had, pound for pound, as much as twice the strength of a human when it came to pulling weights. The apes beat us in leg strength, too, despite our reliance on our legs for locomotion. A 2006 study found that bonobos can jump one-third higher than top-level human athletes, and bonobo legs generate as much force as humans nearly two times heavier.

So the figures quoted by primate experts are a little exaggerated. But it is a fact that chimpanzees and other apes are stronger than humans. How did we get to be the weaklings of the primate order? Our overall body architecture makes a difference: Even though chimpanzees weigh less than humans, more of their mass is concentrated in their powerful arms. But a more important factor seems to be the structure of the muscles themselves. A chimpanzee’s skeletal muscle has longer fibers than the human equivalent and can generate twice the work output over a wider range of motion. In the past few years, geneticists have identified the loci for some of these anatomical differences. One gene, for example, called MYH16, contributes to the development of large jaw muscles in other apes. In humans, MYH16 has been deactivated. (Puny jaws have marked our lineage for as least 2 million years.) Many people have also lost another muscle-related gene called ACTN3. People with two working versions of this gene are overrepresented among elite sprinters while those with the nonworking version are overrepresented among endurance runners. Chimpanzees and all other nonhuman primates have only the working version; in other words, they’re on the powerful, “sprinter” end of the spectrum.

We’re still left to wonder how Bauman managed to be so far off in his calculations. The biologist himself thought that his subjects’ agitation contributed to their exceptional pulls—like an adrenaline-charged mother lifting a bus off her newborn. Later scientists tended to focus on his clumsy measurement procedure. In any case, a modern and accurate comparison of human and chimpanzee strength still has meaning for scientists. By studying the evolutionary changes that made us so much wimpier than our cousins, we may be able to develop new approaches for the treatment of human muscle disorders. We won’t be infusing the elderly with chimpanzee strength any time soon, but a little boost here and there for those who need it? That’s hardly science fiction.

Slate V: Can apes really talk?