Contrary to popular lore that portrays chimpanzees as having “super strength,” studies have only found modest differences with humans. But our closest relatives are slightly stronger by several measures, and now a study comparing the muscle fibers of different primates reveals a potential explanation: Humans may have traded strength for endurance, allowing us to travel farther for food.
To determine why chimpanzees are stronger than humans—at least on a pound-for-pound basis—Matthew O’Neill, an anatomy and evolution researcher at the University of Arizona College of Medicine in Phoenix, and colleagues biopsied the thigh and calf muscles of three chimps housed at the State University of New York at Stony Brook. They dissected the samples into individual fibers and stimulated them to figure out how much force they could generate. Comparing their measurements to known data from humans, the team found that, at the individual fiber level, muscle output was about the same.
Given that different fibers throughout the muscle might make a difference, the researchers conducted a more thorough analysis of tissue samples from pelvic and hind limb muscles of three chimpanzee cadavers from various zoos and research institutes around the United States. Previous studies in mammals have found that muscle composition between trunk, forelimb, and hind limb muscles is largely the same, O’Neill says, so he’s confident the samples are representative across most of the chimp’s musculature. The team used a technique called gel electrophoresis to break down the muscles into individual muscle fibers, and compared this breakdown to human muscle fiber data.
Muscle fibers mostly come in two flavors: myosin heavy chain (MHC) I, which are slow-twitch fibers, and MHC II, or fast- twitch fibers. The latter contract more quickly and generate more force in quick bursts, but fatigue more quickly than slow-twitch fibers. The researchers found that whereas human muscle contains, on average, about 70% slow-twitch fibers and 30% fast-twitch fibers, chimpanzee muscle is about 33% slow-twitch fibers and 66% fast-twitch fibers.
The team ran its data through a computer program that built virtual muscles corresponding to the fiber compositions of humans and chimps, then simulated how much power each muscle could theoretically generate during a single burst. The chimp muscle, they learned, was about 1.35 times more powerful than the human one, they report today in the Proceedings of the National Academy of Sciences.
When the researchers then looked at the muscle fiber breakdown in mammals such as mice, guinea pigs, cats, dogs, horses, lemurs, and macaques, they found that only two animals regularly had more slow-twitch fibers: a small, lethargic primate called the slow loris and humans.
O’Neill says though fast-twitch fibers might give chimps and other mammals an advantage during high-intensity strength tasks like lifting heavy rocks or climbing a tree, humans’ slow-twitch fibers are better suited for endurance tasks like distance running. The researchers propose that early hominins’ muscles gradually became dominated by slow-twitch fibers as they gave up arboreal life and adapted to traveling across long distances to hunt and forage. Another benefit of slow-twitch fibers is they consume less metabolic energy, he adds, potentially freeing the body to devote more resources to other adaptations, like bigger brains.
Anne Burrows, a biological anthropologist at Duquesne University in Pittsburgh, Pennsylvania, whose research focuses on primate biomechanics but was not involved with the work, says the study is well-designed and convincing. "Instead of thinking about the results as pointing to greater strength in chimpanzees, we might instead want to consider … what the greater percentage of slow-twitch fibers in humans means to our unique locomotion method, bipedalism," Burrows says. "I think that's the bigger story here."
Burrows does have reservations about the authors’ evolutionary arguments. “Before I fully buy into that interpretation, I would like to see data from upper limb musculature in chimpanzees and humans, and I would like data from gorillas and orangutans,” she says.
Adrienne Zihlman, an anthropologist at the University of California, Santa Cruz, is more skeptical still of the study’s evolutionary ramifications. There simply isn’t enough known about the musculature of early hominins to speculate about their muscle fiber distributions, she says, so linking slow-twitch fibers to human evolution is a stretch. “The muscle fiber finding is an interesting factoid, but the tale that they spin based on that just doesn’t come from their data.”