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Gary B. Gillis

It was in high school that I first learned of the remarkable genetic similarity between humans and chimpanzees: we share around 98–99% of our DNA with our nearest great-ape relatives. With the recent sequencing of the chimpanzee genome, it is exciting to think that we might soon understand how these relatively small differences between human and chimp DNA translate into rather notable differences in our respective phenotypes. Melanie Scholz of Vrije University and colleages from Amsterdam and Antwerp recently focussed on one currently underappreciated phenotypic difference between chimpanzees and humans: the incredible strength of chimps.

Anecdotal and scientific evidence indicate that humans have inferior strength to chimps, and most would be hard-pressed to win a rope-tugging contest against a chimp half their size. Since chimps aren't overly endowed with muscle mass, Scholz and her colleagues wondered if there might be something special about the intrinsic properties of chimpanzee muscle that sets them apart from humans. To investigate, they compared squat jumping performance between bonobos (Pan paniscus), close relatives of the common chimpanzee, and humans.

During squat jumps, the amount of work generated by the limb muscles closely parallels the potential energy gain of the body. Since a body's potential energy gain is directly related to the height of a jump, estimates of muscle energy output can be made by keeping a close track of a body's vertical movement during a jump, without using any invasive procedures.

To find out if chimps have a superior jumping performance, Scholz and coworkers recorded high-speed videos of squat jumps from three bonobos and four human subjects taking off from a force plate. All three bonobos performed squat jumps higher than 0.7 m. In contrast, the best human subject jumped just over 0.3 m, and the literature reports that top-level athletes jump between 0.4–0.5 m. To estimate the mechanical energy and the power output of the jumps, the team analyzed the movements of the center of mass and the limbs as well as the ground-reaction forces recorded from the force plate. These data were combined with limb anatomical data from previous studies into a mathematical model that determined limb muscle mechanical energy and power output during jumping.

In both humans and bonobos, the mechanical energy and power output required for the best jumps were similar at approximately 450 J and nearly 3000 W, respectively. However, bonobo limb extensor muscle mass is less than half that in a human, suggesting that per gram of muscle, the work and power output of bonobos' muscles are over twice those observed in humans.

The authors suspect that the observed differences in work and power generation could be related to fundamental differences in the ability of a certain mass of muscle to produce force, which could be caused by different forms of the muscle protein myosin. Properties of muscle contraction such as muscle fibre shortening distances or velocities could also be responsible. Regardless, it is possible to make two tentative generalizations. First, some of those small differences in DNA make-up between chimps and humans may well relate to muscle structure and function; and second, assuming chimp muscle properties are widespread among the great apes, King Kong just got a whole lot scarier.