What makes chimps four times stronger than humans?

WASHINGTON - Recently a pet chimp inflicted devastating injuries on a Connecticut woman - a stark reminder that chimps are much stronger than humans, as much as four-times stronger, some researchers believe.

But what is it that makes our close primate cousins so much stronger than we are?

A possible explanation is that great apes simply have more powerful muscles. Indeed, biologists have uncovered differences in muscle architecture between chimpanzees and humans.

But evolutionary biologist Alan Walker, professor at Penn State University, thinks muscles may only explain a part of the story.

Walker argues that humans may lack the strength of chimps because our nervous systems exert more control over our muscles. Our fine motor control prevents great feats of strength, but allows us to perform delicate and uniquely human tasks.

On the other hand, great apes, with their all-or-nothing muscle usage, are explosive sprinters, climbers and fighters, but not nearly as good at complex motor tasks. In other words, chimps make lousy guests in china shops.

Our finely-tuned motor system makes a wide variety of human tasks possible. Without it we couldn’t manipulate small objects, make complex tools or throw accurately. And because we can conserve energy by using muscle gradually, we have more physical endurance - making us great distance runners.

Walker’s hypothesis stems partly from a finding by primatologist Ann MacLarnon. MacLarnon showed that, relative to body mass, chimps have much less grey matter in their spinal cords than humans have.

Spinal grey matter contains large numbers of motor neurons - nerve cells that connect to muscle fibres and regulate muscle movement.

More grey matter in humans means more motor neurons, Walker proposes. And having more motor neurons means more muscle control.

Our surplus motor neurons allow us to engage smaller portions of our muscles at any given time. We can engage just a few muscle fibres for delicate tasks like threading a needle, and progressively more for tasks that require more force, according to a Penn State release.

Conversely, since chimps have fewer motor neurons, each neuron triggers a higher number of muscle fibres. So using a muscle becomes more of an all-or-nothing proposition for chimps. As a result, chimps often end up using more muscle than they need.

‘That is the reason apes seem so strong relative to humans,’ Walker writes.

The study is scheduled for publication in the April issue of Current Anthropology.


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