Beetle of Burden

By Carl Zimmer|Monday, April 01, 1996
RELATED TAGS: UNUSUAL ORGANISMS
Ounce for ounce, rhinoceros beetles are the world’s strongest animals. But just how strong are they? And what makes them so powerful?

Inspiration can come from the strangest places. While perusing the Guinness Book of World Records, Rodger Kram saw a startling entry. They claim that the world’s strongest animal is the rhinoceros beetle, and that it can support 850 times its own body weight, he says. Kram, a physiologist at the University of California at Berkeley, decided to test that claim in his lab.

Kram didn’t simply stack weights on beetles’ backs, since a little tilt would unbalance the load. Instead he created a yoke. He glued a patch of Velcro on each beetle’s back and attached to it a thin lead strip that extended over the beetle’s head and beyond its rear. On the ends he glued lead weights he had bought from a hobby shop (they’re normally used to give model trains traction).

When Kram put the rhinoceros beetles on a treadmill, he discovered that while they were not as strong as the Guinness Book claimed, they were still amazing. They could carry up to 100 times their own weight of one-tenth of an ounce, although under that load they could barely move. With 40 times their own weight, they plodded for about ten minutes before becoming exhausted. But with 30 times their own weight, things changed: after half an hour of walking, they still showed no sign of fatigue. I got bored before they got tired, says Kram.

The beetles’ feat is comparable to a 150-pound man walking a mile with a Cadillac on his head without tiring. To a physiologist like Kram, what was particularly mysterious was what fueled the beetles. In humans and ponies and dogs, he explains, if you have them carry a weight equal to 10 percent of their body weight, their metabolic rate goes up by 10 percent: 10, 10; 20, 20; and so on in a proportional increase.

For the rhinoceros beetles to follow this rule carrying 30 times their weight, though, they would have to speed up their metabolism by a factor of 30--something only a few creatures on Earth, such as bumblebees, can do. Kram didn’t expect ground-dwelling beetles to have a metabolism that could burn at such a high, sustained rate. But he also didn’t expect what he found when he sealed his treadmill in a test chamber and measured how much oxygen the beetles consumed, and thus how many calories they burned. While carrying 30 times their weight, the beetles consume only four times more oxygen than when walking unencumbered--an astonishing display of biomechanical efficiency. If these beetles carry 50 percent of their own weight, it’s absolutely trivial, says Kram. They hardly notice it at all.

Rhinoceros beetles don’t actually carry heavy loads on their backs in nature, but they do engage in strenuous tasks, such as plowing through forest litter to find food and locking horns with rivals. Kram has yet to figure out their secret. The mechanical tricks of other animals don’t apply. Hermit crabs, for example, can carry their heavy shells by dragging their abdomens; with the seafloor supporting most of the load, the crabs just have to move it forward. But rhinoceros beetles keep their bellies high. People practiced at carrying heavy loads on their heads conserve energy by turning some of the energy of their forward movement into the rise and fall of their stride. The beetles, on the other hand, walk too slowly to do anything of the sort. When carrying a load, they don’t change their walking style at all.

Kram plans to explore their skeleton and muscles to get to the bottom of this mystery. Like all insects, the beetles have a stiff exoskeleton, and it’s possible that it is so exceptionally strong that it can bear the beetle’s load without requiring much work from the muscles. But Kram suspects he’ll find the solution in the muscles--and he sees the beetles’ slowness as a clue. Muscles that contract and relax slowly, he says, can be powerful and metabolically efficient at the same time; the ones that clams use to keep their shells shut tight are an example. Rhino beetles may have muscles like that.

Uncovering their full story could be a great help to researchers trying to build many-legged robots for tasks like mining the moon. After all, a robot with legs of hollow steel is essentially a mechanical beetle. The people who make robots haven’t been too concerned with energy consumption. They just plug the robot in, and there’s a nuclear reactor at the other end, and they have plenty of power, says Kram. But when we really start to make a practical, autonomous, payload-carrying robot, energy conservation will become much more important.
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