Limbless Locomotion

According to conventional wisdom, snakes compensated for their lost limbs by evolving a manner of locomotion that was at least twice as efficient as that of limbed lizards, their reptilian relatives. Most snakes, after all, don’t have to lift their weight up and down as they proceed and they don’t have to hold their body erect. Such economy can be a valuable advantage in the predator-prey world, where energy saved is energy available for hunting or escape.

Although this view has often been repeated in textbooks, the only research supporting it was published as a one-paragraph abstract in 1973. And for evolutionary biologists Michael Walton and Bruce Jayne of the University of California at Irvine, that wasn’t enough. In late 1989 the two researchers decided to reinvestigate the science of slither, and they quickly discovered that limbless locomotion isn’t all it’s cracked up to be.

One way to conduct studies of energy expenditure is to put a gas mask on your subject and measure the oxygen it consumes while moving on a treadmill. This yields a standard measure called the net cost of transport (NCT): the milliliters of oxygen needed to move each gram of body weight one kilometer. But with snakes that method presents certain difficulties. "You can’t call up a supply house and say, ‘Deliver me a snake mask,"" notes Walton. "The problem with snakes is they don’t have much of a neck or anything to attach it to."

The researchers’ solution was to cut the end off a plastic test tube that would cover a snake’s nostrils; they then drilled a hole in the end and attached a hose connected to a gas analyzer. not wanting to damage the snakes’ scales with glue, they wedged the mask in place with a small piece of rubber. Next they need a serpent-friendly treadmill. Snakes can make progress only by working against surface friction or pushing on raised objects like roots and stones. The two researchers obligingly covered a standard treadmill with Astroturf, which they then dotted with additional raised squares of the artificial grass.

For their experiment Walton and Jayne chose black racers because they have a high capacity for strenuous aerobic activity. These snakes evidently also have a refined sense of proper dress. Eight of the 15 snakes were so irritated at having a test tube stuffed over their head that they either shook it off or refused to move. Fortunately, the other seven were less fussy and slithered away on the treadmill.

How did they do? When moving in the familiar S-shaped lateral undulations, these snakes expended he same energy as similar-size lizards, with an NCT of 1.15. The earlier study had claimed an NCT of only .52 for garter snakes. Jayne and Walton aren’t sure why the previous figure was so low, but they suspect that the garter snakes were pushed into moving too fast. This might have forced the snakes to tap stored energy reserves, which are metabolized anaerobically - that is, without oxygen. The snakes’ performance would thus have been temporarily boosted by an unmonitored energy source.

Lateral undulation isn’t the only way snakes get around. In narrow spaces like underground burrows, snakes wedge part of their body in one spot and push or pull the rest ahead, folding and unfolding in a style known as concertina motion. To test their snakes in a tight spot, Jayne and Walton built a narrow tunnel over a revolving turntable. Unable to undulate, the snakes pushed off against the walls to keep up with the road beneath them. This jerky style of motion proved to be seven times as expensive as lateral undulation, with an NCT of 8.49.

All in all, Walton and Jayne concluded, snakes work just as hard to get around as comparable-size reptiles, birds or mammals. But this doesn’t mean there’s no good reason for the evolution of limblessness. Raiding burrows to eat eggs, steal food or catch prey, the researchers point out, can be an excellent way to make a living. "Without limbs these animals can get into burrows very easily," says Walton. "You don’t want legs hanging off the sides and getting in the way."

The preceding article was reproduced with permission from the April 1991 issue of Discover magazine.