SCIENCE / LOCOMOTION : Legs Knocked Out From Under Theory on Why Snakes Evolved

Snakes wearing tiny oxygen masks while slithering on treadmills at UC Irvine apparently have refuted some conventional wisdom about evolution.

Scientists measuring the amount of oxygen consumed by the snakes have concluded that, contrary to earlier predictions, limbless locomotion does not conserve energy. The research suggests that snakes spend at least as much energy moving--and in some cases, much more--as legged animals of similar size.

“It’s long been an evolutionary puzzle why many vertebrates lost limbs entirely,” said Bruce Jayne, an evolutionary biologist at UC Irvine and co-author of the research in the journal Science. “The suggestion that the loss of limbs conveys an energetic advantage for locomotion is not the case at all,” he explained.

In 1973, a Harvard University study found that garter snakes consumed 30% of the energy of legged lizards moving the same distance. The preliminary study was never published, but was regularly cited in scientific articles and textbooks to explain that snakes and certain species of legless salamanders and lizards evolved as energy-conserving creatures.

To determine the energy expenditure required for movement, the California researchers placed tiny oxygen masks on black racer snakes of the species Coluber constrictor. The snakes were then put on custom-designed motorized treadmills and the scientists recorded oxygen consumption, speed and endurance during two types of locomotion.

During lateral undulation, a snake slithers back and forth along the ground by moving its entire body simultaneously at the same speed. But when confronted with passage through a narrow tunnel, snakes change their strategy for locomotion. Using concertina locomotion, a snake bunches up its body and then extends its head forward while the rear portion of the body remains still. Then the snake bunches up the front section of its body and pulls its rear section forward.

Snakes moving by lateral undulation used virtually the same energy as that of limbed lizards of identical size, the researchers noted. Moreover, the energy the snakes spent was also similar to that predicted for birds and mammals of the same weight.

In contrast, snakes moving by concertina locomotion required seven times the energy to move the same distance as during lateral undulation, Jayne said. “It is the starting and stopping during concertina locomotion that is responsible for a higher energetic cost,” Jayne said.

Although snakes apparently conserve no energy by slinking along the ground on their bellies, other reasons may have influenced the loss of their limbs during the course of evolution.

“Without legs, snakes--because of their cylindrical shape--can squeeze through very small openings. With legs that might be impossible,” Jayne said.

C. Richard Taylor, a comparative physiologist at Harvard University who conducted the earlier snake studies, said the new research indicates that “snakes don’t optimize for low energy costs, but instead for the flexibility required to move across different terrains.”

Robert Full, a locomotion expert at UC Berkeley, has found in his studies of millipedes, cockroaches, salamanders and a variety of other animals that an animal’s size, not its number of legs, or lack of legs, determines the amount of energy spent during locomotion.