Sand Dune Two-Step

The need for speed may bring these lizards to their (hind) feet.

Story by Adam Summers  ~  Illustration by Sally J. Bensusen

As the sun rises over the Mojave Desert, its pale yellow light casts every irregularity into stark relief. Countless pockmarks and minute tracings on the surface of the dunes provide evidence of a busy night for kangaroo rats and sidewinders-the scuffling of tiny feet, the swishing of curved bodies, and the occasional mad sprint to escape a predator. But one small patch of the Kelso Dunes in the southeastern area of the desert is strangely lacking in such sandy signs, having been blown smooth by biologists wielding gas-powered leaf blowers. These researchers are testing out their theories of lizard locomotion.

As it races off at thirteen feet per second, a fringe-toed lizard briefly rises up on its hind limbs before dropping back down on all fours.

Among the most astonishing residents of the dry and apparently barren Kelso Dunes is the Mojave fringe-toed lizard (Uma scoparia). This small, finely checkered lizard is active during the day, skittering across the sand in search of invertebrate prey. Occasionally it disappears in a flash as it deftly burrows into the sand. For a real show, try catching one of these lizards. It races away in a sudden burst of speed, its front end lifting off the ground like a hot rod, its forelimbs weakly pawing the air. As the four-inch-long creature dashes off at thirteen feet per second, its hind limbs windmill like the legs of a cartoon character making a fast getaway, and its long toes flare, each bearing an expanded row of scales that provide traction in the sand.

Scientists assume that being fleet of foot boosts evolutionary fitness by helping an animal avoid becoming someone's dinner for long enough to produce lots of offspring. As a result, a great deal of research has been conducted on the relationship between speed and such variables as leg length; how long feet stay in contact with the ground; and the degree to which the legs are splayed, as in lizards, or held underneath the body, as in mammals (except for the duck-billed platypus). So, using a lizard-sized treadmill and high-speed cameras, Bruce Jayne, of the University of Cincinnati, and Duncan Irschick, of Tulane University, have been gathering sprinting data for the Mojave fringe-toed lizard as well as for its close relative, the zebra-tailed lizard (Callisaurus draconoides).

In the laboratory, the researchers determined the top speed of both species and also when they use two legs and when they use four. The longer-legged zebra-tailed lizard-which, like its relative, is diurnal but which inhabits a wider range of terrains, from fine sand to the hardpan of desert washes-runs much faster than the fringe-toed lizard, even on loose sand, where the researchers had expected the latter's toe cleats to be a significant advantage. And oddly, though the researchers assumed that running on two legs would be quicker than running on all four, they found that after the first few seconds of movement, when both species can run bipedally, there was little difference in speed between the two modes of locomotion. Running on two legs may facilitate quick acceleration or may simply be an unintended consequence of it.

Laboratory studies like these are useful for purposes such as understanding the mechanics of a rapid, sprawling gait or making an evolutionary comparison between species. However, Irschick and Jayne wanted to know how the lizards perform outside the contrived conditions of a biology lab. In the field, they wondered, are the animals content to creep around most of the time, sprinting only when necessary, or do they regularly dart from place to place?

To circumvent the problems of exposing high-tech electronic equipment to sand and extreme heat, Jayne and Irschick borrowed a tactic from Sherlock Holmes: they measured the footprints left by lizards and then compared the prints with data from the laboratory experiments.

In their early experiments on the dunes, the researchers simply walked up to lizards and observed the animals as they sped off. They found that on gentle slopes, both species were just as likely to run uphill as they were to run down or across, so long as the direction was away. On steeper slopes, the zebra-tailed lizards tended to run across the hill, presumably because uphill would be slower and downhill more apt to result in a tumbling fall. While both species ran on two feet at first, as they did in the lab, it turned out that the fringe-toed lizard resumed quadrupedal running after just a few upright steps, while the gangly zebra-tail remained bipedal for nearly half the escape strides.

The smoothed patch on the Kelso Dunes is one of several sites in the Mojave where Jayne and Irschick have set up a more ambitious experiment so that they can study locomotion in a community of lizards behaving naturally, unobserved and undisturbed. For this experiment, the researchers deploy their leaf blowers at night and then retreat, leaving behind a blank slate ready to record the animals' movements. The lizards are active for several hours during the relative cool of morning and again in the early evening. When the sun is high, they seek cover in burrows or under vegetation. At this point the researchers, braving the heat (up to 115° F), examine the dunes to measure the lizards' tracks.

Over the course of several weeks, the research team measured more than 5,000 footfalls in more than 300 trackways. Calculating the slope of each path and the speed of the animal that made it, they found that the fringe-toed lizard moves remarkably quickly when covering longer distances. In the lab, the lizards displayed a range of speeds, but in the field they preferred just two speeds: an exhausting dash across open ground and a slow, aerobically sustainable walk when foraging near cover. Surprisingly, they rarely ran on two legs out in the Mojave when no six-foot-tall researchers were there to alarm them; the need for rapid acceleration may arise less frequently in the wild than it does on treadmills in a laboratory. In one intriguing trackway, the footfalls of a fringe-toed lizard were paralleled by the prints of a roadrunner, a lizard-eating bird. After several feet, the lizard tracks disappeared abruptly: evidence of a well-timed dive under the sand or perhaps an indication that no matter how fast the lizards are, they do not always win the race.

Adam Summers is an assistant professor of ecology and evolutionary biology at the University of California, Irvine (asummers@uci.edu).