For swimming through sand, a slick and slender snake
can perform better than a short and stubby lizard.
That's one conclusion from a study of the movement
patterns of the shovel-nosed snake, a native of the Mojave Desert of the
southwest United States. The research shows how the snake uses its slender
shape to move smoothly through the sand, and how its slippery skin reduces
friction -- both providing locomotive advantages over another sand-swimmer: the
sandfish lizard native to the Sahara Desert of northern Africa.
The study provides information that could help explain
how evolutionary pressures have affected body shape among sand-dwelling
animals. And the work could also be useful in designing search and rescue
robots able to move through sand and other granular materials.
Using X-ray technology to watch each creature as it
moved through a bed of sand, researchers studied the waves propagating down the
bodies of both the snakes and sandfish lizards. Granular resistive force
theory, which considers the thrust provided by the body waves and the drag on
the animals' bodies, helped model the locomotion and compare the energy
efficiency of the limbless snake against that of the four-legged lizard --
which doesn't use its legs to swim through the sand.
"We were curious about how this snake moved, and
once we observed its movement, how it moved so well in the sand," said Dan
Goldman, an associate professor in the School of Physics at the Georgia
Institute of Technology. "Our model reveals how both the snake and the
sandfish move as fast as their body shapes permit while using the least amount
of energy. We found that the snake's elongated shape allowed it to beat the
sandfish in both speed and energy efficiency."
Information about the factors enabling the snake to
move quickly and efficiently could help the designers of future robotic
systems. "Knowing how the snake moves could be useful, for instance, in
helping robots go farther on a given amount of battery power," Goldman
said.
Supported by the National Science Foundation and the Army
Research Office, the research was published online December 18, 2014, in the Journal
of Experimental Biology. The study is believed to be the first kinematic
investigation of subsurface locomotion in the long and slender shovel-nosed
snake, Chionactis occipitalis.
Measurements made by former Ph.D. student Sarah Sharpe
revealed that the snake propagates traveling waves down its body, from head to
tail, creating a body curvature and a number of waves along its body that
enhance its movement through the sand. As a consequence of the kinematics, the
snake's body travels mostly in the same "tube" through the sand that
is created by the movement of its wedge-shaped head and body.
Because the snake essentially follows its own tracks
through the sand, the amount of slip generated by its motion is small, allowing
it to move through the sand using less energy than the sandfish (Scincus
scincus), whose movement pattern generated a larger fluidized region of sand
around its body.
Overall, the research showed that each animal had
optimized its ability to swim through the sand using its specific body plan.
"For each body wave the snake generates, it moves
farther than the sandfish does within a single wave of motion of its
body," Goldman noted. "Having a long and slender body allows the
snake to bend its body with greater amplitude while generating more waves on
its body, making it a more efficient sand swimmer."
The snake's skin is also more slippery than that of
the sandfish, further reducing the amount of energy required to move through
the sand.
Scientists had suspected that long and slender animals
would have a sand-swimming advantage over creatures with different body shapes.
The research showed that the advantage results from a high length-to-width
ratio that allows the formation of more waves.
"If you have the right body shape and slick skin,
you can get a very low cost of transport," explained Goldman.
To study the snakes as they moved through sand, Sharpe
-- from Georgia Tech's Interdisciplinary Bioengineering Program -- and
undergraduate Robyn Kuckuk, from the Wallace H. Coulter Department of
Biomedical Engineering at Georgia Tech and Emory University, glued tiny lead
markers onto the scales of the snakes. The markers, which fall off when the
snakes shed their skin, allowed the researchers to obtain X-ray images of the
snakes moving beneath the surface of the sand. Sharpe, now a biomechanical
engineer with a research and consulting firm in Phoenix, created detailed
videos showing how the snakes moved.
Associate professor Patricio Vela and graduate student
Miguel Serrano, both from Georgia Tech's School of Electrical and Computer
Engineering, developed software algorithms that allowed detailed analysis of
the wave-forms seen on the X-ray movies as a function of time.
Stephen Koehler, a research associate in applied
physics at Harvard University, applied resistive force theory to obtain data on
the snakes' movement and energy efficiency. Animals swimming in sand can only
move if the thrust provided by their bodies exceeds the drag created. The
theory predicted that the snakes' skin would have about half as much friction
as that of the sandfish, and that prediction was verified experimentally.
Joe Mendelson, director of research at Zoo Atlanta,
assisted the research team in obtaining and managing the snakes.
Understanding how animals move through granular
materials like sand could help the designers of robotic systems better
understand how to optimize the use of energy, which can be a significant
limiting factor in robotics.
"This research is really about how body shape and
form affect movement efficiency, and how we can go between experiment and
theory to improve our understanding of these issues," said Goldman.
"What we are learning could help search and rescue robots maneuver in
complex terrain and avoid obstacles."
Beyond the robotics concerns, the work can help
scientists understand biological issues, such as how the body plans of
desert-dwelling lizards and snakes converge to optimize their ability to move
through their environment.
"These granular swimming systems turn out to be
quite useful for understanding fundamental questions about evolutionary
biology, biomechanics and energetics because they are simple to analyze and
they can describe a good number of systems," Goldman added.
Citation
Sharpe SS., Koehler SA, Kuckuk RM, Serrano M, Vela PA, Mendelson J, Goldman DI. 2014. Locomotor benefits of being a slender and slick sand-swimmer." The Journal of Experimental Biology (2014): jeb-108357.
Sarah Sharpe, et al. 2014 Locomotor benefits of being
a slender and slick sand swimmer. Journal of Experimental Biology, 2014
DOI: 10.1242/jeb.108357