In addition to extreme temperatures and sustained drought, desert-dwelling
animals have also had to adapt their movement to the peculiarities of the desert
terrain. The classic terrain of the desert is sand, and this presents hindrances to
free move-ment familiar to anyone who has taken a walk on coastal dunes.
Animals of sandy deserts experience the same problem and have developed an
array of methods, including anatomic and behavioral adaptations, for moving
more easily over this shifting and blazing-hot substrate.
• Hopping or leaping. Powerful hind legs and large, long rear feet can leap
over the substrate effectively. The surface area of the rear foot spreads the
animal’s weight over a long strip oriented in the direction of travel, causing a
reduction in the rearward slippage of soil particles or sand grains on takeoff.
This method of movement is used by various mammals, from jerboas to
kangaroos.
• Sandshoes. A large surface area on feet helps to prevent the body from
sinking into soft sand, much as snowshoes do in soft snow. The broad feet of the
camel perform this function, while many types of desert lizard, such as the
whiptail and collared lizard, have very long toes, often with fringes or large
scales that similarly extend body-to-sand contact.
• Shuffling. While walking, diurnal desert lizards often shuffle their feet at
each step, pushing
allowing their feet to rest briefly on the slightly cooler grains beneath.
• “Swimming.” Numerous species of snakes and lizards are able to “swim”
through very soft sand or loose soil, making sideways undulating movements of
the body to tunnel just below the surface. The sandfish lizard (Scincus scincus)
of Middle Eastern deserts is expert at this type of propulsion.
• Sidewinding. The sidewinder rattlesnake (Crotalus cerastes) of North
American desert regions is named for its distinctive method of locomotion,
which leaves in its wake a striking trail of elongated S- or J-shaped marks across
the sand. The sidewinder moves by throwing its body into a series of sideways
waves, lifting its head from the substrate, throwing it diagonally forward, and
then setting it down again several centimeters farther on in the direction of
travel. This elaborate method of propulsion allows the snake to push rearward
with its whole body length against the sand for extra grip or purchase and with
each loop to lift every part of the body clear of the hot sand, giving precious
cooling-down time.
• Rolling. This method of movement is unique to the solifuge, or sun spider
(Galeodes arabs, for example; see also p. 98). The solifuge has a remarkable
emergency escape mechanism. By curling its legs together under its body to
form a cartwheel or ball shape, it is able to roll at speed down a dune or hillside.