Desert-dwelling lizards have evolved adaptations that allow them to cope with wide temperature variations, dryness and sandiness. The challenges they face include maintaining body temperature, getting enough water and moving through loose sand. Their behavioural and physical characteristics make them one of the few types of animal able to live successfully and permanently in deserts.
Daily body temperature
Unable to regulate their body temperature internally, desert lizard adaptations include using external sources of heat and cold, such as the sun, ground and air, instead. Lizards that are awake during the day lie in the sun to warm up. Some nocturnal lizards, such as the banded gecko (Coleonyx sp.), maintain their temperature through managing their contact with hot and cool soil and air. Lizards also use shade to cool down, or change their skin colour, darkening it to absorb more of the sun's radiant energy. Body position is another adaptation: some lizards changing orientation to the sun to warm up or cool down.
One behavioural adaptation lizards use to overcome desert-living challenges is avoidance. Through hibernation in winter and estivation in summer, lizards avoid environmental stresses by reducing activity during periods of extreme temperature. To help survive while inactive, lizards can store water and fat in their body tissues. The spiny chuckwalla (Sauromalus hispidus) and giant Isla San Esteban chuckawalla (Sauromalus varius) store fluid in lymph sacs along the sides of their bodies, and fatty tissue in their tails. During periods of inactivity, lizards live in burrows underground where temperatures remain relatively constant.
Body shape adaptations help lizards cope with living in sandy deserts. Some lizards have no external ear openings or have evolved scaly ear flaps to keep sand out. Very smooth scales, narrow heads and small eyes make it easier for lizards to move and breath, and elongated, scaly, hind toes help them burrow or dig quickly to escape predators. Other adaptations to living in sandy deserts include nasal valves, ringed eyelids and wedge-shaped heads.
Some lizard adaptations allow them to drink dew and rainwater that collects on their bodies. The Australian thorny devil (Moloch horridus) and the Texan horned lizard, Phrynosoma cornutum, have adapted scales that funnel dew and rainwater to the corners of their mouths. Then, through moving their tongues and lower jaws, they draw the water into their mouths and drink it. Phrynosoma cornutum adopts a posture thought to be an ancient defence pose to help move water into its mouth. Other lizards lacking specialised skin, such as Phrynocephalus helioscopus and Trapelus pallidus, also move water in this way, though their postures are considered to be former threat poses.