Nocturnal desert animals
A few desert reptiles are nocturnal; the Moorish gecko (Tarentola mauretanica), is found in arid regions in North Africa (also in Spain, France and Greece, so it is not restricted to deserts).
Tarentola is most active for a few hours after sunset. During the night, itsTb is as low as 18°C, and can fluctuate by up to 11°C. Those lizards that tolerate wide fluctuations in Tb, even when they could use features of the environment to maintain a steady Tb, are known as thermal generalists. The Moorish gecko is a thermal generalist at night, when it is active rather than resting in its burrow. During the early morning the Moorish gecko basks in the sunlight and its skin darkens until almost black. At night the gecko is very pale.
What functions and advantages do the changes in skin colour give?
Dark colours absorb and radiate heat better than light colours. At night a light colour should reduce heat loss by radiation, and there is not much heat available to absorb. During the day, dark skin promotes absorption of solar heat. Although radiation to the atmosphere by the dark skin is also promoted, the energy so lost is of little significance compared to the large amount of solar heat absorbed.
The advantage to the gecko of warming up in the morning is uncertain, but it is possible that a physiological process such as digestion of the food eaten during the night requires a higher Tb than the gecko can maintain at night.
The ability of the gecko to vary skin colour shows that behavioural thermoregulation in reptiles is supplemented by physiological mechanisms.
Sheltering in the available shade in the desert, or being active at night, are simple strategies for keeping Tb below lethal levels. In sandy desert areas, the sand itself plays an important role in behavioural thermoregulatory strategies. The Mojave fringe-toed lizard (Uma scoparia) is restricted to fine, wind-blown sand, e.g. in dunes, dry lake beds and desert scrub in the Mojave desert. Burrows in sand collapse immediately or soon after the animal has moved on, so animals buried in sand rely on air trapped between sand particles for breathing. Uma is a ‘sand-swimmer’ and its dorsoventrally flattened body and shovel-shaped head facilitate movement through the sand, which is especially important when escaping from predators such as snakes and badgers.
The eyelids are protected from sand by large eyelid fringe scales. The digits have large lamellar fringes, elongated scales, especially long on the hind feet, which enable the lizards to run at speed on the sand surface. Uma grows up to about 110 mm in length, and its activity pattern is diurnal, varying according to ambient temperature. In March and April Uma is active for short periods because of the low spring temperatures in the Mojave. In summer, from May to September, the lizards are active during mornings and late afternoons, feeding on insects and plants. Sand-swimming lizards are also found in the Namib desert and include the wedge-snouted sand lizard (Meroles cuneirostris).