Insects, spiders and scorpions are invertebrates - they do not have an internal skeleton with a backbone. In the desert the most numerous vertebrates - creatures with a backbone and internal skeleton - are reptiles. Unlike other vertebrates such as birds and mammals, they are cold-blooded. This means that they cannot generate their own body heat, so they rely on the external environment in order to maintain a relatively constant body temperature.

All manner of reptiles are found throughout the world's deserts - in the Sahara alone there are nearly 100 species. Reptiles have a natural advantage over other animals in the form of a thick skin that minimises water loss. Many reptiles are more active during the day than at night because they are cold-blooded. During the night, when temperatures often fall to around freezing point, reptiles lose their body heat and consequently have very little energy for hunting. During the day they can take full advantage of the sun's warming rays, and this is the time when they have the energy to look for prey. However, the desert heat can be too much even for reptiles, so they avoid over-exposure by moving from one shadow to another.

There are three main types of reptile found in the deserts - lizards, snakes and tortoises. The first two are almost exclusively carnivorous, but tortoises are herbivores. Smaller lizards obtain their food and fluids from insects while snakes and large lizards feed on small mammals, eggs and other lizards. Desert reptiles can range from a few centimetres in length to over 1.5 metres. The largest is the desert monitor lizard which is found in the Sahara among other places and can attain lengths of up to 1.6m.

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photo courtesy of WWF-UK