Homeostasis in Desert Reptiles

Deserts, whether hot or cold, are considered to be one of the most difficult environments for living systems, lacking the essential free water which ac- counts for approximately 60-70% of their body mass and more than 98% of their constituent atoms {Macfarlane 1978}. Amongst vertebrates, reptiles are usually thought of as the animals most adapted or suited to such environments because of their diurnal habit, based on a need for external heat, and their ability to survive far from obvious sources of water. This impression is rein- forced when one examines the composition of vertebrate faunae characteristic of deserts and arid zones: reptiles predominate and they are often the only vertebrates to be found in hyper-arid areas, such as some parts of the Sahara {Monod 1973}. I recently had occasion to examine this assumption carefully, however, and was led inexorably to the conclusion that reptiles represent a particularly successful desert group, not because of their evolution of superior adaptations, but because of their possession of a basic suite of behavioural and physiologi- cal characteristics that suit them uniquely to this very resource-limited environment {Bradshaw 1986a}. These fundamental reptilian characteristics are: 1. their low rates of metabolism, compared with birds and mammals, which result in extremely low rates of resource utilisation and lead to considerable economy in the handling of water 2.