Issues: Ectothermy vs Endothermy
A modern example of how reptiles manage to survive in hot dry conditions can be found in the marine iguanas (Amblyrhyncus cristatus) which are able to survive on barren larva fields on the tropical arid islands of the Galapagos Islands. These animals bask in the sun which helps raise their body temperatures without the risk of desiccation. Physiological processes of animal's body, like other chemical reactions, are affected by temperature. Up to about 40oC the higher the temperature the quicker the physiological processes and the more energy they produce and the more active the animal can be. Neither reptiles nor amphibians generate their heat internally like we do, but they draw it directly from the environment usually in the form of solar radiation. The daily activity cycle of these marine iguanas maintains the body at the most efficient temperatures. At dawn, when ambient temperatures are lowest, they climb to ridges and expose themselves broadside to the rising sun. As temperatures rise, the risk of overheating increases, the iguanas respond by lifting their bodies off the ground and positioning themselves so that air currents can pass below them. They can also pack themselves into the few shady places that exist (such as rock crevices). The sea surrounding the islands is influenced by the cold Humboldt Current, and is only entered to feed on green alga at the hottest time of day (noon). During foraging their bodies would cool-off rapidly and they will need to conserve as much heat as possible. These animals therefore constrict the arteries near the surface of their bodies so that blood circulates only in the centre of the body.
Marine Iguana (Amblyrhynchus cristatus)
Nevertheless body temperatures will drop up to 10oC before they have to return to land. On land they stretch-out their bodies and absorb warmth from the black larva surfaces. With the setting sun they again congregate on the ridges with their bodies' broadside to the last of the solar radiation for the day. These behavioural sequences maintain the body temperatures close to 37oC, although it varies considerable more than in endothermic animals (e.g. our bodies). Animals like iguanas are ectothermic since their body temperatures tend to fluctuate. Endothermy has advantages since it permits greater independence of the prevalent temperatures (e.g. can maintain activities at night and in cold regions), but is energetically expensive. About 80% of daily calories is invested in maintaining body temperatures constant in endothermic animals. In contrast an ectothermic animal uses only 10% of the energy that an equivalent endothermic animal would use. As a consequence they survive in desert conditions were endothermic animals would have more difficulty surviving.