Most mammals (such as the cat shown in the picture) and birds are endothermic, as opposed to ectothermic. Ectothermic organisms gain most of their heat from the surrounding environment; most reptiles are ectothermic. Endothermic organisms, on the other hand, receive their heat from internal metabolism. The advantage of being an endotherm is that they can maintain stable body temperatures even when their external environment changes. In both hot and warm environments, endotherms have mechanisms to sustain their stable temperature. On the other hand, ectotherms are usually not able to survive in temperatures below freezing. The advantage of being an ectotherm is that they do not need as much food as endotherms that are their same size because ectotherms get most of their heat from external environments. Although they cannot create enough heat for thermoregulation, ectotherms have methods of maintaining a stable temperature. For example, ectotherms can cool off in the shade or warm up by laying in the sun.