Endothermic organisms can regulate their own body temperature via internal regulation mechanisms independent of environmental factors. For example, a dog is able to cool itself down in the heat of day by panting. Exothermic organisms cannot internally regulate their own body temperature. Rather, they have external regulation, and they depend on environmental factors. The lizard mentioned cannot regulate his own body temperature. In order to warm up, he must find sunlight and heat to increase his body temperature; in order to cool down he may find shade or seek refuge underground. Exothermic organisms are extremely dependent on climate to carry out life functions. 

The principle of allocation, in regards to energy, states that energy spent doing one activity takes away the energy that could be spent doing other activities. For example, if an organism expends energy chasing prey, the organism will be tired and will have less energy to carry out other functions. 

The Second Law of Thermodynamics states that during an reaction that some energy will not be completely transferred and will ultimately be lost as heat to increase the entropy of the universe.

Endotherms regulate their body temperature, keeping it within a narrow range. To do this, they require a large amount of energy (food), more than exotherms which do not regulate their own body temperature and thus have a lower energy requirement.

Ectotherms do not regulate their body temperature; their body temperature fluctuates due to their external environment.

Vasodilation allows more heat to escape via the blood vessels. Thus, in a cold environment, this would be counterproductive. Vasoconstriction shrinks the blood vessels so that less heat escape. Shivering thermogenesis produces heat by converting ATP to kinetic energy. Thermoregulation is the term describing organisms maintaining their internal body temperature.

An ectotherm relies on external sources to regulate its body temperature. The snake lies on the dark rock to absorb heat energy from the sun, to warm its body during the cool day. Animals that hibernate, such as bears, cannot be ectotherms. Hibernating animals downregulate their body’s metabolism and thus body temperature; ectotherms cannot control their body temperature. Humans are endotherms, and like other endotherms, shiver when cold to increase their body temperature through expending energy (“shivering thermogenesis”). Dogs are endotherms, and pant when hot to cool their body temperature by expelling heat.

Endotherms maintain a relatively constant body temperature, independent of their external environment. They can shiver when cold to increase their body temperature through expending energy (“shivering thermogenesis”).

An ectotherm relies on external sources to regulate its body temperature. Endotherms maintain a relatively constant body temperature, independent of their external environment. To regulate their body temperature, endotherms have a higher