|A caricature (Ѻ) of French physiologist Jules Beclard holding a thermometer, conducting what seems to be one of his circa 1860 animal heat and muscular work and temperature experiments, shown with a physiology book in the background, standing above a dead (deceased) animal, and in front of a live (existive) animal (cat or dog).|
One of the earliest postulates of animal heat was made by Greek physician Hippocrates who in c.490BC stated that: “heat, a quantity which functions to animate, derives from an internal fire located in the left ventricle”. 
In the late 18th century, French chemist Antoine Lavoisier introduced the "combustion theory of animal heat", the dominate theory of animal heat or how heat is produced in animals. In Lavoisier's animal heat model, body is warmed by combustion of carbon from food, with the incidental consequence of changing red arterial blood coming from the heart into the darker venous blood that carries the ashes back to the lungs in the form of carbon dioxide. 
Mechanical equivalent of heat
One of the early stimulators to the development of the science of thermodynamics was the synthesis of the mechanical equivalent of heat in relation to the need to understand animal heat. In particular, two German physicians and physicists, Robert Mayer and Hermann Helmholtz, in particular, wanted to understand the reason and operation behind the production of animal heat and how this related to other physical heats, such as solar heat, electrochemical heat, chemical heat, combustion heat, frictional heat, etc. 
In the late 1840s, debates on the sources and character of animal heat renewed impetus to use the metaphor of the animal machine. A key player in these debates was German chemist Justus Liebig who used a language of vital forces and in his 1842 Animal Chemistry drew on the metaphor of the furnace to describe the “metamorphosis” of nutrition and oxygen.  His student German physician Robert Mayer, influenced by these views, used the metaphor of combustion in his early publications on the mechanical equivalent of heat. 
In particular, during a series of blood-letting procedures, while working as a ship’s physician in 1840 aboard a Dutch merchantman ship destined for a round-trip to Java, Mayer noted that since less body heat is required in the tropics, he reasoned, based on the Lavoisier combustion theory of animal heat, that less combustion will take place, less ash will be produced, and the venous blood will retain the bright red color of fresh arterial blood. 
● Animal mechanism
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9. Baeyer, Hans C. von (1999). Warmth Disperses and Time Passes - the History of Heat (pg. 20-22). New York: The Modern Library.
● Leslie, Patrick D. (1778). A Philosophical Inquiry into the Cause of Animal Heat: With Incidental Observations on Several Phisiological and Chymical Questions. London.
● Lewes, G.H. (1858). “Animal Heat”, Blackwood’s Edinburgh Magazine, 84: 414-30.
● Animal heat – Encyclopedia Britannica, 11th ed. (1911), LoveToKnow.