Comparative physiology
Comparative physiology is a subdiscipline of physiology that studies and exploits the diversity of functional characteristics of various kinds of organisms. It is closely related to evolutionary physiology and environmental physiology. Many universities offer undergraduate courses that cover comparative aspects of animal physiology. According to Clifford Ladd Prosser, "Comparative Physiology is not so much a defined discipline as a viewpoint, a philosophy." History. Originally, as narrated in a recent history of the field, physiology focused primarily on human beings, in large part from a desire to improve medical practices. When physiologists first began comparing different species it was  sometimes out of simple curiosity to understand how organisms work but also stemmed from a desire to discover basic physiological principles. This use of specific organisms convenient to study specific questions is known as the Krogh Principle. Methodology. C. Ladd Prosser, a founder of modern comparative physiology, outlined a broad agenda for comparative physiology in his 1950 edited volume (see summary and discussion in Garland and Carter): 1. To describe how different kinds of animals meet their needs. 2. The use of physiological information to reconstruct phylogenetic relationships of organisms. 3. To elucidate how physiology mediates interactions between organisms and  their environments. 4. To identify "model systems" for studying particular physiological functions. 5. To use the "kind of animal" as an experimental variable. Comparative physiologists often study organisms that live in "extreme" environments (e.g., deserts) because they expect to find especially clear examples of evolutionary adaptation. One example is the study of water balance in desert-inhabiting mammals, which have been found to exhibit kidney specializations. Similarly, comparative physiologists have been attracted to "unusual" organisms, such as very large or small ones. As an example, of the latter, hummingbirds have been studied. As another example, giraffe have been studied because  of their long necks and the expectation that this would lead to specializations related to the regulation of blood pressure. More generally, ectothermic vertebrates have been studied to determine how blood acid-base balance and pH change as body temperature changes. Funding. In the United States, research in comparative physiology is funded by both the National Institutes of Health and the National Science Foundation. Societies. A number of scientific societies feature sections on comparative physiology, including: Biographies. Knut Schmidt-Nielsen (1915–2007) was a major figure in vertebrate comparative physiology, serving on the faculty at Duke University for many years and  training a large number of students (obituary). He also authored several books, including an influential text, all known for their accessible writing style. Grover C. Stephens (1925–2003) was a well-known invertebrate comparative physiologist, serving on the faculty of the University of Minnesota until becoming the founding chairman of the Department of Organismic Biology at the University of California at Irvine in 1964. He was the mentor for numerous graduate students, many of whom have gone on to further build the field (obituary). He authored several books and in addition to being an accomplished biologist was also an accomplished pianist  and philosopher.