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 Prosser, "Comparative Physiology is not so much a defined discipline as a viewpoint, a philosophy."
Originally, 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.
1. To describe how different kinds of animals meet their needs.
- This amounts to cataloging functional aspects of biological diversity, and has recently been criticized as "stamp collecting" with the suggestion that the field should move beyond that initial, exploratory phase.
2. The use of physiological information to reconstruct phylogenetic relationships of organisms.
- In principle physiological information could be used just as morphological information or DNA sequence is used to measure evolutionary divergence of organisms. In practice, this has rarely been done, for at least four reasons:
- physiology doesn't leave many fossil cues,
- it can't be measured on museum specimens,
- it is difficult to quantify as compared with morphology or DNA sequences, and
- physiology is more likely to be adaptive than DNA, and so subject to parallel and convergent evolution, which confuses phylogenetic reconstruction.
3. To elucidate how physiology mediates interactions between organisms and their environments.
- This is essentially physiological ecology or ecological physiology.
4. To identify "model systems" for studying particular physiological functions.
- Examples of this include using squid giant axons to understand general principles of nerve transmission, using rattlesnake tail shaker muscles for measurement of in vivo changes in metabolites (because the whole animal can be put in an NMR machine), and the use of ectothermic poikilotherms to study effects of temperature on physiology.
5. To use the "kind of animal" as an experimental variable.
- "While other branches of physiology use such variables as light, temperature, oxygen tension, and hormone balance, comparative physiology uses, in addition, species or animal type as a variable for each function." 
- 25 years later, Prosser put things this way: "I like to think of it as that method in physiology which uses kind of organism as one 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 specializaitons 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.
A number of scientific societies feature sections on comparative physiology, including:
- American Physiological Society
- Australian & New Zealand Society for Comparative Physiology & Biochemistry
- Canadian Society of Zoologists
- Japanese Society for Comparative Physiology and Biochemistry
- Society for Integrative and Comparative Biology
- Society for Experimental Biology
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 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.
Some journals that publish articles in comparative animal physiology
- American Journal of Physiology - Regulatory, Integrative and Comparative Physiology
- Annual Review of Physiology
- Comparative Biochemistry and Physiology
- Integrative and Comparative Biology
- Journal of Comparative Physiology
- Journal of Experimental Biology
- Physiological and Biochemical Zoology
- Barrington, E. J. W. 1975. Comparative physiology and the challenge of design. Journal of Experimental Zoology 194:271-286.
- Clark, A. J. 1927. Comparative physiology of the heart. Cambridge University Press, London.
- Dantzler, W. H., ed. 1997. Handbook of physiology. Section 13: comparative physiology. Vol. I. Oxford Univ. Press, New York.
- Dantzler, W. H., ed. 1997. Handbook of physiology. Section 13: comparative physiology. Vol. II. Oxford Univ. Press, New York. viii + 751-1824 pp.
- Feder, M. E., A. F. Bennett, W. W. Burggren, and R. B. Huey, eds. 1987. New directions in ecological physiology. Cambridge Univ. Press, New York. 364 pp.
- Garland, T., Jr., and P. A. Carter. 1994. Evolutionary physiology. Annual Review of Physiology 56:579-621. PDF
- Gilmour, K. M., R. W. Wilson, and K. A. Sloman. 2005. The integration of behaviour into comparative physiology. Physiological and Biochemical Zoology 78:669-678.
- Gordon, M. S., G. A. Bartholomew, A. D. Grinnell, C. B. Jorgensen, and F. N. White. 1982. Animal physiology: principles and adaptations. 4th ed. MacMillan, New York. 635 pages.
- Greenberg, M. J., P. W. Hochachka, and C. P. Mangum, eds. 1975. New directions in comparative physiology and biochemistry. Journal of Experimental Zoology 194:1-347.
- Hochachka, P. W., and G. N. Somero. 2002. Biochemical adaptation — mechanism and process in physiological evolution. Oxford University Press. 478 pp.
- Mangum, C. P., and P. W. Hochachka. 1998. New directions in comparative physiology and biochemistry: mechanisms, adaptations, and evolution. Physiological Zoology 71:471-484.
- Moyes, C. D., and P. M. Schulte. 2006. Principles of animal physiology. Pearson Benjamin Cummings, San Francisco. 734 pp.
- Prosser, C. L., ed. 1950. Comparative animal physiology. W. B. Saunders Co., Philadelphia. ix + 888 pp.
- Randall, D., W. Burggren, and K. French. 2002. Eckert animal physiology: mechanisms and adaptations. 5th ed. W. H. Freeman and Co., New York. 736 pp. + glossary, appendices, index.
- Ross, D. M. 1981. Illusion and reality in comparative physiology. Canadian Journal of Zoology 59:2151-2158.
- Schmidt-Nielsen, K. 1972. How animals work. Cambridge University Press, Cambridge.
- Schmidt-Nielsen, K. 1984. Scaling: why is animal size so important? Cambridge University Press, Cambridge. 241 pp.
- Schmidt-Nielsen, K. 1997. Animal physiology: adaptation and environment. 5th ed. Cambridge University Press, Cambridge. ix + 607 pp.
- Schmidt-Nielsen, K. 1998. The camel's nose: memoirs of a curious scientist. 352 pp. The Island Press. Review
- Somero, G. N. 2000. Unity in Diversity: A perspective on the methods, contributions, and future of comparative physiology. Annual Review of Physiology 62:927-937.
- Willmer, P., G. Stone, and I. Johnston. 2005. Environmental physiology of animals. Second edition. Blackwell Science, Oxford, U.K. xiii + 754 pp.
- August Krogh
- Claude Bernard
- Evolutionary physiology
- Human physiology
- John Speakman
- Knut Schmidt-Nielsen
- Krogh Principle
- Lancelot Hogben
- Peter Hochachka
- Phylogenetic comparative methods
- Raymond B. Huey
- Theodore Garland, Jr.
- Prosser (1950, p. 1)