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." 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. 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. 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.