Abstract
Many biologists appeal to the so-called Krogh principle when justifying their choice of experimental organisms. The principle states that “for a large number of problems there will be some animal of choice, or a few such animals, on which it can be most conveniently studied”. Despite its popularity, the principle is often critiqued for implying unwarranted generalizations from optimal models. We argue that the Krogh principle should be interpreted in relation to the historical and scientific contexts in which it has been developed and used. We interpret the Krogh Principle as a heuristic, i.e., as a recommendation to approach biological problems through organisms where a specific trait or physiological mechanism is expected to be most distinctively displayed or most experimentally accessible. We designate these organisms “Krogh organisms”. We clarify the differences between uses of model organisms and non-standard Krogh organisms. Among these is the use of Krogh organisms as “negative models” in biomedical research, where organisms are chosen for their dissimilarity to human physiology. Importantly, the representational scope of Krogh organisms and the generalizability of their characteristics are not fixed or assumed but explored through experimental studies. Research on Krogh organisms is steeped in the comparative method characteristic of zoology and comparative physiology, in which studies of biological variation produce insights into general physiological constraints. Accordingly, we conclude that the Krogh principle exemplifies the advantages of studying biological variation as a strategy to produce generalizable insights.
Highlights
Classical physiologists, such as Claude Bernard (1813–1878) and August Krogh (1874–1949), emphasized the importance of organism choice for observational, experimental, and comparative studies in biological research. Krogh’s 1929 formulation of this idea has become known as the Krogh principle, which claims that: “for a large number of problems there will be some animal of choice, or a few such animals, on which it can be most conveniently studied” (Krogh 1929)
Krogh organisms are typically not standardized; little effort is put into building specialized infrastructures around them; and they are often chosen for specialized features that allow for investigation of specific biological problems, 3 This claim is based on our examination of over 250 papers that refer to the Krogh principle
This focus is not necessary biased, but some have appealed to the Krogh principle when highlighting that an overly narrow focus may lead to missed opportunities: Today, the main part of biomedical research is limited to very few species
Summary
Classical physiologists, such as Claude Bernard (1813–1878) and August Krogh (1874–1949), emphasized the importance of organism choice for observational, experimental, and comparative studies in biological research. Krogh’s 1929 formulation of this idea has become known as the Krogh principle, which claims that: “for a large number of problems there will be some animal of choice, or a few such animals, on which it can be most conveniently studied” (Krogh 1929). Krebs and Krebs regard the debate as futile because it resulted from problematic extrapolations from studies of different animals (birds and insects) that respond to different physiological and environmental constraints They caution against the application of the Krogh principle to complex problems and recommend that the principle should only be ‘Extreme’ organisms and the problem of generalization:. Some authors frame the problem as “the dilemma of which animal model(s) most accurately represent(s) and reproduce(s) the human condition being investigated” (Arnoczky et al 2009: 32) While this problem is common to all animal experiments, the use of non-standard organisms following the Krogh principle may be problematic as in their view it may lead to “fallacious generalizations”. He argued for the relevance of comparative physiology for medicine, and for the independence of the discipline.
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