Putting the August Krogh principle to work in developmental physiology.

The August Krogh principle has guided many comparative physiological studies, being especially useful for developmental physiology. Several attributes of unusual, if not unique, animals enable researchers to understand developmental phenomena more generally - the essence of the Krogh principle. This article provides examples of unusual traits of animals currently being used to understand development and reproduction. 1) Accelerated development greatly minimizes time spent examining how animals develop across time from egg to adult. For example, the tropical gar begins to breath air within as little as 2.5 days after hatching - much faster than other air-breathing fishes - facilitating study of the development of respiratory reflexes in fishes. 2) Transparency of the body wall has been exploited to image cardiac output in near-microscopic larvae of the zebrafish and mahi mahi, and to capitalize on bacterial biosensors to investigate development of in vivo digestive function in Caenorhabditis elegans. 3) Gigantism, as in the chicken-sized embryos of the emu, or the larvae of the paradoxical frog, allows surgeries not otherwise feasible. 4) Reproductive traits such as polyembryony in armadillos and parthenogenesis in planaria have informed us about classic gene vs. environment questions. Finally, 5) large body mass range enables clearer allometric analyses. Insects like the silk moth, show a more than a 1000-fold difference between eggs and adults. The August Krogh principle, then, is not simply to justify the study of exotic animals (as interesting as that is!), but has been used to generate a broader synthesis and understanding of all taxa.