These data received surprisingly sparse mechanistic interpretation, but Robin remarked, ‘it seems clear that the maxim that life can only exist within a relatively narrow pH range is not applicable to the turtle and may only be true for homeothermal animals'. However, in 1962, Eugene Robin published careful measurements of arterial blood gases from freshwater turtles maintained at 10, 24 and 37☌ that revealed a consistent rise in arterial P CO 2 and a fall in pH over this temperature range. Yapp's contemporary textbook was even less informative: ‘The normal pH varies from one animal to another but is usually between 7 and 8’ ( Yapp, 1961). Ladd Prosser's influential textbook from 1950 merely stated that ‘land vertebrates have a blood pH usually near 7.4’ ( Prosser, 1950), whereas William B. In the early 1960s, relatively little was known about acid-base physiology of ectothermic vertebrates. Nevertheless, the α-stat hypothesis was also met with critiques that were difficult to resolve and may explain why the topic has received remarkably limited attention in recent years. This hypothesis, which suggested that it is the ionisation of proteins, particularly the α-imidazole groups on histidines, that is being regulated must certainly be regarded as one of the major contributions of the comparative approach to general physiology. During the ensuing decades, a series of comparative studies, particularly on fish, frogs and turtles, led to the formulation of the α-stat hypothesis. This fundamental question occupied Herman Rahn and Robert Blake Reeves, both at the Department of Physiology at The State University of New York at Buffalo, USA, from the late 1960s. The delicate regulation of pH, however, does not reveal why pH is regulated at 7.4 in mammals, whilst many ectothermic vertebrates defend a pH much closer to 8. Here, it is typically emphasised that arterial pH is tightly regulated by the concerted effort of transepithelial movements of acid-base equivalents (through structures such as the kidneys, skin or the gills) and the arterial partial pressure of CO 2 ( P CO 2) as dictated by ventilation of lungs or gills in relation to metabolic CO 2 production. The exquisite maintenance of pH of bodily fluids is a hallmark of homeostatic regulation that occupies numerous lectures of any decent university course in human or comparative physiology.

What is phstat