Metabolism and circadian rhythms of the European blind cave salamander Proteus anguinus and a facultative cave dweller, the Pyrenean newt (Euproctus asper)

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Abstract
Comparisons of circadian rhythmicity, behavior, and metabolism between surface- and cave-dwelling salamanders allow evolutionary trends in these processes to be elucidated. The proteid Proteus anguinus, an obligate cave-dweller, showed no apparent daily rhythm of activity or resting metabolic rate. In contrast, the salamandrid Euproctus asper, a surface-dweller/facultative cave-dweller, had a circadian resting metabolic rate and activity cycle. These circadian rhythms had an endogenous component. The lives of both studied salamanders were characterized by long periods of inactivity punctuated by bouts of foraging or exploratory/predatory behavior. Proteus anguinus had reduced resting metabolic and spontaneous activity rates (considerably lower than those of most surface-dwelling amphibians), and therefore appears to be a good example of a vertebrate as a low-energy system. The low metabolic and activity rates of P. anguinus are interpreted as adaptations to a subterranean environment, where a poor and discontinuous food supply and (or) intermittent hypoxia may be present for long period