Oxygen Supply and Demand at the Incipient Lethal Oxygen Level.

Several prominent physiological and ecological theories posit that oxygen supply limits metabolic rate and aerobic scope for performance. Here I show, in contrast, that oxygen supply evolves to meet the ecologically mandated maximum demand at the prevailing environmental oxygen partial pressure. Exercise and environmental hypoxia pose distinct oxygen supply challenges that must be met by a common oxygen supply system. As a result, maximum and basal metabolic rates, their respective critical oxygen partial pressures, and the temperature and size coefficients for each metric, are mechanistically and quantifiably linked in animals across the tree of life. From this relationship I estimate the response of vertically migrating animals, with focus on the jumbo squid, Dosidicus gigas, to correlated oxygen and temperature changes with depth. Relaxed selection for active metabolism at depth, rather than oxygen limitation per se, results in a strong temperature sensitivity of metabolism and enhanced tolerance to hypoxia during daytime forays into the oxygen minimum zone. Specific selection for low oxygen tolerance results in the capacity for metabolic suppression. Vertically migrating animals are effectively shallow-living species for which active metabolism occurs only in warm, oxygenated waters.