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Oxygen levels in the Earth's atmosphere fluctuated dramatically during the Phanerozoic eon, and may have reached concentrations that were 60% higher than current levels. These atmospheric changes have engendered much speculation about the relationship between O 2 availability and major evolutionary events among animals (eg, insect gigantism, flapping flight, endothermy). With the exception of studies of exercising humans, a limited number of investigations have examined the potential effects of hyperoxic conditions on physiologic performance variables in animals. Given that the energetic costs of maintenance and digestion can account for the majority of the total energy budget of snakes, this study was conducted to determine the effect of simulated paleoatmospheric oxygen concentration on the metabolic rates of postabsorptive and postprandial snakes. A series of three repeated-measures trials conducted on Western diamondback rattlesnakes under oxygen concentrations ranging from 21% to 50% revealed that hyperoxia did not affect resting metabolic rates, but that 35% O 2 was sufficient to reduce specific dynamic action by an average of 11%. Significant increases in respiratory exchange ratios were also observed in postprandial snakes. Given the degree to which extant snakes are energy-limited, such an energetic benefit may have had significant implications for the feeding behaviors and life histories of ancient snakes as they radiated from

Hyperoxia reduces costs of digestion in snakes: potential bioenergetic consequences of the paleoatmosphere