The metabolic cost of breathing in red-eared sliders: An attempt to resolve an old controversy.

Accurately measuring the metabolic cost of breathing in turtles has been a challenge with cost estimates varying greatly between different studies and/or methods used. To determine the source of discrepancy, we calculated costs using two methods in a single group of red-eared sliders (Trachemys scripta elegans). The unidirectional ventilation method yielded an estimate of 3.3ml O2/L air ventilated while the regression method (using hypoxia as a respiratory stimulus) produced an estimate of 0.8ml O2/L air ventilated when corrected for hypoxia-induced metabolic suppression. Cost differences may be in part due to the non-linear nature of the relationship between metabolic cost and ventilation. They are also likely due to the challenge of accurately estimating costs from irregular, episodic breathing pattern of turtles and the buffering capacity of their large lungs that lead to inconsistency in the amount of O2 extracted from each breath/breathing episode. Given the difficulty in obtaining consistent measures, the values reported here must be taken cautiously.