We (Seibel et al., 2021) proposed a novel method to quantify the oxygen supply capacity (α), which defines the maximum metabolic rate (MMR) that can be achieved at a given oxygen pressure. In their Correspondence article, Farrell et al. (2021) suggest that our method lacks ‘empirical realism’. The core issue is whether the MMR is linearly related to PO2 and whether that line intercepts the origin as required for the constancy of α, from which its predictive power derives. However, Farrell et al. (2021) overlooked the published derivation and validation of α, which is based on decades of empirical physiology and which demonstrates convincingly that α is constant from rest to maximum exertion (Deutsch et al., 2015; Seibel and Deutsch, 2020).

For a given species, α, calculated as the standard metabolic rate (SMR) divided...

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