The recent paper in Journal of Experimental Biology by Seibel et al. (2021) claims that a new term, the oxygen supply capacity (α), breathes new life into critical oxygen partial pressure (Pcrit). Despite the authors' good intentions, their idea does not resuscitate the 88 year old concept of Pcrit (Tang, 1933). It would be wonderful if the α-line were prescriptive and biologically meaningful across species and environmental conditions, but we suspect that it is not. We argue for what the data have told us for years: the relationship between any metabolic rate (MR) and PO2 is not adequately described by straight lines.

We raise several problems with their overly simple linear model: maximum metabolic rate (MMR)=PO2×α. This linear line is anchored at the origin, must pass through any...

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