Critical PO2 is size-independent in insects: implications for the metabolic theory of ecology

• Why metabolic rates scale hypometrically remains unclear.
• The metabolic theory of ecology suggests metabolism is constrained by O2 delivery in larger animals.
• Critical PO2s for growth and metabolic rates of resting or flying insects are size-independent.
• Direct O2-limitation of larger insects cannot explain hypometric scaling of metabolic rates.
• Incomplete data also do not support evolutionary models for O2-supply constraints in larger animals.

Insects, and all animals, exhibit hypometric scaling of metabolic rate, with larger species having lower mass-specific metabolic rates. The metabolic theory of ecology (MTE) is based on models ascribing hypometric scaling of metabolic rate to constrained O2 supply systems in larger animals. We compiled critical PO2 of metabolic and growth rates for more than 40 insect species with a size range spanning four orders of magnitude. Critical PO2 values vary from far below 21 kPa for resting animals to near 21 kPa for growing or flying animals and are size-independent, demonstrating that supply capacity matches oxygen demand. These data suggest that hypometric scaling of resting metabolic rate in insects is not driven by constraints on oxygen availability.

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