Fig. 3. Temperature-compensated aerobic scopes depending on standard metabolic rates (SMRs) in warm-adapted fish or other ectotherms compared with cold-compensated SMRs in eurytherms (Northern hemisphere) and reduced SMRs in Antarctic (and possibly Arctic) polar stenotherms. As a consequence of metabolic adjustments to cold, active metabolic rates at maximum aerobic activity (given as the metabolic rate at the critical swimming speed, U_crit) may be cold-compensated in eurytherms, whereas lower rates may result for those Antarctic stenotherms with low SMRs (see, however, Fig. 2A and the text for a balanced view of these patterns). The low SMR in polar stenotherms despite high mitochondrial densities suggests that capacities are downregulated to levels expected from the Q₁₀ relationship, possibly because of high Arrhenius activation energies (see text). For each of the four groups (warm-adapted versus temperate eurythermal versus cold eurythermal versus polar), straight vertical arrows depict the relationship between standard and maximum aerobic metabolic rate. The warm-water situation is interpreted to be the original situation (on evolutionary time scales) and, accordingly, to represent `the hub of metabolic cold adaptation' according to Pörtner et al. (2000).