Activity alters how temperature influences intraspecific metabolic scaling: testing the metabolic-level boundaries hypothesis

A common belief is that the body-mass scaling of metabolic rate is the result of intrinsic (physical) constraints related to body design. However, many recent studies have shown that extrinsic (ecological) factors significantly affect metabolic scaling relationships, both within and among species. One of these factors is ambient temperature (T), but its influence on the intraspecific (ontogenetic) scaling slope (b) of metabolic rate varies widely. I tested whether the metabolic-level boundaries hypothesis (MLBH) can explain this variation, at least in part. The MLBH predicts that b should correlate negatively with T for resting metabolism, but show variable associations (from negative to positive) for metabolism measured during varying levels of locomotor activity. I tested the MLBH by using a literature survey of T effects on b for resting or routine metabolic rates of 179 animal and plant species. As predicted, sedentary species of ectothermic animals and plants exhibiting no locomotor activity mostly show negative associations between b and T, whereas mobile species exhibiting various low levels of uncontrolled, spontaneous, routine locomotor activity during measurements of resting or routine metabolism show varied associations. A similar difference occurs between teleost fish species exhibiting no locomotor activity versus varying levels of routine locomotor activity. These results show that intrinsic and extrinsic factors (e.g., activity and ambient temperature, respectively) can interactively affect metabolic scaling. Metabolic scaling is highly malleable, and not the simple result of universal physical constraints.