The Metabolic Theory of Ecology
Energetics of stress: linking plasma cortisol levels to metabolic rate in mammals.
Physiological stress may result in short-term benefits to organismal performance, but also long-term costs to health or longevity. Yet, we lack an understanding of the variation in stress hormone levels (i.e. glucocorticoids) that exist within and across species. We linked the primary stress hormone in most mammals (i.e. cortisol) to metabolic rate and showed that baseline concentrations of plasma cortisol vary with mass-specific metabolic rate among cortisol-dominant mammals, and both baseline and elevated concentrations scale predictably with body mass. We used phylogenetic analyses to consider non-independence of shared evolutionary histories. The results quantitatively link a classical measure of physiological stress to whole-organism energetics, providing a point of departure for cross-species comparisons of stress levels among mammals. This work was published in Biology Letters and can be found here.
Metabolic constraints on dive duration in endothermic and ectothermic vertebrates
Dive duration in air-breathing vertebrates is thought to be constrained by the volume of oxygen stored in the body and the rate at which it is consumed (i.e., “o xygen store/usage hypothesis” ). The body mass-dependence of dive duration among endothermic vertebrates is largely supportive of this model, but previous analyses of ectothermic vertebrates show no such body mass-dependence. We showed that dive duration in both endotherms and ectotherms largely support the oxygen store/usage hypothesis after accounting for the well-established effects of temperature on oxygen consumption rates. Analyses of the body mass and temperature dependence of dive duration in 181 species of endothermic vertebrates and 29 species of ectothermic vertebrates show that dive duration increases as a power law with body mass, and decreases exponentially with increasing temperature. Thus, in the case of ectothermic vertebrates, changes in environmental temperature will likely impact the foraging ecology of divers. This work was published in PeerJ and is available here.
The natural logarithm of median (A) and maximum dive duration (min.; B) as a function of the natural logarithm of body mass (g) for air-breathing endothermic (birds and mammals; closed points, solid line) and ectothermic vertebrates (reptiles and amphibians; open points, dashed line). Data were normalized to 30°C assuming a Q10 of 2.5.