Effects of Contingency versus Constraints on the Body-Mass Scaling of Metabolic Rate

Abstract

I illustrate the effects of both contingency and constraints on the body-mass scaling of metabolic rate by analyzing the significantly different influences of ambient temperature (T a ) on metabolic scaling in ectothermic versus endothermic animals. Interspecific comparisons show that increasing T a results in decreasing metabolic scaling slopes in ectotherms, but increasing slopes in endotherms, a pattern uniquely predicted by the metabolic-level boundaries hypothesis, as amended to include effects of the scaling of thermal conductance in endotherms outside their thermoneutral zone. No other published theoretical model explicitly predicts this striking variation in metabolic scaling, which I explain in terms of contingent effects of T a and thermoregulatory strategy in the context of physical and geometric constraints related to the scaling of surface area, volume, and heat flow across surfaces. My analysis shows that theoretical models focused on an ideal 3/4-power law, as explained by a single universally applicable mechanism, are clearly inadequate for explaining the diversity and environmental sensitivity of metabolic scaling. An important challenge is to develop a theory of metabolic scaling that recognizes the contingent effects of multiple mechanisms that are modulated by several extrinsic and intrinsic factors within specified constraints.

Rediscovering and Reviving Old Observations and Explanations of Metabolic Scaling in Living Systems

“…In addition, high metabolic activity in resting organisms (as related to energetic lifestyles or high temperatures) may often cause SA (and possibly RT) mechanisms to influence metabolic scaling more than RD mechanisms (Figure 4). This view is supported by frequently observed inverse correlations between the slopes and elevations of metabolic scaling relationships, both within and among species, as predicted by the MLBH [19,20,46,56,129]. Experimental manipulations of environmental temperature and oxygen concentrations, as well as of respiratory surface areas, also support primary effects of SA mechanisms on metabolic scaling in specific contexts (see reviews in [19,20]).…”

“…In particular, metabolic heat production compensating for surface-area-related heat loss figures importantly in various heat-flow models [41,122,124], the metabolic-level boundaries hypothesis (MLBH) [19,39,46,56], and the heat dissipation model [123], as applied specifically to endotherms. Multiple observations support thermoregulatory SA models, including the close similarity between the body-mass scaling of body surface-area, heat loss and metabolic heat production in resting birds and mammals and those exposed to cold in a He-O 2 atmosphere with high thermal conductivity [20,39,41,46,56,127,128], the close similarity between the scaling of basal metabolism in huddled mammals and that of the exposed surface area of a huddle [20], and the similar scaling of metabolic heat production and thermal conductance in birds and mammals at low ambient temperatures (e.g., 0 • C) [129].…”

“…First, changes in temperature can affect not only the elevation, but also the slope of metabolic scaling relationships, both within and among species (see Section 2.1.4), as predicted by the MLBH [19,46,54,56,57,59,129] and the viscosity hypothesis of Verberk and Atkinson [256]. Second, several other kinds of abiotic and biotic factors may influence metabolic scaling slopes (reviewed in [19,20,56,103,121]).…”

Commentary: On the Interpretation of the Normalization Constant in the Scaling Equation

“…The MLBH further predicts that any factor that affects the elevation of metabolic scaling relationships (quantified as β or L), including temperature, physiological state, and activity level, will in turn affect the slope via shifts in the relative influence of various metabolic processes that relate to either surface area or volume. Many studies have tested the MLBH, often with confirming results (reviewed in Glazier, 2008Glazier, , 2009aGlazier, , 2010Glazier, , 2014bGlazier, , 2018b.…”

The Biological Clock in Gray Mouse Lemur: Adaptive, Evolutionary and Aging Considerations in an Emerging Non-human Primate Model

“…Since the basal metabolic rate is inversely correlated with body mass in endotherms, important energetic demands and costs are extremely marked in small eutherians (Gillooly et al, 2001; Glazier, 2018). That is the reason why a lot of small species use adaptive physiological mechanisms to reduce their energy consumption during inactive times of the day (Martin and Yoder, 2014).…”