Abstract
Food availability varies substantially throughout animals' lifespans, thus the ability to profit from high food levels may directly influence animal fitness. Studies exploring the link between basal metabolic rate (BMR), growth, reproduction, and other fitness traits have shown varying relationships in terms of both magnitude and direction. The diversity of results has led to the hypothesis that these relationships are modulated by environmental conditions (e.g., food availability), suggesting that the fitness consequences of a given BMR may be context-dependent. In turn, there is indirect evidence that individuals with an increased capacity for aerobic work also have a high capacity for acquiring energy from food. Surprisingly, very few studies have explored the correlation between maximum rates of energy acquisition and BMR in endotherms, and to the best of our knowledge, none have attempted to elucidate relationships between the former and aerobic capacity [e.g., maximum metabolic rate (MMR), aerobic scope (Factorial aerobic scope, FAS; Net aerobic scope, NAS)]. In this study, we measured BMR, MMR, maximum food intake (recorded under low ambient temperature and ad libitum food conditions; MFI), and estimated aerobic scope in the leaf-eared mouse (Phyllotis darwini). We, then, examined correlations among these variables to determine whether metabolic rates and aerobic scope are functionally correlated, and whether an increased aerobic capacity is related to a higher MFI. We found that aerobic capacity measured as NAS is positively correlated with MFI in endotherms, but with neither FAS nor BMR. Therefore, it appears plausible that the capacity for assimilating energy under conditions of abundant resources is determined adaptively by NAS, as animals with higher NAS would be promoted by selection. In theory, FAS is an invariant measurement of the extreme capacity for energy turnover in relation to resting expenditure, whereas NAS represents the maximum capacity for simultaneous aerobic processes above maintenance levels. Accordingly, in our study, FAS and NAS represent different biological variables; FAS, in contrast to NAS, may not constrain food intake. The explanations for these differences are discussed in biological and mathematical terms; further, we encourage the use of NAS rather than FAS when analyzing the aerobic capacity of animals.
Highlights
Identifying and understanding the factors driving inter-specific variation in metabolic rates is an important area of research in energetics, and, there is increasing interest in how inter-individual variation in energy metabolism is maintained within populations (Careau et al, 2008; Burton et al, 2011; Maldonado et al, 2011)
Regarding maximum rates of energy expenditure, we found a positive correlation between the residuals of Maximum metabolic rate (MMR) with both measures of aerobic scope
We essentially demonstrate that aerobic capacity is positively correlated with maximum food intake in an endotherm
Summary
Identifying and understanding the factors driving inter-specific variation in metabolic rates is an important area of research in energetics, and, there is increasing interest in how inter-individual variation in energy metabolism is maintained within populations (Careau et al, 2008; Burton et al, 2011; Maldonado et al, 2011). Aerobic scope is a measure of aerobic capacity that describes the extent to which metabolic rate can be increased above baseline energy requirements to drive a range of key functions, including digestion, locomotion, growth, and reproduction (Guderley and Pörtner, 2010). Variation in aerobic scope among populations has been linked to differences in geographic distributions (Naya and Bozinovic, 2012), ability to cope with environmental extremes (Pörtner and Knust, 2007; Kassahn et al, 2009), and migratory effort (Tudorache et al, 2007; Eliason et al, 2011), suggesting that it may be both a trait of ecological relevance and a measure of aerobic capacity
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