Flexibility in metabolic rate confers a growth advantage under changing food availability

Abstract

1. Phenotypic flexibility in physiological, morphological and behavioural traits can allow organisms to cope with environmental challenges. Given recent climate change and the degree of habitat modification currently experienced by many organisms, it is therefore critical to quantify the degree of phenotypic variation present within populations, individual capacities to change and what their consequences are for fitness. 2. Flexibility in standard metabolic rate (SMR) may be particularly important since SMR reflects the minimal energetic cost of living and is one of the primary traits underlying organismal performance. SMR can increase or decrease in response to food availability, but the consequences of these changes for growth rates and other fitness components are not well known. 3. We examined individual variation in metabolic flexibility in response to changing food levels and its consequences for somatic growth in juvenile brown trout (Salmo trutta). 4. SMR increased when individuals were switched to a high food ration and decreased when they were switched to a low food regime. These shifts in SMR, in turn, were linked with individual differences in somatic growth; those individuals that increased their SMR more in response to elevated food levels grew fastest, while growth at the low food level was fastest in those individuals that depressed their SMR most. 5. Flexibility in energy metabolism is therefore a key mechanism to maximize growth rates under the challenges imposed by variability in food availability and is likely to be an important determinant of species' resilience in the face of global change.

Keywords: energy metabolism; fitness; intraspecific variation; phenotypic flexibility; somatic growth; standard metabolic rate.

Figure 1

Relationship between standard metabolic rate (SMR; mg O₂ h⁻¹) and body mass (mg) of juvenile brown trout. SMR was measured after fish had been on an intermediate ration for 28 days (initial rate) and then measured again (final rate) after they were switched to a low, intermediate or ad libitum ration for an additional 28 days.

Figure 2

Change in standard metabolic rate (SMR) of juvenile brown trout as a function of food availability. SMR was measured after fish had been on an intermediate ration for 28 days (initial rate) and then measured again (final rate) after they were switched to a low, intermediate or ad libitum ration for an additional 28 days. Plotted are back-transformed SMR values standardized for the mean body mass of 10·2 g.

Figure 3

Growth in fork length as a function of change in standard metabolic rate in juvenile brown trout. SMR was measured after fish had been on an intermediate ration for 28 days and then measured again after they were switched to a low, intermediate or ad libitum ration for an additional 28 days. Plotted are partial residuals of growth rate evaluated at mean fork length = 91 mm and mean initial rSMR = 0 as a function of back-transformed values for the change in SMR standardized for the mean body mass of 10·2 g.