PLASTICITY IN ENERGY METABOLISM
is metabolic rate a plastic trait?
Metabolic rates reflect the energetic cost of living but exhibit remarkable variation among conspecifics, partly as a result of the constraints imposed by environmental conditions. Metabolic rates are sensitive to changes in temperature and oxygen availability, but effects of food availability, particularly on maximum metabolic rates, are not well understood. Here we show in brown trout (Salmo trutta) that maximum metabolic rates are immutable but minimum metabolic rates increase as a positive function of food availability. As a result aerobic scope (i.e. the capacity to elevate metabolism above baseline requirements) declines as food availability increases. These differential changes in metabolic rates likely have important consequences for how organisms partition available metabolic power to different functions under the constraints imposed by food availability.
is plasticity in metabolic rate adaptive?
Phenotypic plasticity in physiological, morphological, and behavioral 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.
Plasticity 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.
We examined individual variation in metabolic plasticity in response to changing food levels and its consequences for somatic growth in juvenile brown trout (Salmo trutta). 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.
Plasticity 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.
Plasticity in metabolic rate reduces weight loss during tough times
Energy stores are essential for the overwinter survival of many temperate and polar animals, but individuals within a species often differ in how quickly they deplete their reserves. These disparities in overwinter performance may be explained by differences in their physiological flexibility in response to food scarcity. However, little is known about how these phenotypic changes affect their winter energy use.
We examined individual flexibility in standard metabolic rate in response to food scarcity and their consequences for depletion of lipid stores among overwintering brown trout (Salmo trutta). Metabolism tended to decrease, yet individuals exhibited striking differences in their physiological flexibility. The rate of lipid depletion was negatively related to decreases in metabolic and rate, with the smallest lipid loss over the simulated winter period occurring in individuals that had the greatest reductions in metabolism.