As the “fire of life”, metabolic rate is critical to animal performance. As energy availability and oxygen transport potential are not limitless, all animals face trade-offs between competing demands. A key interest of eco-physiologists is to understand these trade-offs and determine which strategies might prove the most efficient in the face of environmental challenges like climate warming.
Here, we take three large datasets on a diadromous fish species, Galaxias maculatus, to assess metabolic rates of individuals as they transition from juveniles to reproductive adults across four acclimation temperatures (15, 18, 20, and 23 °C). We use natural variations in growth rate and reproductive maturation across each individual’s life history to test for relationships with different levels of metabolic performance from resting (standard metabolic rate, SMR) to maximal (maximum metabolic rate, MMR). We predict that if growth and/or the level of reproductive maturation are significant drivers of metabolic variability, there will be positive relationships between mass-standardised metabolism and individual growth phenotypes and/or reproductive investment.
We find that allometry of SMR differs predictably with warming temperature, whereby the intercept increases and the slope decreases. This generates a trend in which across-treatment variance in SMR declines as mass increases, with all temperatures converging to a similar metabolic level for the fish with the highest body masses. MMR allometry shows no difference in slope with temperature, but intercepts differ in a manner that suggests a performance “ceiling” is reached across the temperature range.
Linear regression analyses on mass-standardised variables show that individual growth phenotypes have a weakly positive influence on the magnitude of SMR and MMR at some temperatures, suggesting that SMR and MMR of fasted fish are slightly impacted by growth. However, we find no consistent directional correlation between level of reproductive maturation and SMR or MMR at any temperature, raising questions around the allocation of energy to reproduction in fish. We suggest that metabolism, growth and reproductive investment may not trade-off as clearly as theory predicts, and we encourage further research into the complicated balance of these traits in maximising lifetime fitness in an era of climate change.