Effects of temperature acclimation and diet on energetics and behavior of the Mexican axolotl (Ambystoma mexicanum)

Many organisms exhibit physiological responses to monthly or seasonal temperature shifts, but mechanisms driving these “thermal acclimation” effects remain unclear. Prior work on Mexican axolotls (Ambystoma mexicanum) revealed that warm-temperature exposure led to reduced metabolic performance at a new test temperature. We postulated that this pattern was driven by a thermal mismatch between metabolic energy expenditures and energy assimilation (i.e., digestion), resulting in energetic stress at warm temperatures. The metabolic theory of ecology (MTE) predicts higher metabolic rates at warmer temperatures, whereas the dynamic energy budget (DEB) theory predicts that animals with a reduced energy reserve should have reduced metabolic performance. Linking these frameworks together might explain the observed thermal acclimation effects seen in axolotl metabolism. To test the proposed linkages between temperature, metabolism, energetics, and physiological stress, I exposed axolotls to two acclimation temperatures (7°C and 25°C) and three levels of food availability. I then measured their effects on metabolic rates, behavior, and blood lipids. I used linear regression models to test for main and interactive effects of acclimation temperature, performance temperature, and food availability on axolotl metabolic rate, activity levels, and blood lipids. Results generally showed support for the prediction of higher metabolic rates with warmer temperatures and higher levels of reserve energy, with the exception of warm-acclimated fasted animals that exhibited higher than expected metabolic rates at the cooler performance temperature. This result was driven by increased activity levels in this treatment combination, possibly due to increased foraging behavior in animals with the lowest levels of reserve energy. These results support the hypothesis that axolotl thermal responses are driven by thermal mismatches between energy assimilation and expenditure, as predicted by MTE and DEB.