Bacterial populations in long-term stationary phase laboratory cultures can provide insights into physiological and genetic adaptations to low-energy conditions and population dynamics in natural environments. While overall population density remains stable, these communities are very dynamic and characterized by the rapid emergence and succession of distinct mutants expressing the Growth Advantage in Stationary Phase (GASP) phenotype, which can reflect an increased capacity to withstand energy limitations and environmental stress. Here we characterize the metabolic heat signatures and growth dynamics of GASP mutants within an evolving population using isothermal calorimetry. We aged Escherichia coli in anaerobic batch cultures over 20 days inside an isothermal nanocalorimeter and observed distinct heat events related to the emergence of three mutant populations expressing the GASP phenotype after 1.5, 3, and 7 days. Given the heat produced by each population, the maximum number of GASP mutant cells was calculated revealing abundances of ∼2.5 x 107, ∼7.5 x 106, and ∼9.9 x 106 cells in the population, respectively. These data indicate that mutants capable of expressing the GASP phenotype can be acquired during the exponential growth phase and subsequently expressed in long-term stationary phase (LTSP) culture.