Intrapopulational variation in the standard metabolic rate of insects: Repeatability, thermal dependence and sensitivity (Q₁₀) of oxygen consumption in a cricket

Studies focusing on physiological variation among individuals, and its possible evolutionary consequences, are scarce. A trait can only be a target of natural selection if it is consistent over time, that is, a trait must be repeatable. In ectotherms it has been suggested that standard metabolic rate (MR) is related to Darwinian fitness, since it reflects energy usage and expenditure. The metabolic rate of the cricket Hophlosphyrum griseus was determined at three ambient temperatures. Repeatability of MR was estimated by product-moment correlation on residuals of body mass, as well as the thermal sensitivity of MR on an individual basis (individual Q₁₀). The MR of H. griseus was significantly repeatable (r=0.53) and highly dependent on ambient temperature, and its sensitivity (Q₁₀) was dependent on the temperature range. Our estimation of MR repeatability was high in comparison to published studies in vertebrates. Ours is the second report of repeatability (i.e. consistency over time of an individual's performance ranking within a population) of any aspect of energy metabolism in an insect, and also the first study to report significant repeatability of MR. Individual Q₁₀ values revealed important interindividual variation, which reflects the existence of intrapopulational variability in the thermal sensitivity of MR. In addition, individual Q₁₀ values were negatively correlated between temperature ranges. This means that crickets having low Q₁₀ at low temperatures, presented high Q₁₀ at high temperatures, and vice versa. Our results suggest that MR could be of selective value in insects, showing consistency over time and intrapopulational variability in its thermal dependence. Nevertheless, its heritability remains to be determined.