Age modifies metabolic but not survival responses to immune challenge in female yellow mealworm beetles (Tenebrio molitor)

Immune activation is energetically costly, yet the extent to which physiological costs translate into consequences for organismal performance remains unclear. Because immune responses and energy allocation can vary with age, we experimentally manipulated the type and intensity of immune stimulation (injection versus implantation) in young and old female Tenebrio molitor and quantified the effects on metabolic rate and survival under food limitation. Immune stimulation elevated metabolic rate, with treatment explaining approximately 30% of the variance in log-transformed metabolic responses, particularly following injection-based challenges. In contrast, survival under food limitation was only weakly affected by immune treatment (∼2% variance explained), but was strongly influenced by body mass (∼15%). Metabolic responses differed between young and old females, indicating age-dependent physiological sensitivity to immune challenge, whereas the effects of immune treatment on survival were similar across age classes. These findings demonstrate that immune activation produces pronounced physiological costs, but these costs do not necessarily translate into reduced starvation tolerance. Age modifies physiological responses to immune challenge, yet energetic reserves remain the primary determinant of survival under food limitation.