Life history trade-offs in ectotherms impart a pseudo-hormetic response, not hormesis

Abstract

All organisms are faced with survival and fitness challenges. However, differences in how ectotherms and endotherms deal with these challenges causes confusion when theoretical explanations are proposed. Herein, an immunochallenged, immature ectothermic vertebrate increased growth in the face of immunochallenge, reminiscent of up-regulated physiological efficiency termed hormesis. The immunochallenged subjects increased food intake relative to control animals, a largely ignored possibility in previous studies. This likely led to an energy surplus that fueled additional growth. Although there was increased resource demand from the immune response that exceeded internal stores, the acceptably sized food items contained more resources than immunologically-driven demand required. We theorize that because ectotherms lack significant internally-stored resources compared to endotherms, they must feed to fuel increased physiological demand. This can lead to excess resource intake because the minimum acceptably sized prey contains more available resources than upregulation required. This creates a pseudohormetic hormetic response, fueled by excess food intake rather than significant improved physiological efficiency. Further, we speculate that lifespan and/or maturity may interact with resource management ectotherms, though our data are inconclusive on this matter. Ultimately, our data suggest additional growth when ectotherms face stressors is a pseudo-hormetic response stemming from increased food intake instead of upregulated physiological efficiency.