Quantum-Biological Transduction via Ratio-Controlled Regimes and Robust Response Plateaus

Quantum biology has identified nontrivial quantum effects in living systems, yet how quantum dynamics are converted into robust biological function under physiological noise remains unclear. We argue that the key challenge is transduction—how quantum dynamics produce stable biological outputs—rather than coherence preservation. Using radical pair magnetoreception as a model system, we find a pronounced response plateau in magnetic anisotropy as a function of the recombination-rate ratio, robust across modelling choices and absolute rate scaling, while disappearing under hyperfine ablation. Moderate dephasing enhances the response within a bounded noise window, offering a natural account of the mismatch between in vivo robustness and in vitro fragility. Spectral analysis identifies mode identity exchange as a signature of the plateau boundary. A minimal open-quantum toy model reproduces the key qualitative features, suggesting broader applicability. Together, these results support a view of quantum-to-biological transduction as a problem of regime stability rather than signal optimisation.