Light-Gated, Vibration-Amplitude–Dependent Action Selection in Stag Beetles: Threat Display versus Tonic Immobility

Abstract

Beetles, especially nocturnal Lucanidae, when confronted with danger will alternate between conspicuous threat display (head elevation, mandible opening, stridulation) and tonic immobility (TI). This review synthesizes evidence into a compact gate × branch model: light acts as an activity gate while substrate-borne vibration supplies the branching signal. Decreasing luminance opens the gate and elevates readiness; within the active state, low to moderate vibration favours display whereas high amplitudes recruit TI. The mechanistic substrate comprises highly sensitive leg mechanoreceptors—the subgenual and chordotonal organs and campaniform sensilla—whose macro- and micro-mechanical filtering encodes amplitude and spectrum. Behavioural switching shows asymmetric entry and arousal thresholds (hysteresis), explaining history-dependent onset and recovery of TI. Artificial light at night can delay, damp or mistime the gate, shifting decision boundaries for identical mechanical inputs. Additional modulators include genetic and morphological variation, energetic state, substrate transfer properties and vibrational noise. The synthesis specifies measurable axes (illuminance, peak acceleration/displacement, response class, latency and TI duration) and predicts that shielded, dim lighting with low-amplitude disturbance promotes display/locomotion whereas bright, unshielded light and impulsive shocks increase TI frequency and duration.