Torpor-Bout Timing and Integrated Physiological State Assessment: A Feedback Model of Mammalian Hibernation

Periodic torpor–arousal cycles are a characteristic feature of mammalian hibernation, yet the mechanisms determining the timing and termination of individual torpor bouts remain unresolved. Several mechanisms have been proposed, including metabolic hourglass processes, accumulation or depletion of metabolites, circadian mechanisms and other endogenous timing processes. Here, we propose a melatonin-linked feedback model in which the endogenous process terminating a torpor bout initiates interbout arousal (IBA), but does not itself determine the subsequent integrated physiological state of the animal. Instead, arousal initiates recovery of metabolic, neuroendocrine and circadian processes. As physiological function is restored during IBA, melatonin-associated photoperiodic signalling may become functional again, allowing the organism to integrate information about the prevailing environmental photoperiod with its recovered physiological state. This process could influence whether the animal re-enters torpor or remains euthermic. The central hypothesis is therefore not that melatonin directly triggers arousal, but that recovery of melatonin-associated signalling during IBA may provide a feedback link between external photoperiodic information and subsequent torpor–arousal cycling. The model generates experimentally testable predictions concerning the temporal relationship between arousal, melatonin signalling, photoperiod and subsequent torpor re-entry.