Models toward understanding torpor–arousal cycles during hibernation

Hibernation is an adaptive strategy that allows animals to survive periods of food scarcity and harsh environmental conditions by entering a state of reduced metabolic activity. A key characteristic of hibernation in many species is the torpor–arousal cycle, where body temperature fluctuates with periodicity ranging from days to weeks. These cycles, while conserved across many mammalian hibernators, exhibit species-specific variation in amplitude. Despite decades of research, the mechanisms underlying torpor–arousal cycles remain unresolved. This review examines current hypotheses, including the hourglass, water balance, clock, molecular, two-process, and frequency-modulation (FM) models, to elucidate the drivers of torpor–arousal cycles. I highlight how the FM model captures key features of body temperature fluctuations in species such as Syrian hamsters ( Mesocricetus auratus (Waterhouse, 1839)) and 13-lined ground squirrels ( Ictidomys tridecemlineatus (Mitchill, 1821)), revealing two hidden periodicities—one spanning a few days and the other about a year—both present in these animals. My review provides perspective on the advantages and limitations of various mathematical models for understanding the mechanisms underlying the torpor–arousal cycles.