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.
Gen Kurosawa (Mon,) studied this question.