The same countdown underlies both strategies
The leading explanation for why hibernators wake up at all is that something accumulates or runs out during torpor. A two-process model formalizes this as an hourglass: a process H steadily declines (or a metabolite steadily builds), and once it reaches a threshold, arousal is triggered as a logistic switch [1]. This single mechanism reproduces several observed patterns, including the roughly linear relationship between torpor metabolic rate and bout length, and the way ambient temperature shifts how long a bout lasts [1].
A separate modeling line reaches a compatible picture from body-temperature data. In Syrian hamsters, an unbiased model-selection approach found that a short cycle of several days is modulated by an unexpectedly long period of 120–430 days, and the same model reproduced temperature fluctuations in 13-lined ground squirrels [2]. A 2025 review groups these ideas together, noting that the frequency-modulation model captures two hidden periodicities — one of a few days and one of about a year — in both species [3]. In plain terms, the day-to-week torpor bout sits inside a year-long envelope, and both strategies seem to use this layered arrangement.
Facultative hibernators can be tuned by the environment
The two-process model explicitly allows the threshold to shift with season, and it argues that this design has adaptive advantages: bout length can be adjusted easily to environmental conditions and to the animal's energy reserves, and for species that keep foraging, the cycle can be entrained to the light–dark cycle [1]. That is the mechanistic route by which an externally primed hibernator could have its torpor timing bent by what is happening outside.
The model also permits, but does not require, circadian cycles in the threshold, which would concentrate arousals at particular times of day — a pattern seen in some hibernators — while not requiring circadian rhythms in body temperature or metabolism during torpor itself [1]. The word choice matters: this is a proposed and permitted mechanism, not a demonstrated cause of facultative timing.
At the tissue level, liver sequencing in hibernating Syrian hamsters — a facultative species — found the expected shift in fuel use and suppression of RNA transcription and cell-cycle activity, plus a previously unreported suppression of MAPK and protein phosphatase 1 pathways during torpor, with promoter methylation modulating the expression of genes targeted by MAPK-induced transcription factors [5]. These are molecular changes between hibernation phases; the study does not compare facultative with seasonal animals, so it identifies candidate machinery rather than proving a difference between the two strategies.
Seasonal hibernators keep time without external cues
The clearest contrast comes from the ground-squirrel result. 13-lined ground squirrels can undergo repeated hibernation according to intrinsic circannual rhythms under constant laboratory conditions, and the frequency-modulation model reproduced their temperature fluctuations [2]. That is direct evidence that the year-scale component of the cycle can run without environmental priming.
The broader neuroendocrine picture is less settled. A review of brain adaptations during hypometabolism describes internal regulatory systems controlling torpor–arousal transitions and seasonal rhythms, and notes that both obligate and facultative hibernators show certain changes during torpor [4]. Because the available summary is fragmentary, it supports only the general point that internal control systems are involved in both groups, not a clean division of labor between them.
What the models still leave open
None of these modeling papers directly compares facultative and seasonal hibernators, so the distinction drawn here is an inference from what each model allows and from which species were tested. The two-process model is presented as a hypothesis-and-theory framework, and the 2025 review states plainly that the mechanisms underlying torpor–arousal cycles remain unresolved despite decades of work [1][3].
The practical upshot for a curious reader: expect the difference between the two strategies to lie less in having a clock versus not having one, and more in how tightly the threshold and the long period are coupled to external conditions. The frequency-modulation work is described by its authors as a starting point for quantitative comparisons across hibernating species [2] — which is exactly the comparison that would test the distinction directly.
Sources used in this answer
Hypothesis and Theory: A Two-Process Model of Torpor-Arousal Regulation in Hibernators
A two-process model in which a declining hourglass process triggers arousal at a threshold can reproduce the linear torpor-metabolic-rate/bout-duration relationship, ambient-temperature effects on bout length, and seasonal modulation, and it allows but does not require circadian threshold cycles.
Frequency-modulated timer regulates torpor–arousal cycles during hibernation in distinct small mammalian hibernators
A frequency-modulation model with a short period of several days modulated by a 120–430-day period reproduced body-temperature fluctuations in Syrian hamsters and in 13-lined ground squirrels hibernating on intrinsic circannual rhythms under constant conditions.
Models toward understanding torpor–arousal cycles during hibernation
A review of hourglass, water-balance, clock, molecular, two-process and frequency-modulation models finds the mechanisms of torpor–arousal cycles still unresolved, while noting the frequency-modulation model captures two hidden periodicities of days and about a year in hamsters and ground squirrels.
Brain Dead: The Dynamic Neuroendocrinological Adaptations During Hypometabolism in Mammalian Hibernators
Neuroendocrinological adaptations are described as controlling torpor–arousal transitions and seasonal rhythms, with both obligate and facultative hibernators showing certain changes during torpor.
Liver transcriptomic and methylomic analyses identify transcriptional mitogen-activated protein kinase regulation in facultative hibernation of Syrian hamster
In hibernating Syrian hamster liver, torpor involved shifts in fuel use, suppressed RNA transcription and cell-cycle activity, and previously unreported suppression of MAPK and protein phosphatase 1 pathways, with promoter methylation modulating MAPK-targeted transcription factor expression.