Why the calendar alone doesn't predict torpor
Torpor is a controlled drop in body temperature and metabolism that saves energy. Hibernation is the multiday version; daily torpor is a short bout, typically under a day, used by animals called daily heterotherms [1][3]. Because the challenges an animal faces differ by region, habitat, and diet, the season in which torpor pays off also differs, so the timing of torpor is often less clear-cut than the familiar image of winter hibernation suggests [1].
The two groups part ways here. In hibernators, torpor expression changes significantly with season, and that strong seasonality is most pronounced in sciurid and cricetid rodents (squirrels and their relatives, and hamsters and voles), while it is weaker in marsupials, bats, and dormice [1]. In daily heterotherms, by contrast, torpor is typically not as seasonal, and torpor expression does not change significantly with season [1].
When summer torpor makes more sense than winter torpor
Summer torpor has several plausible advantages: saving energy and water, supporting reproduction or growth when resources are limited, and cutting down foraging time and therefore exposure to predators [1]. When torpor does occur in spring or summer, it is usually shallower and shorter than in winter, both because ambient temperatures are higher and because of seasonal functional plasticity, meaning the animal's own physiology shifts with the season [1].
A few species flip the usual pattern outright. Subtropical nectar-feeding blossom-bats and desert spiny mice use torpor more frequently and more deeply in summer than in winter, which the paper links to seasonal availability of nectar or water [1]. That is a useful reminder that torpor can track a specific resource, not just temperature.
How common is each seasonal timing?
Among hibernators, the cold-season pattern from fall to spring is the most typical, at 48% of species. About 14% hibernate from late summer to spring, 8% can hibernate for an entire year or express torpor throughout the year, and only 6% are rarely restricted to winter alone [1]. So winter-only hibernation is the exception, not the rule.
Daily torpor is diverse across both mammals and birds and, as noted, is generally less seasonal than hibernation [1]. Hibernation itself is known across mammals from all three subclasses and from the arctic to the tropics, but in birds it is known for only one species [1].
What else shapes torpor besides the season
Season is only one input. In juvenile garden dormice, a hibernating species, huddling with others influenced torpor patterns and food intake during the pre-hibernation period, and food availability during hibernation modulated total energy expenditure, particularly in males, possibly through changes in reproductive investment [4][5]. Even so, juveniles reached similar body mass and size at the start and end of hibernation regardless of housing or feeding conditions [4][5]. The authors suggest that combining torpor with huddling gives young dormice developmental flexibility to meet early-life energy demands, a possibility rather than a demonstrated purpose [4][5].
On the question of where data are sparse, these studies do not identify a particular season as under-sampled; the seasonal summaries are broad, and the detailed work here concerns one species in one life stage [1][4]. The clearest gap the evidence supports is uneven coverage across groups: seasonality is well characterized in some rodent families but less pronounced, and presumably less well pinned down, in marsupials, bats, and dormice [1].
Sources used in this answer
Seasonal Expression of Avian and Mammalian Daily Torpor and Hibernation: Not a Simple Summer-Winter Affair†
Seasonal torpor expression differs between hibernators, which change significantly with season (most commonly fall to spring, 48% of species), and daily heterotherms, which typically show no significant seasonal change; a few species such as subtropical blossom-bats and desert spiny mice use more pronounced torpor in summer.
Patterns and expression of torpor
The paper contrasts daily torpor in daily heterotherms with hibernation and discusses seasonal variation between summer and winter torpor patterns.
Hibernation, daily torpor and estivation in mammals and birds: behavioral aspects
The paper covers behavioral aspects of hibernation, daily torpor, and estivation in mammals and birds, noting that daily heterotherm torpor bouts last under a day and that some Australian desert species use daily torpor frequently in winter.
Effects of huddling and food availability on seasonal torpor use and energetics in juvenile garden dormice (<i>Eliomys quercinus</i>)
In juvenile garden dormice, huddling influenced torpor patterns and food intake before hibernation, and food availability during hibernation modulated total energy expenditure, especially in males, while body mass and size at hibernation onset and end were similar across conditions.
Huddling and food availability shape seasonal torpor and energetics of juvenile garden dormice
In juvenile garden dormice, huddling and food availability shaped seasonal torpor and energetics, with the authors suggesting combined torpor and huddling provides developmental flexibility for meeting early-life energy demands.