How does torpor and hibernation use vary across climate zones and taxonomic groups, and does this overturn the idea that torpor is a cold-climate specialization?

Torpor is common in warm climates, not just cold ones

The old picture of torpor as a high-latitude, cold-weather trick does not fit the distribution of the animals that use it. Torpor has been reported in tropical and subtropical bats from 10 of the 18 recognized bat families, which together contain 96.7% of all bat species [3]. In those warm regions, short bouts of torpor with relatively high body temperatures are the most common pattern, and hibernation—multi-day bouts of torpor—has been reported in free-ranging subtropical tree-dwelling, cave-dwelling, and house-dwelling bats [3].

Small mammals in temperate and tropical climates also use hibernation and daily torpor, which suggests the behavior is not primarily aimed at cold defense [4]. Instead, it points to a general role for lowering metabolism as a way to cope with a temporary or seasonal bottleneck in energy supply [4].

Many animal groups use torpor, and it evolved repeatedly

Torpor is not confined to one branch of the animal tree. Daily torpor and hibernation occur in many endotherms—animals that maintain a high body temperature—from diverse taxonomic groups [1]. Across 1,338 living endotherms, the two forms appear to sit on an evolutionary continuum, and early endotherm ancestors likely did not hibernate; instead, the trait evolved multiple times in independent lineages [1].

That repeated evolution matters for the climate question. If torpor were only a cold-climate specialization, we would expect it to cluster in a few cold-adapted lineages. Instead, the variation in torpor patterns across living endotherms cannot be attributed solely to environmental niches and partly arises from independent gains of daily torpor and hibernation in various clades [1].

Daily torpor and hibernation are distinct, and latitude shifts the balance

Daily torpor and hibernation are not just points on a smooth spectrum in practice. In an analysis of 214 species (43 birds and 171 mammals), cluster analysis confirmed the classical distinction between the two [2]. Hibernators are significantly heavier than daily heterotherms, and their torpor bouts last far longer: average maximum bout duration was more than 30-fold longer, and mean bout duration more than 25-fold longer, in hibernators [2].

Geography shifts which form is more common. Hibernators are distributed at higher average latitudes (about 35°) than daily heterotherms (about 25°) [2]. That pattern is consistent with colder, more seasonal places favoring long hibernation, while warmer places more often host short daily torpor—but the warm-climate records show that torpor itself is not absent there [2][3].

The energy savings work in warm conditions too

The reason torpor can pay off outside the cold is that it cuts energy use dramatically. In tropical and subtropical bats, torpor reduces energy expenditure by 50–99% compared with normal active metabolism, even under warm conditions [3]. During entry into torpor, metabolic rate falls to a fraction of normal, along with ventilation and heart rate, and body temperature drifts down toward ambient temperature [4].

Bats illustrate why this is useful in the tropics: they are small, spend large amounts of energy when active, depend on a fluctuating food supply, and have only a limited capacity to store fat [3]. A temporary drop in food or water is enough to make deep energy savings worthwhile, whether or not the weather is cold [3][4].

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