How well do molecular mechanisms of torpor found in one hibernator generalize to species in similar ecological niches?

Why the same ecological challenge does not produce the same molecular solution

Torpor—a state of greatly reduced metabolism and body temperature—has evolved repeatedly across distantly related endotherms, from hummingbirds to ground squirrels to bears [4]. A broad evolutionary survey of 1,338 endotherms found that daily torpor and hibernation form a continuum and that the trait arose multiple times in independent lineages, with only weak associations between torpor and species' physiological or environmental characteristics [4]. In other words, animals facing similar seasonal challenges may arrive at torpor through different evolutionary paths, which makes it less likely that a single molecular mechanism will be shared across all of them.

This matters for generalization because even species that look ecologically similar may differ in how they achieve metabolic suppression. The evolutionary evidence suggests that variation in torpor patterns cannot be attributed solely to environmental niches [4], so a mechanism found in one hibernator should be treated as a hypothesis to test in another rather than an assumed universal rule.

A clear example of divergence: muscle myosin in small versus large hibernators

Skeletal muscle makes up roughly half of body mass and is a major consumer of energy, so how muscle shuts down during torpor is a central question. In small hibernators—13-lined ground squirrels and garden dormice—the metabolic states of resting myosin, the muscle protein that consumes ATP even when muscles are relaxed, changed during torpor [2]. At near-torpor body temperatures (8 °C), myosin ATP consumption in fast-twitch type II fibers was reduced by 77–107% during torpor compared with active periods, and the ground squirrel showed hyper-phosphorylation of the Myh2 myosin protein that was predicted to stabilize the molecule [2].

In the larger hibernators—Eurasian brown bears and American black bears—the picture was different: myosin structure and ATP consumption were preserved during hibernation, with no change in the proportion of myosin heads in the resting states or in ATP turnover [2]. The authors interpret the small-hibernator changes as a possible mechanism to reduce muscle energy expenditure in the cold, and the bear pattern as consistent with their milder body-temperature drop [2]. The contrast is a concrete case where body size and torpor depth, not just ecological niche, track with different molecular outcomes.

Similarities across very different animals: what hummingbirds tell us

Anna's hummingbirds are not mammals, yet their daily torpor shows molecular signatures that overlap with mammalian hibernation, including similarities in gene expression and regulatory pathways downstream of hormonal and neuronal control [1]. That is a meaningful point of convergence: some aspects of the torpor response may be shared even across vertebrate classes.

But the same study found that hummingbird torpor does not involve large-scale transcriptomic remodeling—the broad reprogramming of gene activity seen in some hibernators [1]. So the similarity is partial: certain molecular signatures are recapitulated, while the overall scale of gene-expression change differs. This supports a nuanced answer to the generalization question—some mechanisms travel across species, but the full molecular program does not.

What this means for predicting torpor mechanisms in an unstudied species

If you want to predict how a newly studied hibernator suppresses metabolism, the evidence points to checking body size and torpor depth first. Small hibernators that undergo deep, prolonged torpor with body temperatures near 4–8 °C showed myosin remodeling, whereas bears that experience only mild temperature drops did not [2]. That pattern suggests that the molecular strategy may be tied to the physiological demands of deep cooling rather than to sharing a habitat.

Brown adipose tissue (BAT) offers a complementary caution. Small hibernators rely heavily on BAT to rewarm from torpor many times per season, and recent work has illuminated molecular pathways of BAT regulation in these animals [3]. But that review focuses on small hibernators and does not establish whether the same regulatory players operate in large hibernators or in non-mammalian torpor users [3]. Similarly, leptin-related mechanisms of torpor entry have been studied in mice [5], but that work does not address cross-species generalization. The safest conclusion is that torpor's molecular toolkit is partly shared and partly lineage-specific, and ecological similarity alone is a weak predictor of which parts will match.

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