The mitochondrial physiology of torpor in ruby-throated hummingbirds, <i>Archilochus colubris</i>
Bibliographic record
Abstract
Hummingbirds save energy by facultatively entering torpor, but the physiological mechanisms underlying this metabolic suppression are largely unknown. We compared whole-animal and pectoralis mitochondrial metabolism between torpid and normothermic ruby-throated hummingbirds (Archilochus colubris). When fasting, hummingbirds were exposed to 10°C ambient temperature at night and they entered torpor; average body temperature decreased by nearly 25°C (from ∼37 to ∼13°C) and whole-animal metabolic rate (V̇O2) decreased by 95% compared with normothermia, a much greater metabolic suppression compared with that of mammalian daily heterotherms. We then measured pectoralis mitochondrial oxidative phosphorylation (OXPHOS) fueled by either carbohydrate or fatty acid substrates at both 39°C and 10°C in torpid and normothermic hummingbirds. Aside from a 20% decrease in electron transport system complex I-supported respiration with pyruvate, the capacity for OXPHOS at a common in vivo temperature did not differ in isolated mitochondria between torpor and normothermia. Similarly, the activities of pectoralis pyruvate dehydrogenase and 3-hydroxyacyl-CoA dehydrogenase did not differ between the states. Unlike heterothermic mammals, hummingbirds do not suppress muscle mitochondrial metabolism in torpor by active, temperature-independent mechanisms. Other mechanisms that may underly this impressive whole-animal metabolic suppression include decreasing ATP demand or relying on rapid passive cooling facilitated by the very small body size of A. colubris.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.001 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".