The hummingbird’s Adipostat: Can a Simple Rule Explain Torpor Frequency and Duration in Hummingbirds?

Because hummingbirds are small and have an expensive mode of locomotion, they have constrained energy budgets. Torpor is used to buffer against these energetic challenges, but its frequency and duration vary. We measured lipid content, metabolic rates, and torpor use in two species of migrating hummingbirds, Calliope (Selasphorus calliope) and Rufous hummingbirds (Selasphors rufus) at a stopover site. We constructed a mass-balance model to predict lipid thresholds for torpor entry, torpor duration, and minimum morning lipid reserves. Hummingbirds entered torpor if their lipid contents were below a sharply defined threshold. Torpor duration increased as initial lipid content decreased, and birds that entered torpor had relatively constant morning lipid reserves. We propose a Minimum Morning Reserve Hypothesis that identifies torpor lipid thresholds and predicts frequency and duration. Several hypotheses were proposed previously to explain torpor’s ultimate function, which can be derived ..., The data used in this research was collected through capture of two species of migrating hummingbirds (S. rufous and S. calliope). Body lipid content was measured using a Quantitative Magnetic Resonance Scanner, and metabolic rates were determined using Flow-through Respirometry. This dataset was used to develop a model to predict torpor thresholds and durations, and minimum morning lipid reserves in migrating hummingbirds., , # The Hummingbird’s Adipostat: Can a Simple Rule Explain Torpor Frequency and Duration in Hummingbirds?

[https://doi.org/10.5061/dryad.c866t1gh2](https://doi.org/10.5061/dryad.c866t1gh2)

## **Description of the data and file structure**

We measured fat content and metabolic rates on 43 captured migrating hummingbirds of two species (*S. rufus and S. calliope*). Metabolic rates were derived using a flow-through respirometry system. We used Carbon Dioxide (CO2) output and chamber flow rates from each bird to compute Vco2 in milliliters per minute, throughout each overnight metabolic trial. We used thermal equivalents (liters of CO2 per gram of fat consumed) to transform Vco2 values to fat consumption rates in milligrams of fat per hour. Fat contents of birds were measured non-invasivley in grams, using a Quantitative Magnetic Resonance (QMR) scanner.

**Files and variables**

**File: The_Hummingbird_s_Adipostat_Supplemental_Data.xlsx**

**Description:** Microsoft Excel file with metabo...