DOI: 10.5061/dryad.c866t1gh2
title: The hummingbird’s Adipostat: Can a Simple Rule Explain Torpor Frequency and Duration in Hummingbirds?

description: 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 as special cases that result from modifying our mass balance
 model’s parameters. Torpor entails a balance between energy savings and
 the non-energetic risks of torpor, such as predation and physiological
 stress. We assessed energy equivalents of the non-energetic costs of
 torpor by accounting for the energetic costs and benefits of torpor, and
 by documenting its occurrence and length.
description: The data used in this research was collected through capture of
 two species of migrating hummingbirds (<em>S. rufous</em> and
 <em>S. calliope</em>). 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.
description: # 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 metabolic and fat data from captured *S. rufus* and *S. calliope* hummingbirds **Variables** * Species: Species and Genus * Bird Id: Unique Bird Identification Number * Capture Date: day/month/year * Age: Hatching-year (HY), After hatching-year (AHY) * Sex: Male (M), Female (F) * Beg Mb: body mass at beginning of trial in grams * End Mb: body mass at end of trial in grams * Torpor: 0: No Torpor, 1: Torpor * rn: Normothermic Metabolic Rate (milligrams of fat per hour) * rt: Torpid Metabolic Rate (milligrams of fat per hour) * pT: Torpor Duration (hours) * C: Arousal Cost (milligrams of fat) * Fbeg: Fat Mass at Beginning of Trial (milligrams) * Fbeg%: Fat Mass at Beginning of Trial (% BegMb) * Fend: Fat Mass at End of Trial (milligrams) * Fend%: Fat Mass at End of Trial (% EndMb) * Fat-Free Mass: EndMb - Fend * T: Trial Duration (hours)  **Code/software** Microsoft Excel was used to manage and collate data. JMP Statistical Software was used to perform all statistical analyses.