Ectotherms overwintering in temperate ecosystems must survive low temperatures while conserving energy to fuel post-winter reproduction. Freeze-tolerant wood frogs, Rana sylvatica, have an active response to the initiation of ice formation that includes mobilising glucose from glycogen and circulating it around the body to act as a cryoprotectant. We used flow-through respirometry to measure CO2 production () in real time during cooling, freezing and thawing. CO2 production increases sharply at three points during freeze–thaw: at +1°C during cooling prior to ice formation (total of 104±17 μl CO2 frog−1 event−1), at the initiation of freezing (565±85 μl CO2 frog−1 freezing event−1) and after the frog has thawed (564±75 μ l CO2 frog−1 freezing event−1). We interpret these increases in metabolic rate to represent the energetic costs of preparation for freezing, the response to freezing and the re-establishment of homeostasis and repair of damage after thawing, respectively. We assumed that frogs metabolise lipid when unfrozen and that carbohydrate fuels metabolism during cooling, freezing and thawing, and when frozen. We then used microclimate temperature data to predict overwinter energetics of wood frogs. Based on the freezing and melting points we measured, frogs in the field were predicted to experience as many as 23 freeze–thaw cycles in the winter of our microclimate recordings. Overwinter carbohydrate consumption appears to be driven by the frequency of freeze–thaw events, and changes in overwinter climate that affect the frequency of freeze–thaw will influence carbohydrate consumption, but changes that affect mean temperatures and the frequency of winter warm spells will modify lipid consumption.
↵† Present address: Department of Entomology and Nematology, University of Florida, Gainesville, FL 32611, USA
Supplementary material available online at http://jeb.biologists.org/cgi/content/full/216/2/292/DC1
This research was supported by a Natural Sciences and Engineering Research Council of Canada (NSERC) Undergraduate Research Award to J.R.S.; an NSERC Discovery Grant, the Canadian Foundation for Innovation and an Ontario Early Researcher Award to B.J.S.; an Ontario Graduate Scholarship to C.M.W.; NSERC doctoral scholarships to K.E.M. and H.A.M.; and an NSERC Discovery grant and Canada Research Chair to K.B.S.
LIST OF SYMBOLS AND ABBREVIATIONS
- thermal sensitivity coefficient
- respiratory quotient
- temperature of crystallisation
- rate of carbon dioxide production
- © 2013.