Seasonal stage differences overwhelm environmental and individual factors as determinants of energy expenditure in free-ranging red squirrels

Quinn E. Fletcher*, John R. Speakman, Stan Boutin, Andrew G. Mcadam, Sarah B. Woods, Murray M. Humphries

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

42 Citations (Scopus)


Despite the central importance of the rate of energy expenditure in the lives of animals, the major drivers of within-species variation in energy expenditure remain uncertain, largely because most intraspecific studies focus on one or only a few potential determinants of expenditure. 2.Here, we examine the determinants of daily energy expenditure (DEE) in free-ranging female North American red squirrels (Tamiasciurus hudsonicus Erxleben) occupying a highly seasonal environment. By relating variation in 260 measurements of DEE from 176 individuals to key sources of seasonal (reproductive and foraging stages), environmental (resources and air temperature) and individual (body mass and individual identity) variation, our comprehensive analysis examines the relative importance of DEE predictors that have been more commonly examined in isolation. 3.Red squirrels demonstrated extensive variation in DEE with 5th (177kJperday) and 95th (660kJperday) percentile DEE levels that would correspond to mammals on an interspecific scale ranging in mass from 148 to 1120g. 4.Seasonal stage differences accounted for most variation in DEE, with high expenditure during lactation and autumn hoarding, and very low expenditure during winter. Contrary to interspecific studies, energy expenditure increased with increasing ambient temperature and it was weakly related to body mass in all seasons except for winter. High resource availability was associated with reduced energy expenditure in winter, but elevated expenditure during lactation and hoarding. 5.Collectively, these results highlight substantial intraspecific variation in energy expenditure, most of which can be explained by a combination of seasonal stages and environmental conditions, and fundamental differences in the importance and direction of determinants of energy expenditure when examined at the intra- versus the interspecific level.

Original languageEnglish
Pages (from-to)677-687
Number of pages11
JournalFunctional Ecology
Issue number3
Early online date15 Mar 2012
Publication statusPublished - Jun 2012

Bibliographical note

We thank all squirrelers, especially Ainsley Sykes and Elizabeth Anderson, for their assistance with field data collection and preparation. We are grateful to Paula Redman and Peter Thomson for technical assistance in isotope analyses for the DLW work. We thank the Champagne and Aishihik First Nations for allowing us to do research on their lands. We thank Francisco Bozinovic and two anonymous reviewers for comments that improved the manuscript.Research support was provided by the Natural Sciences and Engineering Council of Canada, National Science Foundation and Northern Scientific Training Program Grants. An NSERC Postgraduate Graduate Scholarship provided personal support to Q. E. F. and S. B. W. This is paper number 65 of the Kluane Red Squirrel Project.


  • Body mass
  • Doubly labelled water
  • Energetics
  • Food hoarding
  • Lactation
  • Resource availability
  • Sciuridae
  • Thermoregulation
  • Yukon


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