BEHAVIORAL ECOLOGY - ORIGINAL RESEARCH Reproductive timing and reliance on hoarded capital resources by lactating red squirrels Quinn E. Fletcher • Manuelle Landry-Cuerrier • Stan Boutin • Andrew G. McAdam • John R. Speakman • Murray M. Humphries Received: 17 May 2012 / Accepted: 28 May 2013 / Published online: 3 July 2013 Ó Springer-Verlag Berlin Heidelberg 2013 Abstract Successful reproduction in a seasonal environ- ment can be accomplished with resources that are stored before use (‘‘capital resources’’) or resources that are used immediately (‘‘income resources’’). Research examining capital versus income resource usage during reproduction has primarily focused on assigning species to positions along a capital–income gradient. Here, we examine the causes and reproductive consequences of among and within-year variation in hoarded capital versus income resource usage by female North American red squirrels (Tamiasciurus hudsonicus) during mid-lactation in a highly seasonal environment. Among years, the proportion of feeding events that were on capital resources (PROPCAP) averaged 39 % during the yearly median mid-lactation periods, but ranged widely between 2 and 100 %. In years with earlier parturition dates, females primarily used hoar- ded capital resources during mid-lactation, whereas in years with later parturition dates, females primarily used income resources during mid-lactation. Within years, PROPCAP during mid-lactation tended to be greater in early-breeding females than in late-breeding females. Rates of water flux in females during mid-lactation provided further evidence that late-breeding females used more water-rich income resources. The proportion of litters that were partially or completely lost, and the litter mass that lactating females supported, was not influenced by the large among-year differences in hoarded capital resource usage. Red squirrels appear to delay reproduction following years with low cone production to time peak reproductive demands to be late enough to be supported by income resources that only become available later in the season. In conclusion, our results offer a rare example of the capacity of a food- hoarding mammal to support reproduction exploiting a wide range of capital and income resources. Keywords Diet Á Phenology Á Reproductive timing Á Seasonal environments Á Capital and income breeding Introduction Successful reproduction requires nutrient (reviewed in Meijer and Drent 1999; Picciano 2003) and energy resources (Millar 1978; Gittleman and Thompson 1988; Kenagy et al. 1989; Hammond and Diamond 1992; Communicated by Pawel Koteja. Electronic supplementary material The online version of this article (doi:10.1007/s00442-013-2699-3) contains supplementary material, which is available to authorized users. Q. E. Fletcher (&) Á J. R. Speakman Institute of Biological and Environmental Sciences, University of Aberdeen, Aberdeen, AB24 2TZ, UK e-mail: q.fletcher@gmail.com M. Landry-Cuerrier Á M. M. Humphries Department of Natural Resource Sciences, McGill University, Macdonald Campus, Ste-Anne-de-Bellevue, QC H9X 3V9, Canada S. Boutin Department of Biological Sciences, University of Alberta, Edmonton, AB T6G 2E9, Canada A. G. McAdam Department of Integrative Biology, University of Guelph, Guelph, ON N1G 2W1, Canada J. R. Speakman State Key Laboratory of Molecular and Developmental Biology, Chinese Academy of Sciences, Institute of Genetics and Developmental Biology, Chaoyang District, Beijing 100101, China 123 Oecologia (2013) 173:1203–1215 DOI 10.1007/s00442-013-2699-3