DATASET FOR REPRODUCTIVE PERFORMANCE ANALYSIS: Data for: Fitness and fur colouration - testing the camouflage and thermoregulation hypotheses in an Arctic mammal Cecilia Di Bernardi, Anne-Mathilde Thierry, Nina E. Eide, Diana E. Bowler, Lars Rød-Eriksen, Stefan Blumentrath, Lukas Tietgen, Brett K. Sandercock, Øystein Flagstad, Arild Landa Questions to: cecilia.dibernardi@gmail.com or arild.landa@nina.no  The dataset "breeding.csv" is restricted to arctic foxes that passed their first year of life and it was utilized to analyse the probability of breeding propensity (binary, 0 if the fox did not breed, 1 if the fox produced young). The dataset includes, for each arctic fox, only those years when the individual was detected. So, separately for each arctic fox, each row is a year and it will be specificied if the fox was detected breeding or not during that year (0-1). For the analysis of litter size, the dataset was subsetted to those years when the arctic fox produced youngs. The dataset was built based on the detection data obtained by seven different methods: i) individual identification based on genotyping from faecal and hair samples collected during den controls in winter (Ulvund et al., 2019), ii) records of PIT-tags from readers set up at feeding stations based on Biomark (Biomark, Inc., Idaho, USA) and Trovan systems (Trovan Ltd., UK) (Landa et al., 2017; Thierry et al., 2020), identification of ear-tags from iii) wildlife camera traps or iv) opportunistic visual observations, v) live captures of wild foxes, vi) recovery of carcasses of dead individuals, and vii) parentage identification from tissue samples collected from offspring captured during summer marking on active dens (see Table 1 in main text). For more information on the environmental data see Appendix A in Supporting Information. Column description: fox_id = arctic fox id year = biological year (e.g. year 2009 goes from 01-07-2009 to 30-06-2010; infact 1st of July was used as a transition between biological years, corresponding to when the next cohort starts to emerge on the den sites) detected = 1 if the arctic fox was detected in that year detected_breeding = 0 if the fox did not breed, 1 if the fox produced young breeding_id = unique id for breeding event built as a concatenation of the den id and breeding year sizelitter = number of pups seen at the same time, which must be considered as minimum litter size age = arctic fox age. Age of the individuals was determined based on the marking event of juveniles, in the breeding station for the captive-born foxes, and during the summer den surveys for the wild-born foxes colour = fur colour of the arctic fox (white, blue) sex = sex of the arctic fox (Female, Male) origin = origin of the arctic fox, either wild born or captive born at Sæterfjellet breeding station and released (Wild born, Captive born) rodent = rodent phase measured in the mountain area where the breeding den site was located; if in that year the fox was not detected breeding then the column corresponds to the rodent phase measured in the most visited mountain area during the specific year (the mode was used) snow1 = duration of snow cover of the winter preceding the breeding event; if in that year the fox was not detected breeding then the column corresponds to the duration of snow cover measured as the average for the visited den sites during the specific year (the mean was used) snow2 = onset of snow cover of the winter preceding the breeding event; if in that year the fox was not detected breeding then the column corresponds to the onset of snow cover measured as the average for the visited den sites during the specific year (the mean was used) avgwin = average winter temperature of the winter preceding the breeding event; if in that year the fox was not detected breeding then the column corresponds to the average winter temperature measured as the average for the visited den sites during the specific year (the mean was used)