Results
Permanent snow cover date, i.e., 100% snow cover without melting until
the spring, was variable across our autumn seasons, occurring almost 3
weeks later in 2015 (November 3rd) than in 2016
(October 16th) and 2017 (October
17th). Completion of snowmelt date, i.e., no more snow
on ground, was similar across study years (May 6th,
2015, May 1st 2016, May 2nd 2017 and
May 1st 2018). When considering both seasons and all
years together, the prevalence of coat colour mismatched hares that
contrasted with their snowless environment was low (14% of trapping
records) in our population. Mismatch occurred more frequently in the
autumn (19% of trapping records) than the spring (8% of trapping
records). The autumn with the latest permanent snow cover arrival date,
i.e., 2015, had the highest prevalence of mismatch (33% of records).
Prevalence of mismatch in the autumns of 2016 and 2017 were 10% and
13% of trapping records, respectively. Spring mismatch was consistent
across years around 10% (2015-9% of trapping records, 2016-10%,
2018-12%), with the exception of 2017 when only 1% of trapped hares
were mismatched.
Effect of coat colour mismatch on mortality
The CPH model with the strongest support in both seasons included snow
depth, snow cover and mismatch (Table S11, S12 & S13). However, the
second highest ranking CPH model for spring, i.e., the model including
only snow variables, was within 2 ΔAICc (AICc = 0.09) from our top
spring CPH model (Table S11). Mortality risk for mismatched hares in
autumn was significantly reduced (z = -2.43; P=0.02) relative to
matched hares (Hazard Ratio (HR)= 0.135; 95% Confidence Intervals (CI):
0.027, 0.679; Fig. 1a). In contrast, coat colour mismatch was positively
correlated with mortality risk for hares in the spring (Fig. 1b), but
this effect was non-significant (z = 1.60; P= 0.11). Models were
qualitatively similar regardless of our classification of mismatch,
except when considering mismatch as a minimum 40% contrast between coat
colour and snow cover; in this case mismatch significantly increased
mortality risk in the spring (HR= 6.780; 95% CI: 2.390, 19.240;z = 3.60; P<0.001). Snow depth (z = -2.29; P=
0.02) and snow cover (z = 2.98; P=0.003) significantly affected
mortality risk in the top spring model, but not in the top autumn model.
In spring, the risk of dying decreased as snow depth increased (HR=0.95;
95% CI: 0.92, 0.993; Fig S1a) and mortality risk increased as snow
cover increased (HR=1.046; 95% CI: 1.01, 1.08; Fig S1b).
Effect of coat colour mismatch on foraging time
Across our study years, hares foraged on average 706 ± 2.29 minutes per
day in the spring and 751 ± 1.65 minutes per day in the autumn. Coat
colour mismatch was an important predictor of daily foraging time in the
autumn, but not the spring (Table S14 and S15). The top model for autumn
foraging time included coat colour mismatch, temperature, year, and the
interaction between temperature and mismatch (Table 1). As autumn
temperature decreased, mismatched hares decreased daily foraging time,
whereas matched hares increased foraging time (Fig. 2a; Table 1). For
instance, when the temperature was - 8 °C, brown-matched hares foraged
65 minutes more per day than white-mismatched hares (Fig. 2a). The top
model for spring included temperature, year, and sex (Table 1), When
coat colour mismatch was included in our spring foraging models, its
effect on daily foraging time was non-significant (t =-0.759,P >0.05).