Discussion
We used camera trap by-catch observations to provide the first
quantitative assessment of how large-scale environmental variables
correlate with mountain hare moult timing. As predicted, hares at higher
altitudes and latitudes moulted later in spring and earlier in autumn,
keeping their winter white coats for longer, compared to their lowland
and low altitude conspecifics. Contrary to our prediction, we found
support for a later spring moult and earlier autumn moult in areas
characterised by coastal climates, rather than inland climates,
resulting in hares that live in areas with coastal climates keeping
their winter white coats for longer than their inland conspecifics.
The unexpected correlation between climate zone and moult timing (Figure
2.c.) could result from variation explained by climate zone also being
partially explained by altitude (Pearson correlation coefficient
> 0.5 in both seasons) and latitude (Pearson correlation
coefficient < -0.3 in both seasons) (Suppl 6). Additionally,
all camera traps from 63° North are close to the coast which may
confound results. However, in southern Norway there is a coastal to
continental moult timing gradient shown in the prediction maps (Figure
3.a. and 3.f.). Furthermore, it is likely that there will be increased
variability in moult timing in coastal climates as there is increased
among year variation in snow cover and duration compared to continental
climates (senorge.no).
Both altitude (Figure 2.a) and latitude (Figure 2.b) had the predicted
correlation with moult timing. Mountain hares that inhabit areas
characterised by higher altitude and latitude moulted later in the
spring and earlier in the autumn, keeping their winter white coats
longer, likely due to an expectation of longer snow cover duration.
These results are consistent with previous studies of other mountain
hare populations and other hare species, which found that increased
elevation correlated with mountain hares keeping their winter coats for
longer (Watson 1963) and increased latitude correlated with snowshoe
hares keeping their winter white coats for longer (Grange 1932).
However, a more recent study on snowshoe hares found no evidence of
moult timing variation across a latitudinal gradient in autumn and, in
spring, hares that live at high latitudes became brown earlier than
those at lower latitudes (Zimova et al. 2019). The correlations between
moult timing and altitude and latitude indicates that mountain hare
populations have adapted to local conditions, which suggests that gene
flow between populations is insufficient to dilute local adaptations.
This is particularly true for altitude as this variable can change
significantly over a short geographic distance.
The non-overlapping CIs between some years in the spring moult (Figure
4) indicate that there is some between year phenotypic variation.
However, the between year difference in moult timing is small, which is
consistent with photoperiod rather than climate being the main driver of
mountain hare moult timing (reviewed in Zimova et al. 2018). This is
consistent with similar studies conducted on snowshoe hares (Mills et
al. 2013, Zimova et al. 2014) and least weasels (Atmeh et al. 2018),
which found evidence of between year phenotypic variation in spring, but
not in autumn. The limited moult timing variation may reduce fitness as
an inability to change moulting patterns in response to among year
variation in snow extent and duration will increase camouflage mismatch
and, consequently, decrease survival probability (Zimova et al. 2016).
Behavioural responses to mismatch are possible, micro-habitat patch
selection, changed diurnal activity patterns, or manually removing
winter fur during the spring moult. However, there is currently no
evidence of snowshoe hares modifying their behaviour in response to
being mismatched (Zimova et al. 2014).
Moult timing variation could result from population level phenotypic
variation, individual level phenotypic plasticity, or a combination of
both factors. As our methodology did not facilitate monitoring specific
individuals, we could not disentangle the relative importance of
population and individual level variation. Observations were obtained at
specific locations in multiple years (Suppl 8). This makes it likely
that some individuals were recorded in multiple years, increasing the
probability of individual phenotypic plasticity influencing results.
As the camera traps were deployed to maximise the probability of
detecting lynx their locations within the surveyed area were not
randomised and some areas of Norway were not monitored. Additionally, we
obtained three times as many observations in spring compared to autumn
(Suppl 4), which is probably caused by increased hare activity patterns
during the mating season (Pettigrew et al. 2021) and the spring dataset
containing observations for 58 more days. This resulted in the intercept
CIs (Figure 4) being larger in autumn than in spring. Despite these
issues, the prediction maps (Figure 3 and Suppl 10) indicate that the
model produces accurate results.
Understanding the role that environmental characteristics have on moult
timing is vital when assessing the impact that climate change may have
on species that express a seasonal coat colour change. Our study sites
span 1,300 km and 11 latitudinal degrees, from sea level to 841 m.a.s.l.
making this is the first study that investigates moult phenology over a
large, continuous climatic gradient spanning three biomes (temperate
forest, boreal forest, and alpine tundra). Analysing the correlation
between the explanatory variables and mountain hare moult timing will
enable us to predict how the species will react to climate change
induced reductions in snow cover extent and duration. We plan to
investigate this issue in a future paper.