Introduction
For species living in seasonal environments (e.g. from summer to winter
or from dry to wet season) local adaptations to annually changing
environmental conditions may evolve. Numerous species have evolved to
time their life history events to match these changes in local seasonal
conditions (Bradshaw and Holzapfel 2007, Williams et al. 2015). To time
these phenological events, one reliable “zeitgeber” is daylength or
photoperiod (Gwinner 2003, Hofman 2004), either on its own or in
combination with other variables, such as temperature (Watson 1963,
Jackes and Watson 1975, Larkin et al. 2001) and snow cover (Watson 1963,
Flux 1970, Zimova et al. 2014).
Many animal species use photoperiod to time breeding (Goldman 1991,
Gwinner 1996b, Dawson et al. 2001, Coppack and Pulido 2004), moulting
(Lesher and Kendeigh 1941, Lyman 1943, Bissonnette and Bailey 1944), and
migration (Gwinner 1996a), and other life history events. As photoperiod
remains constant between years at specific locations, between year
variation in local conditions could result in photoperiod timed
phenological events being mistimed against the local environment.
Fluctuations in environmental variables, such as precipitation
(Villellas et al. 2014) and temperature (Ashmore and Janzen 2003,
Kreyling et al. 2019), can result in increased within-species phenotypic
variation in a variety of plant and animals species when compared to
individuals of the same species that live in more stable habitats.
Consequently, synchrony in phenological timing of individuals within a
population is expected to increase with climate stability.
Animals occupying areas that are seasonally covered by snow live in
environments that change from dark in summer to white in winter. As a
predator avoidance strategy, at least 21 species (Mills et al. 2018,
Zimova et al. 2018) have adapted seasonal changes in colouration of fur
and feather, which provides camouflage in both a winter white and summer
dark landscapes (Wallace 1879, Cott 1940, Merilaita and Lind 2005). To
provide optimal camouflage, the timing of coat colour change should be
synchronised with the period of continuous snow cover. Individuals would
thus be expected to adapt the timing of their coat colour change to
local conditions. Indeed, mismatched timing of coat colour change is
linked to range contractions and population declines in several species
including snowshoe hares, (Diefenbach et al. 2016, Sultaire et al.
2016), mountain hares (Acevedo et al. 2012, Pedersen et al. 2017), rock
ptarmigan (Imperio et al. 2013), and white-tailed ptarmigan (Wang et al.
2002), showing the importance of correct timing. However, snow
conditions might not be stable from year to year, and there might be
seasonal differences in the predictability of the appearance and
disappearance of snow.
There are some suggestions that snow cover might be more stable in
spring compared to autumn. Snowshoe hares (Lepus americanus )
(Zimova et al. 2014) and least weasels (Mustela nivalis nivalis )
(Atmeh et al. 2018) exhibit limited phenotypic variation in moult timing
in parts of their distribution during the spring moult, when
transitioning from white to brown, but not during the autumn moult, when
transitioning from brown to white. Therefore, the timing of seasonal
coat colour change is expected to be more synchronised in spring
compared to autumn. This could result in clearer differences in the
timing of among years moult timing in the spring compared to the autumn.
However, this is the first long-term study over a geographical area
large enough to test these predictions.
Mountain hares (Lepus timidus ) express seasonal coat colour
change in most of their range, except the subspecies of Irish hare
(L. t. hibernicus ) found in Ireland (Mills et al. 2018). They are
a generalist herbivore inhabiting boreal and alpine areas that occupy a
wide range of climatic, latitudinal, and altitudinal gradients,
experiencing large variations in winter snow cover duration. Coastal
areas in the south and south-west of Norway experience relatively short
snow cover duration compared to inland areas and areas in the north
(Schuler et al. 2006) with coastal areas in the south and south-west
receiving as little as one month of snow cover per year (Tallaksen et
al. 2018). Additionally, coastal areas experience greater between-year
variation in the depth and extent of snow cover than inland areas
(www.senorge.no).
Here, we provide the first quantitative study of variation in mountain
hare moult timing with nine years of data using 678 camera locations
along an extensive geographic gradient in Norway. We studied 1) how the
timing of moult varied with local geographical conditions and 2) how the
timing of moult varied among years and seasons, utilising camera trap
data in a Bayesian multinomial logistic regression model framework.
Snowshoe hares that live at lower altitudes and latitudes displayed
winter coats for a longer time compared to their low latitude (Grange
1932, but see Zimova et al. 2019) and altitude conspecifics (Holmgren et
al. 2001, Nowak et al. 2020, Zimova et al. 2020b). Also, increased snow
cover in continental areas is likely to result in hares living in these
areas keeping their winter coats for longer than hares in coastal areas.
Therefore, we used altitude, latitude, and climatic zone, distinguishing
between coastal and continental climates, as indicators of local
geographical conditions for our first aim. For our second aim, we
predicted that moult timing would be more synchronised among individuals
in spring compared to autumn, based on previous studies on snowshoe
hares (Mills et al. 2013, Zimova et al. 2014). Furthermore, we predict
larger among year variation in the timing of moult in spring compared to
autumn.