Introduction
Phenological mismatch is one of the most documented pathways by which
climate change negatively impacts species (Radchuk et al. 2019; Visser
& Gienapp 2019). Earlier onset of spring and delayed onset of winter
have the potential to cause incongruous timing of seasonal phenotypes
(Møller et al. 2008; Lehikoinen 2011; Kudo & Ida 2013). Mismatch occurs
in the timing of numerous seasonal traits such as calving date with
plant growth onset, and laying date with peak of key food sources, and
has resulted in reduced reproductive success and recruitment (Post &
Forchhammer 2008; Reed et al. 2013). However, the costs associated with
phenological mismatches vary within species across populations (Heard et
al. 2012; Doi et al. 2017). Species are often adapted to broad ranges of
ecological conditions, particularly those with large geographic
distributions (Valladares et al. 2014). Local adaptations and variable
selection pressures across environmental gradients alter the magnitude
of phenological mismatch across populations (Phillimore et al. 2010;
Gordo & Doi 2012; Porkert et al. 2014). Such spatial variability in
phenology across ecological conditions may also involve differences in
the mechanistic pathways governing the demographic costs and benefits
associated with phenological mismatch across species ranges.
An example of phenological mismatch that occurs in species across
multiple taxa is coat and plumage colour change mismatched with snow
onset and melt (Zimova et al. 2016; Pedersen et al. 2017; Atmeh et al.
2018; Melin et al. 2020). At least 21 bird and mammal species in the
Northern Hemisphere change colour biannually and improved camouflage is
considered the primary function of this change (Mills et al.2018; Zimova et al. 2018) As snow cover duration is forecasted to
decrease across the Northern Hemisphere (Danco et al. 2016), coat and
plumage colour mismatch is likely to increase. Mismatch may reduce
survival due to decreased camouflage (Atmeh et al., 2018; Zimova et al.,
2016; Melin et al., 2020). However, aside from colour change,
high-latitude species benefit from other winter acclimatization
strategies meant to increase cold tolerance and endure periods of food
shortage, including increasing insulation, decreasing lower critical
temperature, altering activity patterns, and, ultimately reducing daily
energy requirements (Humphries et al. 2005; Fuglesteg et
al. 2006; Sheriff et al. 2009b). Accordingly, coat colour
transitions coincide with multi-trait change that differentiates long
photoperiod, i.e., summer, from short photoperiod, i.e., winter,
phenotypes (Lovegrove 2005; Boratyński et al. 2016). As such, the
thermal and energetic benefits provided by a more insulative, white coat
and associated metabolic and thermoregulatory adaptations may outweigh
the negative costs of colour mismatch in colder conditions.
The snowshoe hare (Lepus americanus ) is a keystone species
distributed across the boreal forests of North America (Krebs et
al. 1995) that undergoes seasonal coat colour change to match the
presence of snow (Ferreira et al. 2017). The initiation of coat colour
change in snowshoe hares is likely affected by photoperiod (Nagorsen
1983) and in the absence of evolutionary change, is predicted to become
increasingly mismatched with anticipated reductions in snow cover
duration (Brown & Mote 2009; Mills et al. 2013). Coat colour mismatch
may impact snowshoe hare demography, as recent studies have reported
high mortality rates in mismatched snowshoe hares at multiple locations
in the southern extent of their range, presumably due to increased
conspicuousness to predators (Zimova et al., 2014; Wilson et al., 2018).
However, the thermal benefits of winter acclimatization in hares,
including reduced metabolic rate (Sheriff et al. 2009a), may also
affect susceptibility to predation and ultimately survival.
White winter-acclimatized snowshoe hares benefit from lower energetic
demands compared to brown summer-acclimatized hares. Indeed, while
temperatures below 0 °C increase energetic requirements for summer
hares, white winter hares remain in their thermoneutral zone until
temperatures below -10 °C (Sheriff et al. 2009a). As such, lower
energetic demands reduce foraging requirements for winter-acclimatized
hares (Balluffi-Fry et al. , In Review). Balancing the trade-off
between obtaining sufficient food to meet energetic requirements and
avoiding predators is a central assumption of prey behaviour theory
(McNamara & Houston 1987; Lima & Dill 1990). Therefore, white
mismatched hares may benefit from lower energetic requirements, reduced
foraging time, and thus reduced predator exposure. These benefits could
compensate for the adverse effects of conspicuousness, particularly when
seasonal temperatures remain low and the energetic demands for brown
summer acclimatized hares are elevated (Balluffi-Fry et al. , In
Review). Geographic variation in winter adaptations and acclimatization
exists across the broad geographic range of the snowshoe hare (Sheriff
et al. 2009b; Gigliotti et al. 2017). As such, the effects of coat
colour mismatch may vary across populations according to the relative
importance of the reduced camouflage cost relative to energy
conservation benefits in different ecological contexts.
Here, we test the hypothesis that reduced foraging requirements with
winter acclimatization reduces the costs of coat colour mismatch in
snowshoe hares. To examine this, we monitored the survival, coat colour,
and foraging time of individuals over the autumn and spring in southwest
Yukon, Canada. First, we predict that mismatched white hares will spend
less time foraging than matched brown individuals, particularly below
the thermoneutral zone of summer brown hares (i.e. 0 °C; Sheriffet al. 2009a). If this foraging difference and thus reduced time
spent vulnerable to predation outweighs the costs of conspicuousness, we
further predict no difference in survival between matched and mismatched
individuals. However, if camouflage loss is the primary driver of
predation risk during coat colour change, regardless of foraging
differences, we expect that mismatched hares are more likely to be
predated than camouflaged individuals, echoing results from previous
studies in the southern extent of their range (Zimova et al. 2016;
Wilson et al. 2018). We found that white mismatched snowshoe hares
experiencing cold temperatures in snowless environments benefitted from
reduced foraging time and thus increased survival relative to brown
matched hares.