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.