Discussion
Phenotypes and climate change can vary widely within a species’ distribution, as can phenological mismatch and its consequences on survival. Elucidating potential unifying mechanisms is crucial to reconcile varied responses to phenological mismatch. We evaluated the effect of coat colour mismatch on snowshoe hare survival in a northern population and further tested a potential mechanism that may influence this effect. We hypothesized that the thermal and energetic benefits of winter acclimatization in white hares, i.e., increased coat insulation and reduced metabolic rate (Sheriff et al. 2009a; Gigliottiet al. 2017), ultimately reduce their foraging requirements (Balluffi-Fry et al. In Review) and thus predation risk, which may influence the costs of coat colour mismatch. Surprisingly, we found that mismatched hares had a higher survival than matched hares in the autumn (Fig. 1a) but that survival did not differ between matched and mismatched hares in the spring (Fig. 1b). Although this result contradicts previous studies that link coat colour mismatch in snowshoe hares to reduced survival (Zimova et al. 2016; Wilson et al. 2018), our proposed mechanism for why this might be the case is supported. Mismatched white hares spent significantly less time foraging than matched individuals in the autumn (Fig. 2a), presumably due to the thermal and energetic benefits of winter acclimatization. Indeed, reduced foraging time likely decreases exposure to predators and subsequently improves survival (Fig 1a). We reconcile our findings with those of previous studies with a unifying factor: temperature.
Matched hares foraged longer than mismatched white individuals in the autumn, and this difference was pronounced at lower (< –3°C) temperatures (Fig. 1a). Given the wide range of ecological contexts, selection pressures, and local adaptations that exist across the distribution of snowshoe hares (Gigliotti et al. 2017), the cost-benefit ratio of lost camouflage versus energy conservation may vary across populations experiencing different temperatures. For example, northern populations experiencing cold temperatures benefit from the energetic advantages of winter coats despite mismatch during snow-free periods, whereas southerly populations experiencing warmer temperatures may not. Indeed, adverse survival effects associated with mismatch in southern snowshoe hare populations in Montana (Zimovaet al. 2016) and Wisconsin (Wilson et al. 2018) occur in regions that experience warmer temperatures than those in southwestern Yukon (Fig. 2). During the period when mismatch is possible in Montana, autumn temperatures can range from ~ 3°C to 17 °C and spring temperatures can range from ~ 4°C to 20 °C.
The seasonal differences in mismatch effects on survival and foraging time that we found within our study population highlight temperature as a unifying factor affecting the survival costs of coat colour mismatch. In spring, mismatch did not influence mortality risk (Fig. 1b) and matched and mismatched hares spent similar amounts of time foraging (Fig. 2b). Mismatched hares in the spring occurred at temperatures (-0.5 °C to 11°C, Fig. 2b) that were approximately within the thermoneutral zone of both summer and winter-acclimatized hares (Sheriff et al.2009a). In contrast, mismatched hares in the autumn experienced temperatures between -7°C and 4°C (Fig. 2a) which fall below the lower critical temperature for summer-acclimatized brown hares, but not winter-acclimatized white hares (Sheriff et al. 2009a). Animals must increase their energetic expenditure when they are exposed to temperatures outside of their thermoneutral zone (Kingma et al.2012), which may represent a likely mechanism explaining the longer foraging time in matched brown hares in the autumn relative to mismatched white hares (Fig. 2a). These results further support that the thermal and energetic benefits of winter acclimatization may outweigh the costs of coat colour mismatch at cold temperatures.
Although camouflage is thought to be the primary adaptive benefit of coat colour polymorphism, like most traits, alternate benefits, e.g., thermal and physiological, exist (Caro 2005; Duarte et al. 2017; Zimova et al. 2018). We found that these alternate benefits offset the costs of camouflage loss at cold temperatures. Our proposed hypothesis, whereby the thermal and energetic benefits of winter acclimatization may influence coat colour mismatch effects through reduced time spent foraging, has the potential to reconcile intraspecific variation among other snowshoe hare populations and merits testing in other colour changing species, i.e. arctic hares (Lepus arcticus ), mountain hares (Lepus timidus ). Climate change-induced variation in temperature and precipitation regimes are likely to vary across species ranges (Loarie et al. 2009). Such variation in climate change effects will be particularly large for species with broad distributions, i.e., circumboreal colour-changing species. Ultimately, as temperatures in the Northern Hemisphere are projected to warm (Danco et al. 2016), northern snowshoe hare populations are likely to reach the threshold (> –3°C) at which the energetic benefits of white coats are lost, and survival costs driven by coat colour mismatch could occur (Zimova et al. 2016; Wilson et al. 2018). However, elucidating the mechanisms through which phenological mismatches may be operating is essential to enable predictions on broad-scale changes in species distributions.