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.