Introduction
Animals exhibit great variation in coloration both within and among species. Which colors persist and how these colors are arranged reflects an optimization of competing adaptive functions including sexual selection, thermoregulation, and camouflage (Walsberg et al. 1978, Endler 1981, Andersson 1994, Stuart-Fox et al. 2017). Often, coloration covaries with the environment when the adaptiveness of coloration depends on how color and environment interact to influence individual fitness (Cott 1940, Cuthill et al. 2017, Barrett et al. 2019). Over evolutionary time, spatial and temporal changes in environmental conditions can produce multiple color phenotypes in a population, or color polymorphisms (Bell 2010, McLean and Stuart‐Fox 2014). Stable color polymorphism is defined as the persistence of intraspecific variation in coloration between sympatric individuals of the same age group that is genetically inherited and the expression of which is not sensitive to changes in body condition, diet, or environment (Huxley 1955, Roulin 2004). Color polymorphism occurs across a myriad of vertebrates (e.g., mammals, reptiles, and birds) and is considered essential for enabling species to respond to environmental gradients associated with habitat type (Hoekstra et al. 2004), vegetation cover (Baling et al. 2020), weather (Galeotti et al. 2009), and pollution (Kettlewell 1955, Bishop 1972).
In high-latitude ecosystems, winter is characterized by freezing temperatures and snow cover, a seasonal and dynamic environmental feature that is important habitat for winter-adapted species. Snow has implications for enhancing crypsis for polymorphic species and species that exhibit seasonal coat color change. Twenty-one species of birds and mammals exhibit seasonal molt changes from brown to white during the winter allowing for greater camouflage on the snowy landscape (Grange 1932, Cott 1940, Mills et al. 2018). This phenotypic trait is an adaptation to seasonal snow cover duration with potentially strong fitness consequences. In some cases, when the rate of seasonal molt change no longer matches the rate of snow cover change, a phenological mismatch occurs. For example, seasonally molting rock ptarmiganLagopus mutus and snowshoe hares Lepus americanus that are phenotypically mismatched with a snowless environment (due to the loss of snow cover) exhibit lower survival during the winter months (Wilson et al. 2019, Melin et al. 2020). This phenotypic mismatch between white body coloration and snowless habitat has potentially strong implications for population persistence, especially along southern range boundaries where climate change-induced snow loss is shortening snow season length and increasing periods of mismatch (Montgomerie et al. 2001, Mills et al. 2013, Wilson et al. 2019). Similarly, for species that do not undergo seasonal molts, environmental gradients (e.g., precipitation, land cover), and periods of rapid environmental change are thought to be maintaining forces for color polymorphism (Galeotti and Sacchi 2003, Roulin 2004). For example, variability in annual snowfall and temperature is likely an important selective force on plumage polymorphism in the tawny owl Strix aluco , where survival of the gray morph is higher than that of brown morphs during snowier winters (Galeotti and Cesaris 1996, Karell et al. 2011).
The ruffed grouse Bonasa umbellus is a winter-adapted galliform that occupies young forest across North America (Rusch et al. 2020) and is polymorphic with two predominant color morphs of red or gray, although intermediates frequently occur (Bump et al. 1947, Rusch et al. 2020). The genetic basis of color morphs in ruffed grouse has not been described, though it is thought to be a melanin-based genetic color polymorphism, which is the basis for red/brown polymorphisms across most bird species (Roulin 2004, Mundy 2005). Throughout their range, the survival rates of ruffed grouse vary seasonally and geographically (Devers et al. 2007, Zimmerman et al. 2007, Wilson et al. 2022), but little research has investigated possible demographic differences between grouse color morphs. Winter is an important demographic constraint for grouse populations in the upper midwestern United States where mortality, mostly due to predation, is highest during the winter months (Small et al. 1991, Gutiérrez et al. 2003, Pomara and Zuckerberg 2017). Ruffed grouse burrow in deep snow to evade predators and cold ambient temperatures (Shipley et al. 2019, 2020, 2022), and overwinter mortality is associated with shallow snow cover (Shipley et al. 2020), but little is known about how phenotypic matching with snow might impose differential survival for red and gray morphs.
Geographic clines in polymorphic traits can provide important insights into the selective pressures that drive the persistence of polymorphism as an adaptive strategy (Levins and MacArthur 1966, Takahashi 2015). For ruffed grouse, it has been suggested that there is geographic variation in color morph ratios across their range that covaries with latitude and winter conditions, although this has not been formally described (Bump et al. 1947, Rusch et al. 2020). Past authors have speculated that red and gray morphs are differently sensitive to snow cover; suggesting that there may be higher proportions of gray individuals found at more northerly latitudes whereas red individuals are more associated with southerly and coastal areas with lower snowfall (Gullion 1984, Rusch et al. 2020). There is some evidence to suggest daily ruffed grouse survival differs by season between morphs, but the relationship between winter survival by morphs and snow cover has not been tested in northern, seasonally dynamic ecosystems (but see: Gutiérrez et al. 2003).
Ruffed grouse occupy a range of different forest types and ages but are closely associated with young successional forests (Zimmerman et al. 2009, Tirpak et al. 2010). The presence of young forest and dense cover may offset the costs of phenotypic mismatches during winter. For snowshoe hares, occupying patches of dense young aspen negated the mortality associated with phenotypic mismatch (i.e., white snowshoe hares on bare ground) (Wilson et al. 2019). In addition to this potential buffering of phenotypic mismatch by forest cover, snow presence and duration of seasonal snow cover varies across different forest cover types (Petty et al. 2015). Forest canopy cover accounts for 57% and 72% of the variance in snow accumulation and ablation, respectively (Varhola et al. 2010), and forest understory density can alter ambient temperature and solar radiation that reaches the ground, which in turn, alters snow cover depth and persistence (Lundquist et al. 2013, Petty et al. 2015).
Our goal was to examine phenotype-by-environment interactions between color morph and snow cover and its role in mediating overwinter survival in ruffed grouse. We used a five-year dataset of overwinter survivorship for a population of ruffed grouse in central Wisconsin to investigate potential effects of snow cover and habitat on individual survival of ruffed grouse. We hypothesized that snow cover and habitat characteristics would differently affect red and gray morph winter survival. Under this hypothesis, we predicted that individuals who were phenotypically mismatched to the surrounding winter landscape would have lower survival. In other words, gray morphs would experience lower survival during periods with less or patchier snow cover (Fig. 1). Conversely, red morphs would experience lower survival during periods of more persistent snow cover (Fig. 1). We further predicted that the cost of phenotypic mismatch would be lower for individuals inhabiting areas of dense cover such as young aspen forest (Wilson et al. 2019).