The Colour of Survival

In boreal forests, many hares adopt white winter coats before the snow arrives. In a snowless landscape, these white hares lack camouflage against predators. However, their early moult from brown into white fur can increase their survival and offers an advantage as the snow season becomes progressively shorter with climate change.

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Throughout the year, we wear different clothing to protect ourselves from the cold or heat and for aesthetic reasons depending on the occasion. Likewise, many animals change the colour, thickness and structure of their fur and feathers in tune with the seasons.

_{Snowshoe hare (Lepus americanus) in a snowy (Kluane Lake/Yukon, Canada) and snowless habitat (Seely Lake/Montana, USA). This mammal moults its coat as colder temperatures, shorter days, and snowfall arrive. In the genetic populations of the temperate forests of the Rocky Mountains and the boreal forests spanning the North American continent, hares that moult from brown to white are abundant (20). However, in coastal areas, and in the third genetic population in the North Pacific, snowfall is brief and less intense, resulting in fewer white individuals. This is due to hybridisation with the black-tailed jackrabbit (Lepus californicus) over 3,000 years ago (17). The hare’s coat has an outer layer, where the longer fur gives each individual its colour, and an inner layer of short fur (19). In winter, the outer layer becomes thicker and denser, while the inner layer maintains a consistent thickness but increases in density. By biomass, the snowshoe hare is the primary herbivore in the North American boreal forest and distinguishes the trophic relationships between continents (21). In Europe, much of the boreal understory remains under snow, providing food for rodents with four-year abundance cycles controlled by small generalist predators (mustelids). In North America, the boreal understory grows above the snow and provides food for hares. In this region, snowshoe hare populations follow 10-year abundance cycles regulated by specialist predators (those that feed almost exclusively on hares), primarily the Canada lynx (Lynx canadensis) (6). Photos courtesy of Alice Kenney and Charles Krebs (Yukon) [see their ecological monitoring program here] and Marketa Zimova (Montana).}

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However, as the climate changes, springs arrive earlier, winters are delayed, and the frequency and intensity of precipitation have become highly variable. All of this makes it harder for species to adjust their wardrobe to temperature changes (1).

In this context, body color is a critical factor for birds and mammals that undergo an annual moult (2). In 21 species from the cold latitudes of the Northern Hemisphere, some individuals are brown in summer, but turn white in winter, while others remain brown year round (3). This phenomenon includes weasels, rodents, ptarmigans, foxes, rabbits and hares.

If these vertebrates moult to white when there is no snow, they stick out like dog’s bollocks against forest habitats dominated by greens and browns (leaves, soil, tree trunks) (4). In turn, if snowfall decreases due to climate change, predators are expected to capture non-camouflaged prey more easily, thereby reducing prey population abundance.

White and brown coats

Joanie Kennah and collaborators studied the combined effect of coat color and ambient temperature on the snowshoe hare (Lepus americanus) in the Yukon (Canada) (5). This species inhabits temperate and boreal forests of North America, where the Canada lynx (Lynx canadensis) is its primary predator (6). Hares change their coats during Spring and Autumn. This moult is genetically regulated (7) and synchronised with the photoperiod (hours of daylight) and the duration of the snow-covered landscape (8).

Using a capture-recapture method, Kennah monitored the color (white or brown) of 347 adult hares equipped with radio-transmitter collars over three consecutive autumns (Sep–Dec) and four springs (Mar–May) from 2015 to 2018. In total, 75 hares died during the study, and 14% of the recaptures were white animals in a snowless landscape.

Contrary to expectations, these white, non-camouflaged hares had higher survival rates than camouflaged individuals in autumn. This relationship was driven by their feeding habits. The studied populations spent 11–12 hours/day feeding. As temperatures dropped in autumn, white hares spent less time feeding (on grasses and shrubs) and more time being vigilant in a snowless habitat (9). This seems to have reduced attacks by their natural predators and compensated for their higher visibility (5). Kennah did not observe these relationships in the spring.

_{Effect of coat moult on the survival and feeding of the snowshoe hare (Lepus americanus) in Yukon, Canada from 2015 to 2018 (5). The moult occurs between April and October (15), during which hares display varying proportions of brown and white fur among individuals (19), while snow progressively covers the ground starting in autumn. A “mismatch” defined as a hare having > 50% white fur in a habitat with < 50% snow cover. Grey represents white hares in a snowless landscape (mismatch = no camouflage against predators) and maroon represents hares whose coat matches the landscape color (match = camouflage). Using capture-recapture data from 347 adult hares (5), the bars show that survival probability is higher for non-camouflaged hares in autumn, while the opposite occurs in spring. The lines below reveal that as temperatures drop in autumn, non-camouflaged hares reduce their feeding time, while camouflaged hares increase it. In spring, these herbivores tend to feed more at lower temperatures, but the relationship is weak and lacks statistical support. During the study, 34 hares died in autumn and 41 in spring. Three out of four deaths were attributed to the Canada lynx (Lynx canadensis), with the remainder caused by the northern goshawk (Accipiter gentilis), great horned owl (Bubo virginianus), and coyote (Canis latrans). The mismatch of brown hares in a snowy habitat occurred in only 1% of individuals and was not considered further.}

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Camouflage or thermoregulation

Modifying body color for camouflage against changing backgrounds is a logical prediction (10), but it also affects thermoregulation. Dark colours absorb more radiation than light colours, which is why people tend to wear light (and cool) clothing in summer and dark (and warm) clothing in winter.

However, the shape and composition of a feather or hair confer different optical and insulation properties. For example, the amount of heat transmitted from the air to the skin by black and white plumage converges as wind speed increases (11, 12). The winter fur of hares — whether white or brown — is longer and denser than their summer fur, allowing them to spend less energy (and require less food) to maintain body temperature in the cold (13).

Over the course of the year, a coat color that contrasts with the habitat increases hare mortality (14, 15). However, for individuals that moult from brown to white and live in extremely cold environments, survival should be highest if they turn white when there is no snow, but when the autumn cold has already set in (5), and then take advantage of the white camouflage to evade predators during the snowy conditions of winter (15).

With both added advantages, these mammals might be less vulnerable to the progressively shorter snow seasons widely documented in the Northern Hemisphere. However, the underlying issue is that the reduction in snow (both in amount and duration) appears to be causing hares to become gradually less white, as the value of their winter white camouflage diminishes (15).

Genes and environmental changes

With 32 extant species [see short videos here and here], the evolutionary history of hares includes frequent hybridisation events between species (16). All species that moult from brown to white have interbred, sharing genetic variation related to coat-colour change and thermoregulation. This genetic diversity is essential for adapting to seasonal ecosystems.

The so-called agouti gene promotes lighter colours and inhibits pigment production in mammals. This gene drives the seasonal color variation exhibited by snowshoe hares across their range (17). To keep these evolutionary processes active, the genetic diversity regulating color must be preserved within each species (18).

Unfortunately, the global network of protected areas fails to capture the genetic diversity associated with the colouration of vertebrates that turn white in winter (3). Any expansion of protected areas should consider including populations of different colours as a straightforward criterion to preserve species’ adaptability to a changing climate.

Salvador Herrando-Pérez and Reagan Early

References

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