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.
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).
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 colour 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.
Climate changes exert selective pressures on the reproduction and survival of species. A study of tawny owls from Finland finds that the proportion of two colour morphs varies in response to the gradual decline of snowfall occurring in the boreal region.
Someone born in the tropics who travels to the Antarctic or the Himalaya can, of course, stand the cold (with a little engineering help from clothing, however). The physiology of our body is flexible enough to tolerate temperatures alien to those of our home. We can acclimate and, if we are healthy, we can virtually reside anywhere in the world.
However, modern climate change is steadily altering the thermal conditions of the native habitats of many species. Like us, some can live up to as much heat or cold as their genetic heritage permits, because each species can express a range of morphological, physiological, and behavioural variation (plasticity). Others can modify their genetic make-up, giving way to novel species-specific features or genotypes (evolution).
When genetic changes are speedy, that is, within a few generations, we are witnessing ‘microevolution’ — in contrast to ‘macroevolution’ across geological time scales as originally reported by Darwin and Wallace (1). To date, the detection of microevolution in response to modern climate change remains elusive, and many studies claiming so seem to lack the appropriate data to differentiate microevolution from phenotypic plasticity (i.e., the capacity of a single genotype to exhibit variable phenotypes in different environments) (2, 3). Read the rest of this entry »
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