(Image: Norbert Rosing /National Geographic Creative)
Removing their white winter coat once kept snowshoe hares hidden in spring, but as the snows melt earlier, they are increasingly exposed. Can they fight back?
IT IS midsummer in Montana. Traipsing through the lush, dewy forest undergrowth, the morning mist is lifting and shafts of orange sunlight beam through the trees. To nature鈥檚 soundtrack of a gurgling stream and birdsong, we check for quarry in live traps near Seeley Lake in the Rocky mountains. From the third one we visit, a young snowshoe hare stares up at us, silent, its whiskered nose twitching. This juvenile has unwittingly signed itself up for a cross-continental journey for science.
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The snowshoe hare is one of 11 species worldwide that turns pure white in winter. The regrowth of its brown summer pelt has evolved to synchronise with average snowmelt times 鈥� dates that have been relatively stable for centuries. Not any more. In temperate regions, periods of snow cover are getting shorter 鈥� one of the strongest signals of climate change. Snowmelt times are changing so quickly that hares are being caught out of fashion 鈥� staying white when their snowy background has already melted (see diagram). This young hare will be transported across the country to North Carolina State University, where it will help my companion Marketa Zimova and her colleagues answer an important question: can the snowshoe hare adapt to shifting climate?
杏吧原创s have long been intrigued by animal camouflage. The story of the peppered moth is a classic example of how background matching can, through natural selection, track human-imposed habitat changes. Before the industrial revolution, the vast majority of peppered moths were pale coloured, like the trees and lichen they rested on. Within 50 years, the darker-winged ones had become more prevalent as this provided better camouflage on the soot-covered trees.
However, early investigations into camouflage were often flawed because researchers based their work on their own vision, says Sami Merilaita at 脜bo Akademi University in Finland. What鈥檚 important, he says, is how the animals鈥� predators see.
The snowshoe hare is the primary prey of the lynx and forms part of the diet of a host of other mammals and birds. Avian predators have good colour vision but mammals do not. 鈥淢ammals have dichromatic vision, like colour-blind people,鈥� says Merilaita. 鈥淭hey can鈥檛 distinguish between green and orange very well, and probably green and brown as well.鈥� So, while speedy hops help snowshoe hares evade some predators, for others its camouflage is vital.
But hares have other interests, such as finding food and mates, and these 鈥渕ay conflict with the need to maximise their background matching鈥�, says Merilaita. In fact, some species that turn white in winter appear to delay the change back on purpose.
(Image: Erlend Haarberg/Naturepl.com)
Arctic-dwelling rock ptarmigan (above) have a white winter plumage and a mottled brown summer plumage. As snow melts in spring, females quickly moult and change colour, but males take almost a month to do so. Not surprisingly, the males experience higher mortality than females during this period.
Clues about why ptarmigan remain dangerously white have emerged from by Bob Montgomerie at Queen鈥檚 University in Ontario, Canada, and his team. Their observations suggest that the males flaunt their stark whiteness to woo females. Then, when hens are no longer fertile, the cocks deliberately soil their plumage with mud until they moult.
In an attempt to test this idea, the researchers dirtied the feathers of male ptarmigans in the mating season with a black 鈥渋ndelible鈥� marker. It took the birds just 48 hours to completely remove the ink, says Montgomerie. 鈥淚t was a strong clue that staying white was important, and that the birds were actively keeping themselves as clean as possible.鈥�
Hare-brained
By contrast, snowshoe hares appear to be oblivious to whether or not they are camouflaged. 鈥淭hey do not act in any way to reduce colour mismatch, or to reduce the negative consequences of mismatch,鈥� says Zimova. When mismatched with their environment, they don鈥檛 hide more, flee more or hang out in areas that match their coat. In short, these hare-brained creatures don鈥檛 seem very smart.
鈥淪nowshoe hares seem oblivious to whether or not they are camouflaged鈥�
If they can鈥檛 adapt their behaviour, can they at least shift the timing of their moults? Zimova and her colleagues over three Montana winters starting in 2010. Locating radio-tagged hares weekly and noting what proportion of the fur cover was white, they examined the length of time individuals were mismatched with their background 鈥� defined as a difference of at least 60 per cent between coat whiteness and snow cover. On average, this came in at nine days a year. And mismatched hares paid a price; they were 7 per cent less likely to survive the week to the next assessment than hares whose pelts matched their environment.
The study also revealed that the dates when moulting began in autumn and spring remained stable, despite large annual differences in snow duration. This suggests that the main trigger of moulting is the length of the day, not snow cover.
Other factors may also be involved. Looking closely at the two moulting seasons the team noticed that hares took about 40 days to change colour in autumn whatever the conditions. In spring, however, the rate of change was more flexible, with the moult lasting 16 days longer in the year with the longest period of snow cover compared with the shortest.
The source of this flexibility is unclear. Temperature and duration of snow cover appeared to have little effect in autumn. 鈥淸In spring] it might be the high reflectance of snow that enters the eye and triggers a hormonal cascade that slows down the change to brown,鈥� says Zimova.
The researchers can now test out their theories and investigate the genetics underpinning coat change at a futuristic-looking facility on the campus of North Carolina State University. In a series of stainless-steel-walled chambers, the young hare Zimova and I trapped, together with a dozen or so others, is being exposed to a variety of conditions to see how these influence the timing and length of moult. From a central control system, the researchers can manipulate day length, temperature and light conditions to mimic the reflectance of snow or bare ground 鈥� mechanisms that might drive the hares鈥� twice-yearly wardrobe change.
The team calculates that if the snowshoe hare fails to adapt, by 2099, reduced snow duration will increase mismatch to between 39 and 68 days a year. Theoretically, that could drive the species to extinction. It is unlikely to come to that, however. Given that individuals vary widely in the timing of their moult, and that those with the wrong colour fur face an increased risk of death, natural selection is likely to kick in.
鈥淭here is large potential for adaptation through evolutionary change,鈥� says Zimova. So as long as some snowshoe hares can stay hidden as snow cover decreases, the species isn鈥檛 doomed. Natural selection will favour quick changers. Evolution can hop to it.
This article appeared in print under the headline 鈥淏rown is the new white鈥�
