As climate change scrambles winter weather, scientists find that many wildlife are using multiple survival strategies to endure the cold
High in the French Alps, a red fox (above) curls up to stay warm, its thick fur coat providing much-needed insulation from the region’s frigid temperatures. To escape North American winters, monarch butterflies (below) fly up to 3,000 miles to shelter together in a handful of conifer forests in the mountains of central Mexico. (Photo above by Benjamin Barthelemy/Nature Picture Library)
ON WASHINGTON STATE’S OLYMPIC PENINSULA, a tiny Anna’s hummingbird sits on a frozen branch, puffed up and motionless during a December cold snap. Its summery, iridescent flash of emerald against the stark white of a snowstorm is a startling sight. How could a member of a bird family most often found in the tropics survive these harsh conditions?
It turns out that many animals, including hummingbirds, have an array of highly effective winter survival skills. Scientists are still unraveling details of many of these strategies as well as discovering new ones. Recently, they’ve also learned how some species can combine two or more survival tricks in unexpected ways.
Winter poses a problem to living things for two key reasons: energy and water. When temperatures drop below a species’ optimum temperature range, cellular functions that provide energy falter and eventually fail. If internal temperatures fall below freezing, the water that makes up at least 70 percent of animal cells freezes, and the organism dies.
To avoid suboptimal internal temperatures or freezing, wildlife rely on three categories of survival strategies: migration (getting away from the cold), dormancy (a state of lowered body temperature and energy expenditure, including hibernation, to wait out the cold) and resistance or tolerance (being able to stay active at low temperatures).
For many animals, from elk and Canada geese to dragonflies, migrating away from winter’s cold and dearth of food is worth the energy expenditure. Migrations can be exceedingly short—just 1,000 feet down a mountainside for the dusky grouse, for example. But other species cross entire continents in search of more welcoming climes. Among the most impressive long-distance travelers are monarch butterflies, which migrate up to 3,000 miles one way, and red knots, small shorebirds that each fall travel some 9,000 miles from the Arctic to the southern tip of South America.
Grizzly bears slumbering in mountain dens are familiar examples of hibernation, but many bear species remain relatively active, and even grizzlies lower their body temperatures by only about 10 degrees F. Rodents such as groundhogs, on the other hand, do go into a deep hibernation, their heart rates dropping dramatically, with body temperatures not much higher than the surrounding environment.
In animals that have evolved to tough it out, resilience to the cold can take many different shapes. Polar bears, for instance, build fat, and red foxes grow thick, insulating coats of fur. But some organisms have more unusual—or just plain weird—ways of dealing with the cold.
Abundant in the southeastern United States, particularly the Appalachian Mountains, most salamanders spend the winter underground in a state of low activity, burning as few calories as possible before they reemerge in spring. But a few salamander species with bodies that can tolerate colder (but not freezing) temperatures do stay active during the winter, making up for the energy expenditure simply by eating as much as they can. The red-backed salamander, for example, overwinters near ant colonies to ensure it has a steady supply of food.
Like many other insects, along with a few fish and amphibians, aptly named snow flies—wingless insects that dot the snow of high mountains and frigid forests—produce an antifreeze that drops their blood’s freezing point. But for the animals to remain active even at subzero temperatures, at least four snow fly species possess another remarkable survival tool, as John Tuthill of the University of Washington and his colleagues recently discovered: The flies detach one of their own limbs.
When a fly’s leg remains in contact with freezing ground for too long, the limb can quickly begin to freeze, with ice crystals creeping up the appendage, Tuthill explains. The insect can detect this freezing and self-amputate the limb before ice reaches its trunk, much like a lizard can detach its tail when a predator grabs it.
It all happens in under half a second, “plenty of time for a neuron in the leg to sense that there’s ice forming and then trigger this reflex that makes the leg amputate,” he says. Tuthill had long known that snow flies tended to be missing legs, but he’d chalked it up to “a complex life, losing limbs along the way.” Though other species also categorized as crane flies self-amputate limbs in response to predation, the new finding, published in Current Biology, is the first documented case of the insects doing the same thing to cope with a temperature threat. “They’re escaping the predator of the cold,” Tuthill says.
Perhaps the most surprising survival skill uncovered so far is the ability of some small mammals with exceptionally high energy demands to shrink their skulls and brains in response to seasonal cold and food scarcity. Come spring, the animals regrow them. Many mysteries remain about how and why certain wildlife species employ this uncommon strategy, particularly whether cold temperatures or a lack of food is the primary driver behind the shrinkage.
The European mole, the most recent critter named to the shrink-to-survive club, is helping scientists better understand the enigma. In a 2022 study published in Royal Society Open Science, researchers analyzed extensive museum collections of both European moles and closely related Iberian moles, which face food shortages in summer, not winter. They learned that 20 percent of the European moles’ winter brain shrinkage is due to cold temperatures rather than limited food resources.
Although classic thinking in biology has long assumed a given wildlife species relies primarily on one of the three main strategies for winter survival—migration, dormancy or cold resistance—that paradigm is beginning to change. In 2022, Missouri State University biologist Giorgia Auteri published a paper in Biology Letters presenting a new “conceptual framework” for looking at cold-survival strategies. She suggests that, rather than relying on a single survival tactic, many animals depend on two or three different strategies.
The new framework was prompted by Auteri’s 10-plus years studying bats. “Bats are really cool because a lot of species use at least two strategies pretty heavily: hibernating and migrating,” she says. Big brown and silver-haired bats, for example, undertake southward migrations—either long or short trips, depending on the location of their summer habitats—which means they can decrease the amount of time they spend hibernating in winter.
Auteri talked to scientists studying other wildlife species and realized bats weren’t the only animals using multiple strategies. While some species may rely heavily on a single approach, many others, including birds and small mammals, mix and match, with individuals even of the same species optimizing a balance that works best for them. “I think, if we look closely enough at any animal, most of them are using all three [strategies] to some degree,” Auteri says. “It’s a pretty big shift for us, as scientists.”
Hummingbirds are prime examples of wildlife whose cocktail of winter survival skills scientists are just beginning to understand. “We used to say that birds either migrate out of the cold, or they stay in cold areas and are resistant to it,” Auteri says. “But as we’re looking more closely, we’re seeing that some birds actually shift their internal body temperatures more than we thought they did” by periodically entering short-term spells of dormancy known as torpor. The two bird families now known for adopting this survival trick are the nightjars (nighthawks, poorwills and related species) and hummingbirds.
Hummingbirds, even during good times, live on a metabolic knife’s edge, requiring an immense supply of calories relative to their body size. In winter, hummingbirds at high elevations and latitudes face limited food options along with potentially frigid temperatures that require even more energy to survive. Scientists are discovering that species that do not undertake long, energy-depleting migrations can rely, at least in part, on torpor. “Torpor is a versatile, energy-saving mechanism for them,” says Erich Eberts, a biologist who recently completed his Ph.D. research on hummingbird torpor.
Ranging along the West Coast from northern Mexico to southern Canada, many Anna’s hummingbirds, for instance, do not migrate. Instead, they spend the winter in one place, gambling that it won’t get so cold they can’t handle it. But if temperatures plunge, “torpor basically allows them to roll with the punches and deal with those cold snaps and unexpected, unpredictable situations,” Eberts says.
In many locations, climate change is shifting the shape of winter, whether that’s warmer average temperatures, unpredictable winter storms, shocking cold spells or diminishing snow and ice. To predict if animals can adapt to the new normal, we must understand how they are surviving now.
For wildlife that depend heavily on a single winter survival strategy, options may be limited. That includes the majority of salamanders, which weather the winter by entering a state of dormancy in underground burrows. When the amphibians dive underground to escape the cold, soil acts as insulation, with the snow on top of soil working like a thick blanket, protecting the ground from sudden chills.
But with today’s warmer winters, that snow doesn’t always arrive. At the same time, greater climate variability means a higher chance of temperature extremes. “If there isn’t snow on the ground, which for many salamanders will be the case, they will not be insulated from these cold snaps,” says Eric Riddell, a University of North Carolina, Chapel Hill biologist who studies the animals. “The second [they] get below freezing, ice crystals form inside them, and they die.”
New thinking about winter survival strategies leads to a brighter outlook for other wildlife. “For many species, they’re already using multiple strategies to some degree, so they could possibly compensate for climactic shifts by … starting to use those strategies to different degrees,” Auteri says.
Consider hummingbirds. Already, “they can change their strategies, trade things off and just thrive in so many different environments,” Eberts says. Some species that overwinter, including Anna’s, are expanding their ranges by taking advantage of feeders and heat-trapping cities, demonstrating new behaviors in cold resilience. “They’re so incredibly flexible,” Eberts says, an observation that some scientists expect will apply to more and more wildlife in the future.
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