Bats do it. Groundhogs do it. Even several jumping mice do it. (And one study suggests early humans may have, too.)
Hibernation—where respiration and heartbeat slow dramatically and body temperature falls to near freezing—is just one of many adaptations among plants and animals to enable cold-weather survival. As we approach the solstice, a look beneath the stillness of winter reveals fascinating processes at work.
Many of us were taught that bears hibernate in winter. In fact, only a few of Pennsylvania’s native species are true hibernators: the groundhog, the woodland mouse, the meadow jumping mouse, and all 11 bats common to the state. Many animals instead enter a state of torpor, where their activity, heart rate, and breathing slow to conserve energy. Black bears, chipmunks, skunks, and racoons all employ this strategy.
Several species of frogs, including the common wood frog, go beyond torpor or hibernation and spend winters frozen alive. As temperatures plummet, water flows out of the frog’s internal organs, eventually forming a protective layer of ice. Meanwhile, its body accumulates urea and glucose, which act as an antifreeze, keeping individual cells in vital organs from freezing.
Other animals employ teamwork, huddling or denning to share body heat. Flying squirrels seek refuge together in leafy nests high in trees, beavers curl up in lodges insulated by mud, and snakes coil together in dens located below the frostline to wait for spring.
Songbirds that overwinter in our region rather than migrating to warmer climes will spend especially cold, snowy nights piled together in a hollow tree cavity or empty nest box. During the day, they puff up their feathers to trap more heated air. Birds also have the unique ability to use a countercurrent heat exchange, isolating the blood that flows in their legs rather than circulating it throughout their entire bodies.
Of course, animals that aren’t dormant must continue to find food to replenish the additional energy required to keep warm. Songbirds rely on essential winter nutrition from seeds, insects, spiders, and spider eggs, which they find under bark, in the ground, or even frozen on tree branches.
Other animals stockpile food stores during autumn. Eastern gray squirrels first check acorns for weevils or other boring beetles, eating those affected nuts on the spot as they would rot and spoil their winter stashes. One squirrel can create 1,000 caches in a year’s time. Thanks to their extraordinary sense of smell and memory, these remarkable rodents recover about 80 percent of the food they’ve “squirreled away”; the remainder is left to grow into oak and beech trees.
Red-headed Woodpeckers wedge seeds, nuts, and even captured insects into a tree’s bark, creating “granaries” that can store thousands of items. They also cache food in gate posts, railroad ties, and under house shingles, hammering their finds into crevices so tightly that other animals are unable to remove them.
“A few years ago, we saw an irruption of Red-headed Woodpeckers at Crow’s Nest Preserve. They were caching the tiny acorns from pin oak trees that grow in the wet, lowland woods adjacent to French Creek,” said Preserve Manager Dan Barringer. “Local birders documented 30 individual birds on the Preserve at one time. Their ability to cache food is really quite impressive.”
Plants have decidedly fewer options than animals when it comes to winter survival. Deciduous tree species drop their leaves in fall, shedding themselves of this winter liability. Leaves are used for photosynthesis—absorbing sunlight through chlorophyll and converting it to energy. They also release water, creating a hydrostatic pressure difference that causes tree roots to draw water from the soil. As trees enter dormancy, their demands for water and nutrients—and the leaves that regulate them—drop.
Conversely, evergreen trees and shrubs have adaptations that allow them to keep their leaves and needles. This means they grow year-round, even during winter, as they continue to photosynthesize.
Needles, like those on white pine trees, are covered in a waxy coating that helps the plant retain moisture in the dry winter months. They aren’t tasty to hungry winter animals unless they are desperate. And their long, narrow shape means needles—even when coated in snow or ice—are less likely than broad leaves to catch the wind and cause damage to the tree.
As spring rolls around, woody plants must “decide” when to resume growing. If they emerge from dormancy too soon, they face damage from frost. Too late and they miss taking advantage of the full growing season. Trees and shrubs have built-in warming and light requirements, triggering leaf-out only after a certain number of days of warmth and sunlight.
It’s easy to understand how many plant species, so dependent on these seasonal signals, are being impacted by our rapidly changing climate. As spring temperatures get warmer than in the past, trees may respond by leafing out and flowering several weeks earlier than normal. This can increase their vulnerability to late frosts.
cold truths from a warming planet.
Animals and plants have three options in the face of climate change: adapt, move, or perish.
One Pennsylvania species on the move is the once-ubiquitous Black-capped Chickadee, which is no longer found in the southeastern part of the state. Instead, the Carolina Chickadee, once considered a southern species, has taken its place. Scientists have identified a zone where the ranges of these two birds overlap; it’s moving north at about 0.7 miles per year. In fact, the scientists who first studied the phenomenon—Robert Curry of Villanova University and Scott Taylor of Cornell Lab of Ornithology—ended up adding another site to their study in Schuylkill County, PA, to keep up with the rapid northward movement of the birds.
“The rapidity with which these changes are happening is a big deal,” Scott Taylor said in Current Biology. “If we can see it happening with chickadees, which are pretty mobile, we should think more closely about what’s happening to other species. Small mammals, insects, and definitely plants are probably feeling these same pressures—they’re just not as able to move in response.”
According to a report by World Wildlife Fund, some bird species now arrive in spring breeding grounds earlier. This can have devastating consequences if sufficient food isn’t available when they arrive. Consider the Red Knot, a federally “threatened” species since 2014.
Every February, Calidris canutus rufa leave their winter home at the far tip of South America. They work their way north along the coast until they make it to Brazil when, flying in flocks of thousands, they travel across the Atlantic Ocean to their Arctic breeding grounds. The birds make a critical stopover at the Delaware Bay to refuel before continuing on. An estimated 90 percent of the entire species population can be found on the Bay—including Raybin’s Beach at our Glades Wildlife Refuge—in a single day. They spend two weeks feeding before resuming their migration to the Arctic Circle.
A key component of the Red Knot diet is the eggs of Atlantic horseshoe crabs, whose spring spawn on the moonlit beaches has always timed perfectly with the Red Knot’s arrival. The crab eggs, full of fat and protein, are the ideal fuel for hungry Red Knots, which have lost up to half their body weight by the time they arrive at Delaware Bay.
“It’s a remarkable sight when they arrive by the thousands,” said Brian Johnson, preserve manager at Glades Wildlife Refuge and avid birder. “Their migration route is one of the longest in the avian world, and timing for each leg of their trip is critical. It would be heartbreaking to think they may arrive too soon and not have enough to eat when they get here.”
While birds are not the only members of the animal kingdom to display climate-based disruptions to their behaviors, they are often cited as environmental bellwethers. Because of their rapid metabolism and wide geographical range, they reflect changes in the environment quickly and warn us when things are out of balance.
sleeping through solstice.
While humans don’t hibernate, many of us experience a sense of sluggishness in winter and gravitate to hearth and home as the days get shorter. Centuries ago, the winter solstice was marked with celebration in many cultures—a last great feast before the famine and fallowness of winter. Could earlier human species have taken this a step further?
Evidence of bones found from a mass grave in northern Spain suggests that Neanderthals and their predecessors may have dealt with extreme cold by slowing their metabolisms and going into a state of torpor just like bears. The bones—dating back more than 400,000 years—show lesions and other signs of seasonal growth disruption that scientists assert are strikingly similar to the bones of other animals with this winter adaptation.
While this fossil find and its conclusions remain a subject of debate among researchers, winter’s demands on those who care for the natural world—like our land stewardship staff—is a certainty. Snow removal, hazard tree pruning, and mowing wet meadows while frozen add extra work to shorter daylight hours. Preserve managers also take this time to build nest boxes and plan out spring projects like meadow installations, tree plantings, and controlled grassland burning.
In early 2022, Natural Lands hired a consulting firm, EcoAdapt, to develop a summary of projected climate-driven changes relevant to our nature preserves over the next 80 years. This included vulnerability assessment reports for priority habitats. From these materials, Natural Lands will begin to build adaptation strategies into our land management and stewardship practices.
“We selected two preserves, ChesLen and Bear Creek, to be the focus of the EcoAdapt assessment,” said Senior Director of Preserve Stewardship Gary Gimbert. “Between them, these two properties have a huge variety of habitat types, from cranberry bogs to serpentine barrens. Once we can develop resilience strategies into our day-to-day stewardship practices, we can expand those practices to our other properties with similar habitats.”
Added Luke Hamilton, climate change coordinator for Natural Lands and manager of Saunders Woods Preserve, “The effects of a changing climate are here and will only increase as time goes on. The animals and plants on the 24,000 acres of land we care for must adapt to these changes. So we will adapt as well.”
no winter, no milkweed.
Milkweed seeds, like the seeds of many native plants, need a cold winter to germinate in spring. This process—plants using a prolonged cold period to promote flowering—is called vernalization.
Many of us want to plant more milkweed to help support flagging monarch butterfly populations. “Monarch caterpillars can only eat plants from the milkweed family,” said Ethan Kauffman, director of Stoneleigh: a natural garden in Villanova, PA. “If you purchase seeds, you can replicate a cold winter by putting them in the refrigerator before you sow them.”
- Dampen a paper towel so it’s evenly moist but not sopping wet.
- Sprinkle milkweed seeds over half of the paper towel, then fold it over the seeds so they are in contact with the towel top and bottom.
- Place the paper towel in a zip-top bag labeled with the date.
- Place in the refrigerator for at least 30 days.
- Remove and sow directly into the soil or in seed trays during the growing season.