How do animals survive long winters?

Birds switch their diets, coyotes grow thicker fur, and turtles bury into mud.

A deer in a Pembroke yard was barely visible through pine trees covered with snow from a 2018 blizzard.
A deer in a Pembroke yard was barely visible through pine trees covered with snow from a 2018 blizzard.Debee Tlumacki for The Boston Globe

When winter arrives, people deal with the cold weather in a variety of ways. We wear sweaters and scarves, winter coats and hats and gloves. And we work and live in heated buildings, and drive around in warm cars. But how do animals survive the long Massachusetts winters?

In all kinds of ways, it turns out.

Mammals and birds are endothermic and produce their own heat, allowing them to maintain a fairly consistent body temperature, but reptiles, amphibians, and most fish are ectothermic, which means their body temperatures are essentially the same as the surrounding environment.

When the weather is cold, the metabolism of reptiles and amphibians slows down, and they enter a dormant state called brumation. They can still move around a bit, but are generally inactive. Fish slow down in winter as well, but remain somewhat active, as ice fishermen can attest.


A dark green leopard frog photographed in West Brookfield.
A dark green leopard frog photographed in West Brookfield.Jacob Kubel

Frogs such as green, bull, and leopard frogs tend to burrow into the mud on the bottom of ponds and streams, according to Bryan Windmiller, director of conservation at Zoo New England. They survive under the ice because dissolved oxygen from the water that surrounds them diffuses into their bodies through blood vessels in their mouths and in their cloacas, multipurpose openings used for reproduction and for excretion of waste.

Tadpoles and salamander larvae have gills that allow them to breath in the water like fish, and they remain active under the ice, Windmiller said. In the water, animals are also buffered against really low temperatures, and they avoid places that will freeze, he explained.

Land-dwelling amphibians, like spotted salamanders and toads, try to get underground below the frost line where they will remain inactive for the winter and be protected from freezing.

A spotted salamander in the Middlesex Fells Reservation.
A spotted salamander in the Middlesex Fells Reservation.Matthew J. Lee

“The frost line is not too deep in the forest because leaf litter and herbaceous plant cover insulate the ground,” said Windmiller. “The frost level in your yard is about 2 or 3 feet deep, but in the woods it’s only 6 to 12 inches.”


Snakes also survive the winter by going underground below the frost line, said Anne Stengle, a biology professor at Holyoke Community College. Cold temperatures will cause them to move underground, into rock crevices and into burrows, where their activity, body temperature, heart rate, and respiratory rate drop. Some snakes will spend the winter together in communal denning sites, said Stengle.

“Milk snakes and garter snakes will den with rattlesnakes, copperheads, and other species,” Stengle said.

Wood frogs, spring peepers, gray tree frogs, and hatchling painted turtles are actually able to freeze solid and still survive, Windmiller explained.

A Wood frog in Dracut.
A Wood frog in Dracut.Tim Beaulieu

“Their heart stops and they’re not breathing,” said Windmiller. “They’re as close to being dead as is possible for a living thing to be. In the spring they thaw out and stretch themselves and they’re ready to roll.”

Freshwater turtles spend the winter in the water and survive the cold by slowing down their metabolism. Their heart rate drops down to about one heartbeat every five or 10 minutes, Windmiller explained.

“Blanding’s, wood, and spotted turtles get down in the mud and vegetation at the bottom of a pond or stream where it’s hard for predators to get them,” said Windmiller. “They park themselves in a good spot and hope an otter doesn’t find them.”

Turtles under the ice get what little oxygen they need through mouth gaping — opening their mouths and exposing their mouth linings to the water, which allows oxygen to diffuse across mouth membranes and into small blood vessels, said Windmiller. In the same manner, turtles can also use cloacal gaping to get oxygen, essentially breathing through their rear end.


A painted turtle at Garden in the Woods in Framingham.
A painted turtle at Garden in the Woods in Framingham.David L Ryan/Globe Staff

Turtles do occasionally die under the ice, said Windmiller. It’s more of a problem for small juvenile turtles if the ice freezes straight down to the bottom. But if the ice is thick for a long period of time, aquatic animals can drown because they run out of oxygen, a phenomenon called winter kill. There’s a finite amount of dissolved oxygen in the water, and additional oxygen is not able to get through the ice.

“All the organisms under the ice are using a shared pool of oxygen,” said Windmiller. “Fish, turtles, worms, clams, aquatic insects — everybody’s breathing.”

If the oxygen supply in the water runs out before the ice thaws and air doesn’t mix with the water and replenish the oxygen, aquatic animals can die. Tadpoles and fish are most prone to winter kill, said Windmiller.

“Turtles almost always make it,” Windmiller said. “Turtles are the best at surviving for a long time with little or no oxygen. Painted turtles can go without oxygen for three months!”

Birds deal with winter in a variety of ways. Some birds stick around and switch to a seasonal menu, while others head south to warmer climates.

A downy woodpecker in Norwell.
A downy woodpecker in Norwell.David L Ryan/Globe Staff

“Insectivores get into trouble unless they have a plan B, like altering their diet,” said Wayne Petersen, director of Mass Audubon’s Massachusetts Important Bird Areas program. “Some, like robins, switch from earthworms and insects to fruits and berries. Most warblers and thrushes go to Mexico and Central and South America for the winter.”


Other insect-eating birds, like downy woodpeckers, chickadees, and titmice, can find insect larvae under tree bark in winter, or switch to seeds or bird feeders, Petersen said.

Water birds, like spotted sandpipers, American woodcocks, Wilson’s snipes, and many duck species, also tend to migrate south in winter, said Petersen.

But migratory patterns for some species are changing. Climate change has allowed species like mockingbirds, red-bellied woodpeckers, and cardinals to expand their ranges northward, and join our traditional year-round residents like crows, blue jays, and chickadees, Petersen explained.

“They have become resident species and hardly migrate anymore,” said Petersen.

A bluejay jousts with a cardinal for perch positions on a backyard feeder in Pembroke.
A bluejay jousts with a cardinal for perch positions on a backyard feeder in Pembroke.John Tlumacki/Globe Staff

Urban and suburban sprawl is also contributing to birds deciding to overwinter in Massachusetts.

“Suburban habitat favors winter survival,” said Petersen. “People plant shrubs like crab apples, which serve as food sources for some birds. Birds like robins strip wild food crops in the woods, but as winter goes on they move into the suburbs to eat.”

Mammals have a variety of winter survival mechanisms as well.

Bear, deer, squirrels, and chipmunks feed on food like acorns and beechnuts in the fall to build up their fat stores for winter, according to the Massachusetts Division of Fisheries and Wildlife’s Web page “How Wildlife Handle Winter.”


MassWildlife says some mammals, like skunks, will sleep through much of the winter but may search for food when temperatures go above freezing.

Bears are not true hibernators, even though they spend time in winter dens, said Dave Wattles, black bear and furbearer biologist from the Massachusetts Division of Fisheries and Wildlife.

“They don’t eat, defecate, or urinate, but they can become active at any time,” said Wattles. “For example, when we do den work we have to sneak in. If we make noise they wake up and can run away immediately.”

A black bear in captivity waits for visitors to throw food into his pen at the the Maine Wildlife Park in Gray, Maine.
A black bear in captivity waits for visitors to throw food into his pen at the the Maine Wildlife Park in Gray, Maine. Robert F. Bukaty/Associated Press

And some bears don’t den at all if food is plentiful, he explained. “We’ve seen this in suburban areas where bird feeders provide a reliable source of food all winter,” said Wattles. “Hibernation is essentially an adaptation to the lack of food for a bear in winter. If food is available, there’s no need to den.”

Other mammals, like woodchucks, are “true hibernators,” meaning they experience reduced body temperatures and decreased heart rates, which help them to conserve energy as they use up their fat stores while they slumber through the winter months, says MassWildlife.

Mice, voles, and other small mammals can create tunnels through the snow, which help to insulate them from the cold, protect them from predators, and allow them to feed on plants and seeds, said MassWildlife.

The fur of some mammals, like coyotes and raccoons, grows thicker in winter, which helps them handle the cold weather. MassWildlife said deer and moose have hollow hairs in their winter coats, which provide insulation by trapping air, while aquatic mammals like otter, beaver, mink, and muskrat have a double layer of fur with dense, fine hairs near the body and longer guard hairs on top, which helps keep them warm.

The MassWildlife website said feeding wildlife can do more harm than good by causing creatures to alter natural behaviors and adaptations to cold weather and scarce food supplies, and can cause other problems such as attracting predators and spreading diseases among wildlife. So MassWildlife generally advises against it, even in winter.

A squirrel takes in Boston Common.
A squirrel takes in Boston Common.Craig F. Walker/Globe Staff

MassWildlife said that while backyard bird feeding during winter months is generally acceptable, fallen bird seed can attract other types of wildlife, so if you notice unwanted wildlife in your backyard, such as coyotes or bears, bring your bird feeders in immediately.

Petersen acknowledged some of the potential downsides to feeding birds, but he added that winter is a tough season, so he thinks giving birds some help this time of year is OK.

“If you look at all the things man has done that [have] had a huge negative impact on birds, like cats, habitat destruction, and chemicals,” Petersen said, “if this is one little thing we can do that is not going to harm birds and is going to give them a leg up, do it and feel good about it.”

Don Lyman is a biologist, freelance science journalist, and hospital pharmacist who lives north of Boston. Send your questions about nature and wildlife in the suburbs to donlymannature@gmail.com.