How wild animals survive the winter in Sonoma County

It's always a high-tension moment when a child or hiker goes missing in the Sonoma County outdoors. Search and Rescue teams mobilize and work against the clock, knowing that the longer the person is out in the landscape, the greater the risk of hypothermia.

Hypothermia happens when the body loses more heat to the outside environment than it can generate. The colder and wetter the conditions, the faster that occurs. In short order, shivering can lead to confusion and unsteadiness, even loss of consciousness and eventually, death, unless emergency measures are taken. That's why we need substantial protection against the cold to survive.

So when the weather turns blustery, with driving rain and freezing temperatures, how do local animals, which live outdoors year-round, manage to stay alive?

To get through, they employ an amazing set of varied, sometimes surprising, strategies.

Winter wear

The most common technique is to change clothes, so to speak, with the seasons.

Coyotes and foxes grow fresh sets of insulating winter wear every year. Like many furred animals who stay active in colder months, they add a layer of particularly plush and thick underfur as days grow shorter.

That fur works to keep them warm by trapping air between individual fibers, close to the skin. Air is a good insulator, and a layer of still air trapped in dense fur acts as a barrier, keeping colder air out and warmth in. On top, most animals have an overlayer of stiffer hair, which provides protection against wind and rain.

Birds grow a layer of fine, downy air-trapping feathers for the same reason. Most birds also can fluff out their feathers, making them look like puff-balls, to expand the amount of air they're holding when it's especially cold. To keep that down relatively dry, birds also produce oils that they preen onto their external feathers as waterproofing. Avoiding wet skin is essential to survival out in the wild.

How do ducks, geese and other birds keep their featherless legs and feet from freezing, even in icy water?

“Birds get cold feet,” explained Blake Barbaree, an Avian Habitat Ecologist with Point Blue, a Petaluma-based wildlife conservation and research organization. A bird's body is covered with feathers for insulation, but their legs and feet are usually featherless and can get pretty cold. To keep a safe internal temperature, they use an elaborate and specialized plumbing system of sorts.

To keep icy cold blood from heading straight to their heart and organs, where it could send them into shock, birds have evolved an interwoven network of veins and arteries between their cold feet and warm bodies. Warm blood pumped away from the body passes close enough to cool blood leaving the feet that it warms the cool blood.

Putting on the pounds

Fat is a great heat barrier, said Tony Nelson, Conservation Manager with Sonoma Land Trust. That's why local black bears spend the summer and fall foraging and eating, to build up a good layer of energy-rich fat cells. Then, as every first grader knows, to stay dry and secure bears look for suitable caves or rocky dens to hibernate the coldest months away.

Nelson said the mountainous terrain in Sonoma County's back country offers plenty of opportunities for bears to lay up for winter without the risk of being disturbed.

Hibernation involves a complete change in a bear's metabolism, helped by a slower heartbeat and reduced breathing, which conserve energy.

But, surprisingly, bears maintain a warm core temperature, only several degrees lower than normal, by burning up their stored fat.

Mountain lions and bobcats don't fatten up before winter, so they need to keep hunting in colder months, Nelson said. In winter, they seek shelter out of the weather in rock crevices, caves and covered spaces at the base of rocky cliffs.

Alternate strategies

Bats have a completely different strategy - huddling together, said Trish Tatarian, who runs Wildlife Research Associates, a local biological consulting services firm, with her husband Greg. The couple conducts in-depth wildlife surveys and provides ecology assessments for government programs and business projects.

Sonoma County has twelve resident bat species. “Rather than hibernating,” Tatarian said, “bats handle the coldest months by roosting in carefully selected ‘hibernacula.'” These are roosting spaces that provide shelter from the elements, have the right ranges of humidity and temperature and have enough room for large groups of bats to crowd together.

“Huddling closely helps preserve some body heat,” she said. Between October and March, the number of insects - bats' food - declines, so bats enter a twilight state, called ‘torpor,' she said.

Unlike hibernation, torpor is a temporary state. The bat's metabolism and heart rate slows and its activity declines. Their body temperature can fall dramatically, by 24 degrees or more, which saves energy. The little mammals can remain in the torpor state for hours or weeks and rouse when conditions improve.

Some small rodents and birds, as well as some snakes and bees, employ the same torpor state to wait out cold snaps, heavy rain or even limited food supplies. Rattlesnakes and garter snakes gather in dens to wait out the winter, Tatarian said, which is why you won't see them on trails now.

Ground squirrels and pocket gophers burrow underground before entering torpor, since it leaves them defenseless against predators. Pacific tree frogs will burrow under leaf litter.

Moving south

Hundreds of bird species wing through Sonoma County to migrate for the winter to warmer environments, sometimes thousands of miles away. In the fall, we see monarch butterflies, which are drawn by instinct to fly south from northern locations where they've been feeding all summer. Some settle in trees near the coast here or continue to Monterey or Mexico to huddle together by the thousands for warmth, until the seasons change.

Other creatures stay closer to home - perhaps your home. Ants, mice and rats will often seek shelter at this time of year by moving under our homes and other structures, as many local residents can attest.

Of course, cold, rainy weather isn't a challenge at all for a whole crew of creatures, said Dr. Wendy Trowbridge, director of Restoration and Conservation Science Programs at Laguna Foundation.

“Winter is an opportunity, too.

There is enough water in the creeks for salmon and steelhead to head upstream to spawn. The additional rain connects once-isolated pools together and brings more food and oxygenated water to our resident fishes.”

Trowbridge said after the last round of rains, she spotted four river otters enjoying the additional water in Piner Creek, playing and swimming.

Trish Tatarian of Wildlife Research Associates celebrates the emerging activity as the winter rains descend and newts, endangered tiger salamanders and red-legged frogs appear.

You might see some of this cold weather activity the next time you're on a winter walk. But, like many of us, wild animals have tucked in and bundled up, awaiting spring.

Stephen Nett is a Bodega Bay-based Certified California Naturalist, writer and speaker. Contact him at

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