Pretty Warm Small Mammals

Just before the recent heavy snows deepened our drifts, I led a long line of third graders from the Hayward Elementary School into the woods at the North End Trailhead near Cable, WI. A thin layer of fluff covered the hard-packed trail. It recorded the recent passing of a fox, as well as the traffic of squirrels. Off in the woods, I could see little chains of mouse tracks from where they hopped through snow—trail dragging—before disappearing down a hole.

“Subnivean Zone.” I made the students repeat, as I described to them the magical, Narnia-like world beneath the snow where little critters carry out unseen dramas. The Earth gives off warmth. The snow becomes a blanket. Between the two, a realm expands. Mice sniff for food. Weasels hunt for mice. Voles convert vegetables into meat, and owls transform voles into feathers. Shrews hurry scurry in a constant search for snacks. We look out on a featureless façade and assume the winter world is asleep.

With Ally and Sarah—two scientists who have been doing research on small mammals in the nearby national forest—staying at my house, I’m reminded every day that much still moves this time of year. Each evening I burst through the front door asking “What did you catch today?” Their eyes light up and an account of how many mice, voles, shrews, flying squirrels, and red squirrels they caught spills out.

Ally holds a cute deer mouse. Photo by Emily Stone.

Scientists though they are, Ally and Sarah always have at least one story ready to illustrate the cuteness of their catches. There was the mouse that ran up to Sarah’s shoulder before jumping to a tree and posing—Vogue style—on a low branch. Another mouse leapt from Ally’s glove and disappeared instantly into the fluff, leaving only a mouse-shaped hole in the surface of the snow. Chubby voles received cooing and grandma-like comments about their size. Flying squirrels dashed up the nearest tree, then soared gracefully into the woods. And the shrews began foraging for food immediately—by nosing into the top layer of fluff and leaving a wiggly trail behind them.

Many little critters, including mice, prefer to spend winter in the Subnivean Zone where heat from the Earth and a blanket of snow help regular temperatures. This mouse wasted no time in getting back under the snow when it was released from the clutches of science. Photo by Sarah Nagel.

Cute does not even begin to describe these citizens of the subnivean, though. Persistent, tenacious, calculating, skilled, and constantly on the verge of disaster might be more accurate.

A few weeks ago, Dr. Paula Anich came down to the Museum’s annual “Wild about Winter Ecology” weekend workshop to talk about the adaptations of small mammals. She recently became famous when her research group at Northland College in Ashland, WI, discovered that flying squirrels glow hot pink in UV light.

With a few calculations, she illustrated the problem of being small in the winter. First, it’s helpful to realize that there is an 80,000-fold difference in body size between a shrew and an elk. In her estimation, those are the smallest and the biggest mammals in Northern Wisconsin. Size matters, because it impacts the surface area to volume ratio, and we lose heat through our surfaces. As a result, little critters lose more heat, and have to generate more heat in order to maintain a healthy body temperature of about 98 degrees.

Shrews burn twelve times more energy per unit of body mass than an elk. They are constantly racing toward the edge of starvation. With that knowledge, it’s easy to understand why a shrew would start foraging for food immediately when Ally and Sarah release it from a trap. Sadly, not all shrews were able to survive an entire night with only the food inside the trap, and they presented the highest death toll of the research. Death is inevitable for these critters, though, and a high birth rate is part of their survival plan.

Short-tailed shrews were plentiful in Ally and Sarah’s small mammal traps. They are 80,000 fold smaller than elk, and so much burn a lot of calories to stay warm. Photo by Emily Stone.

While chubby-looking voles are slightly bigger than shrews, they also must eat to stay warm. Some of the herbivorous vole’s food gets stored as brown fat—a handy type of tissue that can generate heat without shivering.

Of course, eating isn’t the only thing standing between a mouse and its maker. The advantage of being small is that these creatures can take advantage of microhabitats and microclimates. The Subnivean Zone is replete with cozy, windless, nooks and crannies. The mouse that leapt from Ally’s hands dove straight through the wardrobe into a protected place where no elk could follow. The mouse that scurried up a tree may also have had a microclimate in mind—deer mice are excellent tree climbers and often snuggle up together in an old woodpecker hole. There, the heat one loses soaks right into its neighbor, and windchill becomes a non-issue.

Deer mice are excellent tree climbers and will often use cozy cavities to snuggle in with their friends and stay warm. Photo by Ally Moser Scott.

Flying squirrels use the huddle technique, too, which I have the third graders experiment with on-trail. Small groups smush together with a thermometer in their midst, and I put one minute on the clock to see how warm they can get.

From hearing Professor Anich summarize the body of research in her field, to giggling over field observations with the student scientists, and to sharing just the tip of what is known with the next generation, I’m fully immersed in the Subnivean Zone. Layers of learning pile up just as quickly as the snow.

Emily’s book, Natural Connections: Exploring Northwoods Nature through Science and Your Senses is here! Order your copy at http://cablemuseum.org/natural-connections-book/.  Listen to the podcast at www.cablemusum.org!

For 50 years, the Cable Natural History Museum has served to connect you to the Northwoods. Come visit us in Cable, WI! Our new exhibit: "Bee Amazed" is now open!