Very Cold Small Mammals

My car shuddered and groaned before turning over reluctantly and revving to life. The thermometer had displayed negative 32 degrees as I headed out the door, and my weather app countered with negative 37. Looking back through my kitchen window, I could see Ally Moser Scott filling up thermoses with hot water, and organizing her hand warmers, balaclava, and boots. I was headed to a comfortably heated office. She was preparing to be outside all morning in this potentially dangerous cold.

Ally is a master’s student in Professor Jon Pauli’s lab down at the University of Wisconsin Madison. Her thesis is “assessing the population ecology of small mammals in northern Wisconsin to evaluate prey availability for the American marten.” The field part of her research entailed setting up several 25-trap grids in different forest types, and then checking the live traps for nine days straight in order to estimate the numbers of small mammals in each habitat.

Ally Moser Scott (back) and Sarah Nagel continued to check their small mammal trap line despite the recent bitter cold. Photo by Emily Stone.

With 75 traps to check per day, it’s a good thing that Ally has help. Sarah Nagel, who earned a degree in Ecology and Evolutionary Biology from Minnesota State University Moorhead, is gaining field experience by being Ally’s research technician. Ally and Sarah are staying at the Cable Natural History Museum’s Jackson Burke House, which is convenient to their research sites near Clam Lake and Mellen in the Chequamegon-Nicolet National Forest.

Sarah Nagel takes care of recording data on what the traps hold, the snow depth, and snow density. Photo by Emily Stone.

Although Ally and Sarah are willing to brave our recent polar vortex in the name of science, I looked at the weather forecast closely and picked a slightly warmer day to join them on their trap lines. It was barely below zero with a light drizzle of snowflakes when we loaded up in the work truck at 7:00 a.m.

I wasn’t sure what to expect as we walked single file into the woods at the first study site in a spruce swamp. Because the small mammals that Ally is trying to catch spend most of the winter hiding out and staying warm beneath the snow, it’s important to her research that humans don’t tramp all over the trap grids and mess with mouse habitat. We stepped as exactly as possible into the boot prints that were established the first time someone walked among the traps. 

We all walked in a single file line to minimize compaction of the subnivean zone.

Sprigs of Labrador tea, a common bog plant, poked out of snow, which drifted among the scaly trunks of spruce trees. Also scattered among the trees in a 5 x 5 grid, were 25 wide, black, corrugated tubes, each about two feet in diameter and 30 inches tall. Each tube was covered by a square board and a rounded cap of snow. 

This research takes a village. The Great Lakes Indian Fish & Wildlife Commission (GLIFWC) spearheaded this research in collaboration with UW-Madison. Last fall, a crew of GLIFWC staff helped place these tubes. Then, staff from GLIFWC returned regularly to put bait in the tubes, just to make sure that the small mammals were comfortable using these odd habitats.

Research Tech Sarah Nagel shows off a trap tube (the tube protects an area from snow, and smaller live traps are set inside. The pink bucket holds fresh bait, her clipboard, and tools for weighing and measuring the small mammals they catch. Photo by Emily Stone.

The live trap is set inside the protected space of the tube. Otherwise, deep snow would make it diffulcut to set the traps and have them shut properly. 

Finally, when Ally came up to start her 9-day trapping session here, she and Sarah baited and set a metal live trap inside each tube. The bait packets are an ingenious system of a thin plastic bag filled with a little stuffing and sunflower seeds. The small mammals can easily chew into the bag, eat the seeds, and make a warm nest with the stuffing. Although the plastic may seem dangerous, it can add a layer of warmth, and doesn’t cause suffocation. With the bitter cold, the researchers also strap a big air-activated heat packet to each trap. It must be a fairly comfortable set-up for the critters, because some get “trap happy” and come back day after day for a free lunch.

The live trap gets a little bag of bait in the way back, as well as a giant handwarmer attached to the outside. 

For example, when Ally pulled a little deer mouse out of one trap, it was already sporting tiny metal ear tags. It had already been weighed, its hind foot measured, and its ears pierced, on a previous day. Sarah recorded the ear tag numbers on her data sheet, and the wide-eyed mouse was free to scurry away.

Ally Moser Scott holds up a re-captured deer mouse so that she can read the numbers on its ear tags and then set it free. Photo by Emily Stone.

You may be wondering why scientists would go to so much trouble, and brave the recent bitter cold, just to count the little critters in an area. The answer lies in a slightly bigger critter: American martens. These beautiful animals have been reintroduced to Wisconsin, but aren’t thriving as hoped. Resource managers want to know why. A few years ago, Phil Manlick, now a PhD student in Professor Pauli’s lab, stayed at the Museum’s staff house to study American martens in the area through DNA samples from hair and scat. His results showed that the local martens are eating mostly shrews and road-killed deer instead of their preferred food of red-backed voles.

Phil Manlick set out hair snares for American martens in 2013 and 2014. Photo by Emily Stone.

Ally’s research is following up on those findings. Maybe martens are eating shrews because voles aren’t available? During my single day on the trap line, we certainly caught more shrews than mice, and Ally has only caught a few red-backed voles so far.

Ally holds up a short-tailed shrew that was caught in a trap. As insectivores with super high metabolisms, they are on a constant search for food and don't put on any fat stores. This makes them poor food for martens. Photo by Emily Stone. 

My toes were chilled by the end of that morning afield, but the temperatures I endured were nothing compared to the deep freeze that Ally and Sarah navigated safely three days in a row. Her research will eventually earn her a master’s degree. In the meantime, Ally and Sarah have earned some bragging rights for toughing out the bitter cold.

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!

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