Winter Survival Champions

“It’s amazing. I can’t believe there’s anything here,” exclaimed Dr. Joseph Merritt.

Resident Director of Powdermill Nature Reserve, the biological field station of Pittsburgh’s Carnegie Museum of Natural History and author of Guide to the Mammals of Pennsylvania, Merritt is a specialist in small mammals. To learn more about the lives of such creatures as deer and white-footed mice, woodland jumping mice, southern red-backed voles, southern flying squirrels, eastern chipmunks and short-tailed shrews, Merritt has been live trapping them for over sixteen years. On a cold afternoon in February, with two inches of fresh snow on the ground, my husband Bruce and I were accompanying him on his rounds.

The January flood of 1996 had occurred less that a month before our visit. A portion of Merritt’s trapping site along Powdermill Run had been under water and the traps and their protective chimneys had washed away. Merritt had quickly replaced them and, to his amazement, was finding new animals, specifically short-tailed shrews, in that area. Somehow they had learned that the previous residents were gone (presumably drowned) and were already claiming new territory in the winter. Of the eight short-tailed shrews he trapped that afternoon and the following morning, four were new animals in the flood zone and four were re-captures from higher ground. He also found two flying squirrels and two red-backed voles in traps further up the slope, the latter among large rocks that we clambered over and around.

Merritt sets two metal Sherman box traps, protected from the elements by a wooden chimney, ten meters (about 33 feet) apart in his wooded study site of one hectare (about two and half acres) by one hectare, and has a total of 200 trapping sites in all. Baited with sunflower seeds and padded with synthetic-fiber nesting material, the traps are monitored in early morning and late afternoon three days of one week, every month of the year. The short-tailed shrews are toe-clipped because of their tiny ears, the other mammals are ear-tagged for identification so Merritt can distinguish each captive. He also weighs and sexes them and checks their reproductive status.

In the summer he may find as many as 100 traps occupied; in the winter the number is much smaller. That’s because the woodland jumping mice are hibernating, having reduced their body temperature from a normal 98 degrees to 33. Many of the others, such as chipmunks, flying squirrels, and mice, undergo temporary torpor, reducing their temperature to 60 degrees and then periodically arousing to eat stored food. Southern red-backed voles and short-tailed shrews, however, are active all winter long.

Although Merritt studies small mammals year round, he is particularly interested in their winter survival techniques. Years ago, as a graduate student at the University of Colorado in Boulder, he had monitored the individual development and population changes of wintering southern red-backed voles at 11,000 feet in the spruce-fir forests of the Rocky Mountains. Eight days a month he snowshoed into his site and found each trap by digging through snow beneath markers in the trees. He was still snowshoeing in and brushing snow from his traps in early June when he made an electrifying discovery–a two week old vole born under the snow two months before the snow would be gone. In other words, southern red-backed voles not only survived in the winter successfully under a heavy snow pack but they reproduced.

This discovery solidified Merritt’s interest in winter ecology, a discipline pioneered by Russian scientists in the 1930s and 40s. But instead of staying in Colorado, Merritt, a California native, came to Powdermill.

“Once I saw the Appalachian forests, I was hooked,” Merritt told us. Even after sixteen years, he continues to be delighted by the beauty and diversity of eastern woodlands.

His study site at Powdermill is in a mature second growth forest consisting primarily of American beech, yellow poplar or tulip, sugar maple, cucumber magnolia and red oak trees with an understory of striped maple trees, spicebush, witch hazel, and rhododendron. Selectively logged in the early 1900s, the site now has many large trees and a small mammal population of approximately 250.

At Powdermill Merritt is able to study the winter survival tactics of the same southern red-backed vole (Clethrionomys gapperi) that he studied in Colorado, because it ranges in the west along the Rocky Mountains to New Mexico and Arizona, across Canada and the northern United States, and in the east along the Appalachian Mountains south to northern Georgia. This beautiful little creature is easily identifiable by a broad, reddish band running from its forehead to its rump and its bicolored tail, dark brown above and whitish below. It lives in coniferous, deciduous and mixed forests with an abundance of rotting logs, stumps and exposed roots and eats nuts, seeds, berries, mosses, lichens, ferns, mushrooms, plants and arthropods.

After twelve years of live trapping them at Powdermill Merritt determined that they ranged in density from five to 36 voles a hectare. Their winter survival techniques include: reducing their body size in the autumn and winter so they need less food; shifting their food preferences to readily available seeds, roots, bark, and plant parts; foraging under the snow and in subterranean burrows where they are not affected by snow or bad weather and where it is warmer; and engaging in non-shivering thermogenesis (heat production) or NST.

NST is an important winter survival technique not only for red-backed voles but for short-tailed shrews as well. Between their shoulder blades near their spinal cords, they have high energy, heat-producing tissue called brown adipose or brown fat which functions like a blanket to keep them warm. Merritt found that it was especially effective during mid-January, the coldest days of the year at Powdermill.

Despite all these techniques, however, red-backed voles lose weight throughout the winter. In contrast, short-tailed shrews actually gain weight. Merritt calls them the “champions at winter survival.”

Northern short-tailed shrews (Blarina brevicauda) are the largest of North American shrews and one of our most abundant mammals. They live in a wide variety of environments–forests, fields, thickets and grasslands–wherever there is a thick layer of leaf litter and humus where they can construct their intricate underground burrow system. There they hunt invertebrates such as spiders, centipedes, slugs, snails and earthworms as well as salamanders, mice, voles, mushrooms, and plant material. Often, when sitting under a tree in our woods, I can hear the high-pitched squeaking sounds they make. They also use ultrasonic sounds to echolocate objects in their dark burrows.

Short-tailed shrews are smelly creatures, emitting a musky odor produced by oily skin glands on their sides and bellies, which is why cats will kill but not eat them. They are also one of only a few poisonous mammals in the world. Their toxin, similar to that of a cobra’s, immobilizes but does not kill their prey. It slows down their victim’s heart beat, blood pressure, and respiration, making it comatose so it can be stored for three to five days and provide fresh food whenever it is needed. To make sure no other animal eats their prey, they mark it by urinating and defecating on it and cache it in abandoned mice nests. This food caching ability is one of their winter survival techniques.

Until Merritt conducted radiotelemetry studies on short-tailed shrews, they were thought to use huddling as another winter survival technique. Flying squirrels, mice and voles all form group-huddles in a communal nest which help to keep them warm. Not so the belligerent, highly territorial, and individualistic short-tailed shrews. They live alone year round. But the underground nests they build are so well insulated that they are considerably warmer than the above ground temperature. The soil-leaf litter zone where they forage primarily for insects and insect larvae in the winter can be more than 50 degrees warmer than the outside temperature in mid-January and as much as 59 degrees warmer within their tunnels. They also reduce their activity during the coldest periods, staying active only seven to 16 percent of the day. The rest of the time they sleep.

As we accompanied Merritt on his rounds we were impressed by the dedication that drives him on, even in a light snowfall the following morning, even after a couple tumbles on the rocky mountainside, setting and checking hundreds of traps day after day, year in and year out. He admitted that in the milder months he often has help, but in the winter he is usually on his own. He works quickly and efficiently, handling the creatures as little as possible. No matter what the weather, if the traps are set, he checks them, even during the flood when the bridge spanning Powdermill Run was under water.

“I don’t want them to die,” he explained. Unlike some small mammal biologists, he does not kill any of his study animals. In fact, during an earlier visit to his site, in late October, I was struck by the affection he seemed to have for all the small mammals he showed us. He was especially concerned for the wellbeing of his short-tailed shrews.

“Shrews make life real difficult,” he told us. “They’re so temperamental they die if it thunders.” They also chitter loudly the first time they are captured, even before he toe-clips them, something he does not like to do. But it is the only way to get the kind of information on population dynamics that he needs. And after their first capture, the repeats do not protest at all. Many, in fact, are seemingly happy to eat the sunflower seeds in return for a brief minute or two of handling.

During our winter visit, Merritt added to his data. He learned that short-tailed shrews move into a desirable, deserted territory fast, even in winter. One of the newcomers was a big animal–21.5 grams and two weighed 14.5 grams. Although short-tailed shrews cannot be sexed unless they are nursing, he could sex the red-backed voles. One was a male probably born last summer. Another was an unusually big female–30 grams. And one of the flying squirrels, who had never been caught in that particular box before, was a female with a swollen vulva, an indication that she was beginning her reproduction cycle. Like all the flying squirrels he captures, summer and winter, new captures and repeaters, she screamed like a banshee until he released her. Then she flew to one of the 30 flying squirrel boxes hammered on to the trees in the study site.

“I never get tired of seeing them fly,” Merritt commented. Despite what seems at times, to be repetitive, difficult work, Merritt gets more joy from his work than most people, I suspect. To interact with such a charming cast of characters as the diverse small mammals of an Appalachian forest has its rewards and I felt privileged to have had a close-up view of creatures I have often observed from afar in our own Appalachian forest.