By Scott Lindsay, IFW Wildlife Biologist
In southern Maine, winter rules for about one third of the year. To the north and west, due to latitude, elevation and distance from the moderating effects of the ocean, it can prevail for nearly half the year. We all know the challenges and benefits winter can provide. When the stoves are cranked, the wind is howling and darkness comes at 4:00 p.m., I often think about how tough wild animals must be to make it to the next day. The animals have evolved one or more of three strategies to respond to winter; leave, hibernate or remain active.
Hibernation is a remarkable adaptation which pushes physiology to an extreme. Only a few mammals in Maine have really adopted this strategy; some bats, the meadow jumping mouse, the groundhog, and perhaps the chipmunk.
Onset of hibernation is most influenced by day length and the resulting hormone changes which lead to a significant drop in body temperature, respiration and heart rate. Animals that hibernate fatten up during the summer and fall, find a protected location, idle down their body’s engine and hopefully don’t run out of stored energy before warmer weather returns and food is more abundant.
Other mammals respond to winter through a state of torpor, think “hibernation lite.” Mammals that best exemplify torpor include bear, raccoon and skunk. They take most of the winter off by employing similar physiological strategies as true hibernators, but are more likely to wake up for short periods and perhaps even emerge from den sites during temporary periods of warmer weather.
During March or even February, people have sent me photos of large, unidentified tracks in the snow behind their homes and are surprised to learn they were made by bear. Hibernation by mammals does not mean the animal can’t adapt to the cold, it is more likely the animal cannot access food, so remaining active would mean sure death – evolution clearly selects against this.
Migration is another adaptation in response to winter. Many people employ this strategy as well. Like hibernation, migration is physically challenging and poses many risks. In Maine, this option is limited to birds and insects. The osprey, common in Casco Bay during the summer, travels down to Brazil for the winter. The piping plover, an endangered shorebird that nests on southern Maine beaches, begin migration to the Caribbean as early as August. Another famous migrator is the monarch butterfly. How remarkable that a butterfly weighing less than a gram is able to fly 100 miles a day on a migration to a mountain range in Mexico.
Most animals in Maine have developed physical and/ or social strategies to endure winter and remain active. As with the other two strategies, this one is also physically demanding and presents many risks. Animals that are active need to provide constant energy to fuel heat production and employ a variety of strategies to conserve what is produced.
The ubiquitous chickadee and the tiny kinglet are birds that remain active by fluffing their feathers and shivering practically all the time. As the temperature drops, their little bodies function as highly tuned metabolic machines to conserve heat.
White-tailed deer are not particularly well adapted to the temperature and snow depth typical of this latitude. They enter the winter period at their physical peak (though females much more so than males) and once temperatures have dropped and snow depth builds to a foot or so, many begin a localized migration to a deer wintering area, aka deer yard. By doing so, they will share a packed down trail with many other deer and expend much less energy than they would plowing through snow on their own.
Deer further conserve calories by moving less, finding favorable microclimates under hemlock or balsam branches overnight and during storms and seeking out sunny south facing slopes. Food consumption is lower and food is poorer quality, but everything helps and they will seek out hardwood browse if close enough to the deer yard. The many noses and ears in the deer yard will help identify predators and provide a better chance of living than if they were on their own.
Despite this bag of tricks, winter is still tough on deer, particularly in northern Maine. As March and April come and spring starts to make an appearance, deer are near the end of their energy budget. This is a time when deer are more likely to die; energy in is exceeded by energy out.
As the snowpack melts, it reveals evidence of tunnels and food caches at the ground/snow interface. During winter, this is known as the subnivean zone. We don’t see it much, so it often goes unappreciated, but there is a lot going on there.
As the snowpack develops, the difference in temperature between the ground and snow surface increases, leading to the formation of crystals which make it easier for rodents to establish tunnels and forage beneath the insulative blanket of snow where it remains around freezing, despite much colder air temperatures, above. Anyone who has spent time in a snow cave knows how it can make a cold day feel quite comfortable. Voles, mice and shrews are common residents in this zone. Though they are much better off here than on the snow surface, there are predators including weasels, fox and owls that are equally well adapted to find them.
Currently, as the snowpack is receding and meltwater descends to ground level, these tunnels are no longer as good a refuge and the rodents, like the deer, walk a fine line between life and death until the spring sun prevails.