How do whitetail deer survive
snowy winters with subzero temperatures?
So far this winter has not been especially snowy, but nevertheless since the unusual surprise snow storm of mid-November of last year, we have a solid snow cover on a good foundation of ice. In the last months we have experienced some warm days alternating with record lows and fierce winds giving us a sizable ice foundation under the snow cover.
When I see the deer families passing the house on their way from the hill to the valley, I always wonder how they survive a winter that has been here now for over three months. Those deer families always stop under the bird feeders to lick up some bird feed that has fallen onto the snow before moving on. But these meager morsels can’t possibly help them survive the brutal northeastern winters.
When I see the deer families passing the house on their way from the hill to the valley, I always wonder how they survive a winter that has been here now for over three months. Those deer families always stop under the bird feeders to lick up some bird feed that has fallen onto the snow before moving on. But these meager morsels can’t possibly help them survive the brutal northeastern winters.
Afterall, most woodland creatures in the winter, live on a razor thin caloric budget, balanced between sharply diminished food resources, energy expenditures and fat reserves. Even small variations in any of these areas can be fatal. Yet the whitetail deer thrive everywhere in the region. Despite this year’s hard layer of ice under the snow from earlier warming and refreezing, now in March, we are not seeing any less deer moving through the landscape.
What is their secret? Well it is a combination of biological and behavioral adaptations that allow them to survive.
What is their secret? Well it is a combination of biological and behavioral adaptations that allow them to survive.
Biological Adaptations:
In our post of December 20, 2018, we wrote about deer shedding their summer coat for a heavier winter coat in the Fall. This coat is composed of darker hollow guard hairs and a dense underfur giving them excellent protection. The darker guard hairs increase solar heat gain when they are in the sunshine and because they are hollow, excellent insulation. The fine wolly under pelt is an additional layer of terrific protection.
In our post of December 20, 2018, we wrote about deer shedding their summer coat for a heavier winter coat in the Fall. This coat is composed of darker hollow guard hairs and a dense underfur giving them excellent protection. The darker guard hairs increase solar heat gain when they are in the sunshine and because they are hollow, excellent insulation. The fine wolly under pelt is an additional layer of terrific protection.
But deer prepare for winter in other ways. They are eating more in the fall to add layers of fat. Research has shown, that at the beginning of winter, 25 % of their body weight is fat. Think of all the acorns they will eat in September to reach this weight!
This fat layer is so important, that later-season born fawns will shift from growing muscle mass and height, to creating more fat.
This additional fat layer though is not enough to survive. So their metabolism and digestive systems change as well. Having adapted to less food during the northern winter months, their digestive system slows down markedly. Studies in North America have shown metabolic declines of as much as 50%. Thermal images of their bodies during very cold temperatures, shows that they can throttle back blood flow to their limbs and outer body layers, to conserve core heat and calories.
This fat layer is so important, that later-season born fawns will shift from growing muscle mass and height, to creating more fat.
This additional fat layer though is not enough to survive. So their metabolism and digestive systems change as well. Having adapted to less food during the northern winter months, their digestive system slows down markedly. Studies in North America have shown metabolic declines of as much as 50%. Thermal images of their bodies during very cold temperatures, shows that they can throttle back blood flow to their limbs and outer body layers, to conserve core heat and calories.
European Red Deer Buck Deer automatically decrease their metabolism at the beginning of the cold season, and it does not matter how much food is available. Lower temperatures slow down body functions- such as digestion, but they do survive, which is more important. In fact, as reported in the Stuttgarter-Zeitung.de, 03 January 2016, German studies of European Red Deer, showed that their digestive tracts actually shrink. These organs will actually shrink as much as 15%, before enlarging again in the spring. Over a three year study, a confined herd of does which were given abundant food throughout all four seasons, nevertheless they ate half as much food during the winter months. Indeed the cells in their guts changed to start taking up sugars and protein fragments more efficiently. Their winter feces showed significantly lower amounts of protein. Red deer heart rates can be as much as 60% lower in the dead of winter.
Northern whitetails also have adapted by growing larger. Known as Bergmann’s Rule, it shows that in many species distributed over colder and colder latitudes, the northernmost cousins tend to grow larger in body mass. The larger the body mass, the less surface area there is to lose heat, therefore the more efficient the caloric expenditures. (As body mass increases, the increase in surface area decreases in proportion to the body mass). It probably also helps to have taller and heavier bodies to push through winter snows and reach higher branches to browse as well.
You can think in North America of the northern Moose, then the Elk, Mule Deer, Whitetails and southern Whitetails to see this decrease in size. But you see it in bears as well! The Polar Bear is the largest bear on earth, followed by the Kodiak Island and Kamchatka Peninsula Brown Bears, then the Grizzly Bears of the American west and the eventually southernmost black bears. They all demonstrate Bergmann's Rule.
Northern whitetails also have adapted by growing larger. Known as Bergmann’s Rule, it shows that in many species distributed over colder and colder latitudes, the northernmost cousins tend to grow larger in body mass. The larger the body mass, the less surface area there is to lose heat, therefore the more efficient the caloric expenditures. (As body mass increases, the increase in surface area decreases in proportion to the body mass). It probably also helps to have taller and heavier bodies to push through winter snows and reach higher branches to browse as well.
You can think in North America of the northern Moose, then the Elk, Mule Deer, Whitetails and southern Whitetails to see this decrease in size. But you see it in bears as well! The Polar Bear is the largest bear on earth, followed by the Kodiak Island and Kamchatka Peninsula Brown Bears, then the Grizzly Bears of the American west and the eventually southernmost black bears. They all demonstrate Bergmann's Rule.
Behavioral Adaptations:
But what do they eat when the ground is covered in a thick layer of ice and snow? I saw them attacking the snow and ice with their hooves, making a hole to get to the soil where the grass is. These patches are called ‘scrapes’ by hunters. A scrape can be made by bears, racoons or turkeys- all doing the same thing. They can all smell the newer green growth below the snow, but the snow will tell the tale of who has been rooting around your house.
But what do they eat when the ground is covered in a thick layer of ice and snow? I saw them attacking the snow and ice with their hooves, making a hole to get to the soil where the grass is. These patches are called ‘scrapes’ by hunters. A scrape can be made by bears, racoons or turkeys- all doing the same thing. They can all smell the newer green growth below the snow, but the snow will tell the tale of who has been rooting around your house.
The browse line on our hemlock bush I also saw them eating twigs and evergreens on our bushes around the house. Yes, your landscaping is easy pickings for hungry deer. As you drive along the highway, you might observe that in the bordering woods, the tree branches appear to have been trimmed off neatly up from the ground to an even edge of about four feet above the ground. It appears as if a committed gardner is keeping everything neatly trimmed. In fact, it is evidence of a healthy local deer herd browsing away the understory growth. If you observe their wandering tracks in the snow, you’ll see them trace the edges of protruding patches of twigs and saplings.
Two does and their yearling fawns In this photo (looking north), a mother doe is resting on a berm catching the southern slope sun and the eastern rising solar gain from across the valley. Her fawn is browsing the emerging growth around her bed. As the sun rotates to the west and drops below the trees to the left (west), they will move on to better areas to catch the sun. Energy conservation is paramount to surviving the winter. So although deer are nocturnal, they will bed on southern facing slopes during the day. Wise hunters understand this.
I’ve also noticed that in storms, deer gather around the house and lie down. Perhaps the house gives off some warmth, but they definitely are protected from the wind due to the many trees around our house. I think that the plume of hot gases that leave our woodstove chimney may give a little extra warmth as they buffet about in the wind. Certainly the pine branches provide cover as - like a cloudy night, overhead pine branches, dense thickets or bent cattails form an insulating blanket that reduces heat loss. (Overcast nights are always warmer than nights with clear skies, as a heavy blanket of clouds slows the loss of the day's heat from radiating into space.)
Three deer bedded underneath the pines in advance of a coming storm
When they get up and leave, you see cashew shaped indentations in the snow, but the soil is not showing, as their well insulated bodies did not melt the snow. The big end of the cashew is their hind quarters, the other end their shoulders- about the size of a medium- large sized dog. They tuck their legs into their bellies and lay their head and neck next to their abdomen. Yes, they are well insulated and thus protected from the cold. Moving is expending energy. So they will lay like this as long as makes sense given the weather.
In the spring, deer put on weight again. They have that internal clock: slow down for Winter and spring back in the Spring.
Deer know what to do in winter. Their strategies work, most of the time… without our help!
Deer know what to do in winter. Their strategies work, most of the time… without our help!
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