The Wild Side: Winter survival for deer
I think it is safe to say we are officially experiencing winter now. I have had a few people tell me they are sick of winter and are ready for it to be over. Maybe we just aren’t used to winter anymore; maybe the last couple of years have made us soft. Humans are probably not the best example of living organisms that adapt their bodily functions to fit the climate. There are some exceptions; the Inuit (or Eskimos) function very well in a polar climate that I couldn’t handle. But when you look at animals that live in our temperate regions, they change-sometimes drastically-from summer to winter. Some animals den up or hibernate. Other animals change color, diet, fur coat thickness and even activity or roosting behaviors.
Deer are a good example. In the summer, the peak of the growing season, deer might browse on eight pounds of green forage per animal per day. This would not be sustainable year-round, especially in the Northwoods. They gorge themselves through the growing season to build up body condition and fat reserves to function in the winter. As the snow deepens, deer behavior changes. They reduce movement and activity. Their food intake decreases. Their body adapts from digesting green growing summer forage to digesting woodier stems and buds with lower nutritional content. In some places, they engage in a behavior called “yarding,” where they find a ready food source. If they find a timber harvest location where tree branches are lying on top of the snow in easy reach, they will congregate and munch away, sometimes even tolerating the presence of people or vehicles that normally would scare them off.
As a WDNR wildlife biologist, an important part of my job is documenting the health of the deer herd throughout the winter. This includes operating the Winter Severity Index and observing deer yarding activity on a regular basis. The Winter Severity Index has been recorded across northern Wisconsin since the mid 1980s. This is one of the simplest indexes to construct. For every day that has a low of 0°F, you add a point. For every day with a standing snow depth of 18 inches or more, you add a point. If a day has snow depth over 18 inches and a subzero low, it gets two points. We record these numbers daily from December through April. The resulting number tells us a lot about the health of the overwinter population. We see a notable increase in deer mortality when the cumulative number exceeds 80. If the number stays good and low, say below 50, we normally expect a 10 percent increase in the buck harvest the following fall.
Currently, our winter severity is around 25, very favorable for deer survival and herd growth. The snow depth in the woods is approaching 18 inches, though. What is magical about that snow depth? Basically, it is the depth where deer start to drag their bellies in the snow. Every step becomes a chore. It takes a lot of energy to travel. Predators may also have an easier time getting through (or on top of) the snow pack, putting deer at a disadvantage. Basically, it is going to suck to be a deer soon. We will continue to closely monitor the deer herd and watch for signs of significant stress.
This is a good time to mention that our deer forums have been scheduled. This year, the Oneida and Vilas County Deer Forum will be held at the Woodruff Town Hall at 6 p.m. on March 20. More information on the meeting dates, times and subject matter will be released soon. I expect that there will be information on the plans to implement the Deer Trustee Report recommendations at these meetings as well, so plan to attend and learn more about the future of Wisconsin deer management.
Jeremy Holtz is a wildlife biologist with the Wisconsin DNR and writes a weekly column in the Star Journal. To contact him, call (715) 365-8999.