Outdoors commentary: Deer built to survive winter months

We can handle it, for the most part, and simply go indoors.

But what about our four-legged friends, such as deer, squirrels and rabbits? Some have dens, hollow trees and such, but not deer. Deer can bed down in a valley, near a cedar or pine, and get out of the wind, which helps.

But they can’t go inside.

Turns out, they don’t need to. Whitetails are suited — well, sort of, as their “suit” is a thick, fat-lined hide — to withstand more than you think. In fact, I thought the bitterly-cold days of January and February were crunch time for deer survival until I chatted with Jeff Pritzl, the Wisconsin DNR’s deer guru. Actually, his title is Deer Program Specialist, but in reality, this guy is all things deer, top to bottom, side to side.

He’s the deer man.

Anyway, Pritzl explained that March and even April are more critical months to a deer’s survival than what many of us consider the dead of winter — January and February.

“The most critical month is March. If it has been a winter that has been severe, they (deer) are going to have used up their body reserves (to get that far),” Pritzl said. “Deer have the ability to reduce their metabolism and rely on fat reserves. They will work off of their fat reserves as needed. Every time we get a cold snap it dips into that reserve a little bit.”

Deer have the ability to adapt to harsh winters in a number of ways, including conserving energy, slowing their metabolism and even changing what happens with their digestive system in order to adapt to a diet of twigs and woody material — late-season food sources. With their four-chambered stomachs and different microorganisms working in a highly specialized digestive system, a deer can change its diet — and does — throughout the winter, depending on food sources.

“Mostly, they will balance conserving calories by restricting their movement. In the agricultural part of the state, in some areas, they have access to waste grain products throughout the winter,” Pritzl said.

“In many cases that becomes less favorable as the snow gets deeper. When the snow becomes really hard and crusting from freeze, thaw, freeze, thaw, freeze, thaw conditions, then the main forage in the winter is browsing. They turn to woody vegetation that provides the best calories they can get.”

So we know they smartly conserve energy, change their diet to more readily available twigs, bushes and leaves, but how do they stay warm? If you are a hunter and have skinned a deer, you understand why. The hide is thick, heavy and waterproof because of its course hair. And under that hide, as Pritzl mentioned, are layers of fat that the deer — does more than bucks — have built up in preparation for the winter months.

“The bottom line is we could have a pretty severe cold snap here in the winter, and how it relates to covering the bottom line is they can take just about anything winter can throw at them this time of year,” Pritzl said. “It is a matter of how it will affect them beyond the winter.

“The bigger influence to a deer herd in La Crosse County won’t be direct mortality, but how well did they come out of the winter? How successful they are in raising the fawns in the following spring.

In bucks, it can show itself in antler development.”

OK, let’s circle back to the original question of a deer’s critical winter months. Deer, for the most part, will be able to battle through December, January and February. And does, for the most part, have more layers of fat built up for those months than bucks. Bucks, well, they are busy burning calories during the breeding season, then avoiding hunters during the archery and gun seasons, so their fat reserves are generally less.

When March and April roll around, does and bucks have typically used up what is in their refrigerators and freezers, so-to-speak, and must forage for new food. In other words, they hit the grocery store hard during those months.

And if the store is closed – aka a late spring – then it takes a toll on them and their soon-to-be born fawns.

“If things start to green up beginning of May, does can shift back (their digestive system) and give birth and nurse. If the green-up is delayed because of a cold spring, it puts stress on them,” Pritzl said.

“If a deer does survive, her fawn success is all predicated on her body weight. If the winter conditions stretch out into March in the form of snow depth … it can affect that (body weight).”

While minus 20 and minus 30-degree temperatures grab most of the headlines, Pretzl said snow depth – which can linger into the “spring” months – is more critical than temperature. The DNR uses a winter severity index to measure each day and month, and that severity index really doesn’t kick in until there is 18 inches of snow on flat areas.

In other words, in the Coulee Region that index seldom comes into play. In the northern half of the state it’s a different story. More snow now usually means the longer the ground remains covered in the north in the spring.

“We track the winter effect on deer through our winter severity index, which is based on snow depth and air temperature. It accumulates points as the air temperature drops below zero and when the snow depth is greater than 18 inches,” Pritzl said. “It is a cumulative effect that runs over time. It is a good measurement of what we can anticipate as to any deer loses to the winter weather.

“Honestly we don’t even measure it (winter severity index) in the southern half of the state. It never really becomes a factor in the farmland portion of the state. It is unusual for deer to end up succumbing in any great numbers from severe winters in the farmland area.”

So the bottom line is deer are built to withstand the cold, and then some. It’s snow depth that can have a bigger negative impact, along with late winters that carry over into spring. If that happens, a doe’s health, and in turn, reproduction, can be impacted as well as antler growth on bucks.

So let’s hope for an early spring, for the deer’s sake.

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