Thermoregulation

As the plane touched down in Montgomery I was filled with a sense of renewed hope. By mid January the ponds in northern New England are usually frozen over and the landscape is covered in snow. Alabama is still a faded green. Being able to sneak down south for one more hunt is always a treat, but this one was particularly important. Deer season back home was already a distant, and not particularly fond memory. Poor conditions had subdued deer movement through most of the fall and the larder was poorly stocked for the winter ahead.

I knew this would be a productive hunt. The Hit-N-Miss lodge lies in the epicenter of Alabama's famed Black Belt region, which is most noteworthy for its mid January rut. The grounds are intensively managed for quality deer and according to my guide, my timing couldn't have been better. Pre-rut activity had been on the increase and things were "fixin' to bust wide open."


Thus, no one was prepared for what we awoke to the next day, perhaps most especially the deer. In fact, it was such a rare occurrence that even the guides weren't sure what the result might be. I, on the other hand, was quite sure that the sudden, dramatic drop in temperatures would put the deer on their feet. I couldn't have been more wrong.


The difference in deer movement from one day to the next wasn't subtle. "It was like someone turned off a switch," my guide remarked through chattering teeth. Temperatures can vary widely in January in Alabama with low 30s not exceptionally unusual and mid 20s not unheard of. The mercury on that first morning struggled to reach 20 at dawn, and didn't get much higher until late morning. By then, we were all back at the lodge, standing by the hearth with similar stories of woe. Conditions remained unchanged for the next day and a half. It wasn't until after the third morning, while driving to the airport that we started to see deer movement again. In retrospect, I should have known better; but that just goes to show how much whitetail can differ from north to south.

North vs. South
One of the biggest differences is in terms of thermoregulation--how whitetails from different regions are pre-disposed physically, physiologically and behaviorally to respond to changes in temperature. In some cases it's a matter of comfort, in others it's literally a matter of survival.


There are some things the deer itself has no control over. According to the biological principle known as Bergman's Rule, "warm-blooded animals living in cold climates tend to be larger than animals of the same species living in warm climates." The selective advantage is that larger animals have relatively less surface area per body mass exposed to the elements, and thus lose less body heat in colder temperatures. If you want big deer, go north.


Another adaptation is their winter coats. On average, a whitetail's winter coat has about 2,500 coarse, hollow guard hairs to the square inch. They trap air, providing some insulation. However, it is the finer, wooly under fur that provides the most insulation value by trapping layers of warm air close to the skin. These under-hairs are far more numerous than guard hairs, particularly on northern deer. The winter coats of northern deer also tend to be a darker color, which absorbs more solar energy, and thus helps reduce heat loss.

There is an economic advantage to this. Moving about in cold weather to feed has a cost, in terms of energy loss. If the calories they acquire exceed the energy lost as heat, they remain on the positive side of the cost-benefit ratio. In fact, the colder it gets the more they need to move to gain calories, up to a point. Ask any Saskatchewan outfitter and they'll tell you the colder it is, the more deer move. As food becomes scarce in winter, and temperatures plummet, even northern deer eventually reach a point where it's more energy efficient to lay low and wait.

However, it works against them when autumn temperatures are unseasonably warm, as was the case in New England the fall I went to Alabama. Daytime temperatures seldom dipped below the mid 40s, a critical level. Research on northeastern deer has shown that after they've grown their winter coats, daytime movement drops off precipitously once the mercury rises above 45 degrees. The rut goes on, but most of the movement takes place after sunset.

Down south, almost the opposite is true. The thinner-skinned deer are accustomed (and adapted) to warmer temperatures. Temperatures in the 60s and even 70s will have little negative effect on daytime deer movement. In the southwest even 90s won't shut 'em down entirely. But throw a deep cold snap at them and they hole up. Without realizing it, they're taking the most energy efficient action.

Deer also seem to know where to go when the barometer is low. Wind robs a body of heat so they'll seek shelter in the densest cover available. Up north that may be softwoods. In rough terrain it could be the leeward side of hills and mountains. In open-country it's draws and river bottoms. Conversely, sunlight provides a source of free energy and deer will seek out sunny exposures like east facing slopes in the morning and west facing slopes in the afternoon.

In the final analysis, a lot of it is common sense. If you're hunting mid- to late-season in the north, imagine yourself in a thick fur coat. Then think when and where it would be most efficient to move about. In the south, think in terms of a light sweatshirt. Then consider when you'd rather move versus laying in the shade.

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