February is the perfect month for ice fishing, winter habitat work, and watching a highly-anticipated football game. It’s also the month that after a few hours of good outdoor exercise my dogs sleep more soundly on a warm floor than any other time of year. I’ll admit to an occasional mid-winter nap, too. There’s something about coming in from the cold to a warm house and meal that makes you feel satisfied and, well, a little sleepy.
That’s fine for you and your favorite pet, but how about your favorite game animal? Whitetails obviously don’t join you in the kitchen for some chili, so how do they survive the harshest time of year? The first reader who thinks “They reduce their metabolism” owes me a new pair of hunting boots so I can kick the tail of every outdoor writer that continues to spread this myth! For the record, whitetails do not lower their metabolism during winter.
To be fair, this seems a very logical strategy for whitetails living in harsh winter environments, and writers and even some biologists have perpetuated this falsehood for decades. The truth is a whitetail’s metabolism varies extremely little across the seasons. If that’s true, then how do they survive winter’s harshest weather?
I was fortunate to study under Dr. Peter Pekins and be a part of many metabolic trials on whitetails during my graduate work at the University of New Hampshire (UNH). This gave me a great understanding of this scientific technique and an even greater appreciation for a whitetail’s ability to survive. Let’s look at the science behind whitetail metabolic rates.
Deer In A Box
The most commonly used technique for measuring metabolic rate involves placing a deer in a big enclosed box called a metabolic chamber and measuring the deer’s oxygen consumption. Scientists use oxygen consumption measurements to determine an animal’s minimum fasting metabolic rate (FMR). The FMR provides legitimate comparisons between seasons, sex, age class, and even between species.
To accurately measure FMR, deer must have an empty stomach (no food for at least the last 48 hours), be inactive (bedded), and be exposed to temperatures within their thermal neutral zone (TNZ). The TNZ is the range of temperatures over which an animal’s metabolic rate is nearly constant, minimal, and unaffected by temperature. These requirements are important because deer that are standing, moving, feeding or exposed to temperatures outside the TNZ have higher metabolic rates than bedded, non-fed deer within the TNZ.
On The Wrong Track
Scientists started working on white-tailed deer energetics in the 1950s. In 1969 and 1973, they reported winter metabolic rates that were 80 to 116% lower than summer rates. However, the scientists who conducted these early studies didn’t account for deer activity in the metabolic chamber, and they incorrectly assumed the activity wouldn’t skew the calculations. Since the winter rate was much lower than the summer metabolic rate, biologists started referring to a much-reduced winter metabolism and suggested it was a winter survival strategy.
During the next two decades, Dr. Pekins and others identified many problems with the early research. They determined it costs deer 60% more energy to stand than bed down and that standing and moving in the chamber can increase energetic costs by 88%. These and other findings clearly identified problems with the early studies and highlighted the miscalculations. New research showed the scientists were measuring elevated metabolic rates but unknowingly reporting them as minimum rates.
New Deer Science
In the 1980s, scientists revisited the issue of seasonal metabolic rates and reported vastly different results. These more recent studies accounted for deer activity in the chamber and exposed deer to temperatures within the TNZ. These scientists measured winter metabolic rates similar to earlier studies (80 to 85 vs. 77 to 81 kcal/kg BM0.75/day) but reported dramatically lower summer rates (85 to 95 vs. 146 to 166 kcal/kg BM0.75/day). See the chart below.
These more recent studies followed stricter methodologies, used technology unavailable during the early studies, and described more accurate seasonal metabolic rates of deer. These studies reported summer and winter FMRs of 87 and 85 and demonstrated deer do not decrease their metabolic rate during winter.
How Deer Survive Winter
Since deer don’t dramatically decrease their metabolic rate during winter, how do they survive? Body fat is the key physiological component of winter survival. Northern deer gain as much weight as possible during fall to use as an energy source during winter. In essence, they are similar to bears in this feature, except they don’t hibernate. Winter nutrition comes from stored fat supplemented with browse (buds, twigs, dead leaves).
Additional research at UNH showed adult does can get more than 50% of their daily nutrition from their fat reserves. Even captive deer with unlimited food lose weight during winter. The key to winter survival is depositing as much fat as possible during fall and then reducing activity and thus fat usage during winter. A deer herd’s ability to do this is directly related to herd size and habitat quality. Healthy habitats with abundant food and adequate winter cover can produce healthy and productive deer herds, while poor and overbrowsed habitats typically cannot.
All together now: Do whitetails reduce their metabolism during winter? The answer is an emphatic “No!” So get out there and enhance some deer habitat today.