A Whitetail’s Depressed Winter Metabolism – Fact or Fiction?

Posted on January 29, 2013

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 ( Part 1 of a 2 part series)

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Any way you slice it, the winter season for white-tailed deer is truly an endurance test. This is indeed a time when any miscalculation on their part will assuredly lead to certain demise. During winters fraught with deep snow and extremely low temperatures that extend for more than 100 days, deer mortality will be significant. Fortunately, as noted biologist John Ozoga points out, “Whitetails have endured tough winters for thousands of years. They live at the Northern extremes of their geographic range, and have evolved certain physiological and behavioral adaptations to cope with winter. When healthy, and behaviorally wise, they are hardy creatures. So equipped, they can survive cold temperatures, deep snow and nutritional hardships, even under the eye of wolves, coyotes and wildcats.”

However, in spite of the whitetail’s capacity to endure hardships, it must be asked, has our reverence for deer and their survival mechanisms helped shape a myth when it comes to the animal’s ability to depress its metabolism? Have we, in our exuberance to explain the mystery of their behavioral adaptations created a mythical creature that magically survives harsh, northern winters?

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There seems to be little argument that during winters with high severity that their survival almost seems magical, but is depressed winter metabolism the magic? Here in the Northeast, survival is largely predicated on and dictated by quality and quantity of winter habitat, (abundant nutritional food, efficient winter yards) and length of winter. Body fat reserves are a critical factor for survival along with behavioral adjustments (diminished movements) which conserve energy. But the question that looms is, do whitetails have the innate ability to consciously fast, and do they, like a hibernating bear have the wherewithal to reduce their metabolism?

The Pervasive Theory

The long-standing belief by many is that deer lower their metabolic rate to survive winter. Whitetails with lower metabolic rates would require less food and therefore increase their chance of survival during winter. According to some biologist and wildlife scientists, including Peter Pekins, Professor of Wildlife Ecology at the University of New Hampshire, this long-held belief is incorrect.

In a study of, The Basal Metabolic Cycle in White-tailed Deer conducted by Pekins, William Mautz, and John Kanter from January to August of 1989, they found no conclusive evidence to support any significant drop in a wintering whitetail’s Basal Metabolic Rate (BMR). They used eight does, four of which were pregnant during their research.

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According to their results, “Although our data points to a lack of any inherent basal metabolic cycle or rhythm, it is important to recognize the similarity in winter metabolism reported in all studies. Hence, bioenergetics models and activity budgets built upon the basal energy requirements of deer during winter are little affected by our results. It is understandable, though ironic, that the resource limitations of winter likely led to the interpretation of a depressed winter metabolism.”

The irony to this finding is that from a behaviorist’s vantage it would seem that deer have voluntarily geared their system down. It is well documented that northern whitetails are far less active during winter; they feed infrequently, and consume much less food. Given the fact that winter is indeed a negative energy balance on deer, how is it possible for them to sustain themselves if not for a metabolic drop?

On A Diet

As humans, when temperatures plummet we need to increase our food consumption to stay warm. In essence, our bodies are burning calories much faster under these conditions. Therefore, the thought of a whitetail living in harsh winter conditions reducing their food intake doesn’t make much sense. But that is exactly what transpires. I have personally witnessed deer living on sparse winter rations within deeryards eating no more or less than deer that have an abundant supply of food at their disposal. To explain this phenomenon we need to take a look at the seasonal changes in a deer’s diet and how whitetails use this food.

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In a different study conducted by Peter Pekins and associate Karol Worden on the seasonal change in feed intake, body composition, and metabolic rate of white-tailed deer, they began by introducing the cyclical changes in a deer’s diet. “White-tailed deer experience annual mass and fat cycles that respond closely to seasonal change in forage nutrition and availability. Northern deer gain mass during late spring and summer, deposit fat during fall, and catabolize fat reserves and lose mass through winter and early spring. Winter forage is primarily woody browse, which is low in nutritive quality and digestibility. The reduced availability and nutritional value of winter forage, combined with deep snow that increases energy expenditure, affect the rate of mass loss of deer. Subsequently, deer decrease their activity and food consumption, use optimum thermal cover, and feed during warmer daylight hours. Fat reserves are a critical alternative source of energy that may contribute >20% of the whole body consumption of an adult doe [more for an adult buck due to rutting behavior] during late fall and early winter, and provide up to 30% of the energy requirement throughout winter.”

The Experiment

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Pekins and Worden set out to determine if there were any intrinsic changes in Fasting Metabolic Rate (FMR) and feed intake, and if so, their relationship to fat deposition and mobilization from fall to spring. Five captive adult does were used in the study. From September-to-March the deer were fed a palatized grain consisting of 16.7% protein. Each day the deer’s consumption of food was measured. Once a month the deer were placed in a temperature-controlled chamber and provided only water. Metabolic rates were calculated only for 20-minute intervals in which deer remained 100% bedded. Kip Adams, QDMA’s Northeast Regional Director and former student of Pekins provides a detailed look at how FMR is taken, “Inside the metabolic chamber the deer’s oxygen consumption is measured. Scientists use oxygen consumption measurements to determine an animal’s minimum fasting metabolic rate (FMR). The FMR provides legitimate comparisons between summer and winter, 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, and be exposed to temperatures within their Thermal Neutral Zone (TNZ). The TNZ is the range of temperature over which an animal’s metabolic rate is nearly constant, minimal and unaffected by temperature. These requirements are important because deer that are standing or moving, fed, or exposed to temperatures outside the TNZ have higher metabolic rates than bedded, non-fed deer within the TNZ.”

Body composition was also measured on a monthly basis. Each time this was tested the deer were denied food and water 24 hours prior to the measurement.

Next post we will look at the results of this research and see if there is any correlation that can be found, and/or if a whitetail can indeed reduce their metabolic rate during winter months.

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Posted in: Whitetail Deer