QDMA Articles

By: Kip Adams

The Quality Deer Management Association provides reliable, unbiased information for deer biologists, managers and sportsmen. This information dispels myths such as, “once a spike, always a spike,” and clarifies inconsistencies in the biological arenas such as the widespread misunderstanding regarding seasonal metabolic rates for deer. Biologists and outdoor writers often refer to deer decreasing their metabolic rates during winter as a survival strategy. But do deer actually do this?

Indirect respiration calorimetry is the most commonly used technique for measuring metabolic rate. This involves placing a deer in an open-flow large-animal metabolic chamber (big enclosed box) and measuring the deer’s oxygen consumption. Scientists use the measurements to determine an animal’s minimum fasting metabolic rate (FMR). To accurately measure FMR, deer must be bedded (inactive), fasted (>48 hours), and 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.

Scientists started working on white-tailed deer energetics in the 1950s. In 1969 and 1973, scientists reported a seasonal metabolism cycle for whitetails ranging from 77-81 during winter to 146-166 kcal/kg BM0.75/day during summer. However, these scientists did not account for deer activity in the metabolic chamber and assumed activity costs would be minimal. Also, the TNZ for white-tailed deer had not yet been determined. Scientists assumed the lower limit to be “around 20°C” and exposed deer in summer pelage to 16-21°C.

During the next two decades scientists determined the energetic cost of standing is 60% higher than lying for white-tailed deer, and that activity in a chamber (including standing and moving) can increase energetic costs by 88%. These findings dispelled the assumption that activity costs in a chamber are minimal and insignificant. Scientists then identified the lower limit of TNZ (within a chamber) to be 20-26°C, higher than the assumed “around 20°C”.

This new research suggested earlier reported summer FMRs were suspect because low experimental temperatures induced thermal stress and forced deer to elevate their metabolic rates to maintain body temperature. Deer were also active during metabolic trials with no account for differential energetic costs. So, by exposing deer to low temperatures and permitting activity, scientists were measuring elevated metabolic rates, but unknowingly reporting them as minimum metabolic rates.

In the 1980s, scientists revisited the issue of seasonal metabolism. These studies accounted for activity in the chamber and exposed deer to temperatures within the TNZ. Scientists measured winter metabolic rates similar to earlier studies (80-85 vs. 77-81 kcal/kg BM0.75/day) but reported dramatically lower summer rates (85-95 vs. 146-166 kcal/kg BM0.75/day), and concluded there was no annual rhythm. Measurements since have indicated the maximum cost of gestation for a doe with twins is only 150 kcal/kg BM0.75/day, further indicating the error in earlier summer measurements.

Studies in the 1980s followed stricter methodologies and used technology unavailable in the 1960s, and describe more accurate seasonal FMRs of deer. These studies reported summer, winter and spring FMRs of 87, 85 and 90 kcal/kg BW0.75/day, respectively, and demonstrated deer do NOT dramatically decrease their metabolic rate during winter.

Kip’s Korner is written by Kip Adams, a Certified Wildlife Biologist and Northern Director of Education and Outreach for the Quality Deer Management Association (QDMA). The QDMA is an international nonprofit wildlife conservation organization dedicated to ethical hunting, sound deer management and preservation of the deer-hunting heritage. The QDMA can be reached at 1-800-209-DEER or www.QDMA.com.

This article was taken directly from a previous issue of Quality Whitetails, the bi-monthly journal of the QDMA. The $30 annual membership to QDMA includes a subscription to this acclaimed publication. For information on joining QDMA, click here.