Biologists and outdoor writers often refer to deer decreasing their metabolic rates during winter as a survival strategy. But do deer actually do this?
How metabolic rate is measured
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 (a big enclosed box) and measuring the deer's oxygen consumption. The amount of oxygen a deer consumes is then entered into an equation to determine the amount of energy (calories) the deer used during the measurement period. Scientists use these measurements to determine an animal's fasting metabolic rate (FMR); FMR is the lowest amount of energy a deer needs to survive. To accurately measure FMR, deer must be bedded (inactive), fasted (not fed) for at least 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. When deer are exposed to temperatures below the lower limit of their TNZ they must use energy when they shiver to stay warm. When exposed to temperatures above the upper limit of their TNZ, deer must use energy when they pant to stay cool.
Original metabolic research on deer
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 kcal/kg BM 0.75 /day during winter to 146-166 kcal/kg BM 0.75 /day during summer. (The actual numbers aren't as important as recognizing that the researchers found that deer metabolic rate was lower in winter than in the summer.) However, these scientists did not account for the deer standing and moving in the metabolic chamber because they assumed the energetic costs of these activities 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.
A standing deer burns more calories than a bedded deer
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, up to six degrees higher than the assumed "around 20 ° C".
Errors in the original research
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 actually measuring elevated summer metabolic rates, but unknowingly reporting them as minimum metabolic rates. As a result, the winter metabolic rates appeared to be lower.
Updated research on deer metabolism
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 BM 0.75 /day) but reported dramatically lower summer rates (85-95 vs. 146-166 kcal/kg BM 0.75 /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 BM 0.75 /day, further indicating the error in earlier summer measurements.
Deer do NOT lower their winter metabolic rate
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 BW 0.75 /day, respectively, and demonstrated deer do NOT decrease their metabolic rate during winter.
This article was written by Kip Adams, wildlife biologist and Northeast regional director for the Quality Deer Management Association (QDMA). The QDMA is a non-profit wildlife conservation organization dedicated to promoting sustainable, high-quality, white-tailed deer populations, wildlife habitats and ethical hunting experiences through education, research, and management in partnership with hunters, landowners, natural resource professionals, and the public. The QDMA can be reached at 1-800-209-DEER or www.QDMA.com.
Copyright 2006. Whitetail Stewards, Inc. All Rights Reserved
