How deer cope with winter conditions
Mule deer are well-adapted to survive in Utah's snowy winter weather.
Bill Bates is the wildlife section chief in the Salt Lake office. While working for the DWR, he's had the chance to restore river otter populations, research endangered fish, crawl into bear dens, and transplant big game animals.
JUST AFTER CHRISTMAS, a large storm hit most of Utah. The storm came from the south. When storms come follow that flow, generally, southeastern Utah gets hit pretty hard. This storm was no exception. Temperatures were warm enough in most of the region at mid-elevations that it came as rain at first, followed by snow. As a result, accumulations on most mule deer winter ranges was not excessive; about nine to 12 inches in most areas.
The exception was the south Book Cliffs. Temperatures were cold enough there that precipitation fell as snow. Accumulations were in excess of 30 to 36 inches in some areas, including the Horse Pasture, near Nash Wash, which is a critical winter range. Temperatures immediately plummeted to below zero at night in valley locations. Inversions trapped the cold air and the snow did not melt. Two weeks later, the snow at the Cunningham Ranch had settled to about 20 inches, but it was still over 30 inches on the benches. Temperatures in the day time ranged from nine degrees F to about 20 degrees. Once out of the inversion, on the south-facing slopes, temperatures climbed to a balmy 26 degrees and the snow began to melt, especially in areas near dark boulders.
So, how do severe winter conditions like this affect deer, and how do they cope with it?
First, deep snow can deprive deer access to life-saving vegetation. Mule deer are opportunistic concentrate selectors, meaning they select the most succulent and nutritious portions of the vegetation they consume. The digestive system of mule deer comprises about 10 percent of their body weight; a relatively small percent when compared to other large ungulates such as bison whose digestive system comprises 24 percent of the body weight. Mule deer’s smaller digestive system requires them to be more selective about the forages they consume as their smaller digestive tract limits the amount of forage they can process at one time.
A mule deer’s small muzzle and delicate mouth parts allow it to select the portions of plants it wishes to consume. Thus, while the digestive systems of mule deer are relatively small, it is typically filled with higher quality forage handpicked, or should we say “tongue picked” by the individual deer. The ability of mule deer to select high quality portions of the plants works well for this species until high quality forages species are no longer found within its range. When higher quality forage isn’t available within their range mule deer have difficulties satisfying their nutrition requirements, and sadly the production of the herd declines.
Mule deer in southeastern Utah rely primarily on sagebrush for winter forage. It is high in protein and can help sustain them during the winter. Other highly used shrubs include four-wing saltbush, cliffrose, bitter brush, and mountain mahogany. They also used white stem rubber rabbit brush, and juniper, although forage quality is limited.
Even with deep snow, deer can access shrubs if the snow is not frozen and crusted. These pictures show several deer muzzling through the snow in order to eat. However, frozen and crusted, deep snow can prevent deer from accessing necessary forage. This can occur when snow melts, followed by very cold temperatures which can freeze the melting snow. Cold temperatures compound the problem by increasing the caloric need to produce heat to maintain base metabolism. Deep, crusted snow accompanied by below zero temperatures creates life-threatening conditions for mule deer.
So, what is the Division of Wildlife Resources doing about it? Do we care? While in most cases we prefer to manage wildlife populations by providing suitable habitat and harvesting surplus animals, we do have a policy to address severe conditions. It is called the Emergency Big Game Feeding policy, which provides guidelines on when to feed distressed animals and triggers to initiate action.
To quote the policy:
“Continual supplemental winter feeding of big game is not a part of the Division’s routine management program… big game populations should be maintained under natural conditions and by natural available forage. However,… there are times when unusual weather conditions can create critical times of stress when winter forage becomes extremely limited, unavailable, or animals are forced into areas threatening public safety. Furthermore, we recognize that by providing the proper feed, only during these times of critical stress, the Division may improve the survival of those animals that may have otherwise succumbed to starvation.
“The implementation of widespread feeding, which supports higher population levels than healthy habitat can sustain, is not only prohibitively expensive, but involves serious risks in terms of disease and habitat degradation. Under certain circumstances, supplemental winter feeding can be used as a tool to help accomplish the following, especially in the short-term:
- control big game (primarily elk and deer) damage in agricultural areas, e.g. dairies, feed lots, orchards, until a better long-term solution can be sought;
- promote public safety by drawing animals away from highways and urban areas;
- maintain parent stocks of big game populations; and
- relieve stress on populations in short-term severe emergencies.”
Several triggers have been put in place to determine when to provide supplemental feed. These include: body condition of deer going into the winter and decline throughout the winter. This is determined by measuring fat depth along the xiphoid process of hunter harvested or road-killed deer; availability of shrubs; ability of animals to access forage; and, extreme cold temperatures.
To determine if feeding is necessary, and just to keep tabs on conditions for deer herds in general, the Division monitors these ‘triggers’ weekly. Biologists go into the field to measure snow depth. They monitor temperatures and check for forage availability. Fat measurements are taken from road-killed animals. This information is entered into a spreadsheet and summarized each week. If conditions merit feeding, a plan is implemented.
So, how did deer do in the south Book Cliffs? Quite well, actually. While deep, the snow was not crusted, so deer were able to burrow through the snow to access shrubs. Snow on south-facing slopes on hillsides and in gullies experienced some settling and snow melt. Deer were able to access shrubs in those areas, as these pictures indicate. Finally, temperatures, especially on south-facing talus slopes, were warm enough that much of the snow melted. Most of the deer moved up onto these slopes and found ample forage.
Through diligent monitoring of winter conditions, regional personnel were prepared to address the situation if it became critical. Fortunately, it did not. Temperatures this last week climbed to a point that helped greatly. We will continue to monitor all deer winter ranges throughout the winter. Sportsmen and other wildlife watchers are encouraged to view deer from a distance so they do not increase energy demands on wintering deer.