CRITICAL MONTHS FOR DEER SURVIVAL

Written by: Dick Henry

Aside from the fall hunting season when half a million deer hunters are afield in New York State, the most critical months for deer survival are in late winter, especially if weather conditions have been harsh for an extended period of time.



Deer rely heavily on their fat reserves to survive in northern climates. They are ruminants, similar to cows, and possess a four chambered stomach that progressively breaks down food as it passes from one chamber to the next. During the warmer months a deer’s diet consists of green, succulent vegetation and that allows them to build up fat levels in their bodies. However during the winter that food source is no longer  exists, and deer must rely on natural browse from trees and shrubs. Their winter diet has considerably less nutrition, and accordingly deer will then be forced to rely on their stored fat to survive. The quality of winter habitat varies considerably, and unfortunately much of our winter habitat in southeastern NY is mature forest and/or over-browsed landscape.

Deer store fat in three general locations in their body. The subcutaneous fat, found under the skin is utilized first, followed by the mesentery fat found in the gut cavity. As a last resource fat stored in the bone marrow is used for survival.

The femur, or upper back leg bone is one of the last sources of energy for a starving deer.   It is the leg bone that  attaches to the pelvis and is surrounded by meat tissue and blood vessels that serve to draw off the stored fat.  Pictured below are samples of deer femur fat levels in winter.  As stored femur fat is utilized for winter survival, the bone marrow changes from the normally solid white fat to a red gelatinous condition. Healthy deer going into the winter period will have solid, white marrow as shown in the photo below.  As fat is drawn off and utilized for survival the marrow changes color. When the marrow becomes reddish in color, it is a result of the fat having been heavily utilized by the deer for energy and survival. Once the marrow turns reddish and the last remaining stored fat is utilized,  it is pretty much “game-over” for the starving deer.

Samples of femur fat depicting the levels of fat remaining.


How quickly are fat reserves burned during the winter?

It depends on a number of factors related to deer mobility, wind and cold temperatures and the overall severity of the winter conditions. Snow depth is very critical, especially if deer are forced to wallow through deep snow. Long-lasting deep snows can deplete fat reserves at a much faster rate, especially when deer are forced to drag their bellies through the snow for extended periods of time. Low temperatures and wind will also have a draining effect on fat reserves.

Typically young-of-the-year fawns are the first to perish from winter-kill, and that is because of their smaller body mass and the fact that  they never achieve the same level of fat depositions that adult deer have. The old, injured and crippled are typically next to succumb because of locomotion limitations. Surprisingly yearling bucks are often among the next group to perish because they have a  smaller overall body size and smaller amounts of stored fat as a result of having participated in the fall rut.  In severe winters, prime age does are the last group to succumb  from winter starvation. When the herd suffers wide scale losses in all age classes in really hard and long  winters, the deer population will be dramatically reduced in the upcoming  summer and fall. If starvation conditions occur for any measurable length of time it is critical for deer biologists to be in the field evaluating the impacts of winter. It is not necessary to specifically count the number dead deer but it is very crucial to know what age classes are being impacted by winter-kill in order for deer management permit quotas to be adjusted accordingly.

There are also other times of the year when deer experience increased levels of mortality from predation and roadkill, but none as long lasting or dramatic as the impacts from winter-kill. In our area a handful of past winters stand out as really severe winters for deer survival. Specifically the winters of 1976-77, 1987-88, 1992-93 and 2003-04 had extensive winter mortality.

If you should encounter a dead deer this spring while turkey hunting or trout fishing, you can perform a simple field check to determine if the deer died from starvation. Simply cut through the meat and tissues on the outside of the hind quarter and expose the upper back leg bone that attaches to hip.  Crack the that bone with a rock or small hand tool and note the color of the internal bone marrow.  If it’s deep reddish in color, it is in a good indicator that the deer that died from natural winter mortality.



 Examine marrow from the femur(upper leg bone).

In spite of  all the other forms of non-hunting deer mortality, winter kill remains as the most potentially significant cause of  deer losses. After all of the losses throughout the year from predation, vehicle collisions and hunting season, winter mortality will reduce a deer population to its lowest level at any time throughout the year.

Inspite of the numbers of  deer we take during the hunting season,  how many deer  the predators remove, or how many we hit with our motor vehicles, the impact of Mother Nature are always be  “the low hole in the bucket”.