Winter Mortality and White-tail Deer

WinterDeerHere in the Catskills we are once again having one of the more severe winters that we have seen in a number of years. Heavy snowfall accumulations and bitter cold temperatures should remind us that hard winters are always a possibility in our  northern climate. We have seen almost continuous snowpack since New Years.  While it presents numerous challenges for us, it also complicates the health and survival of our local deer herds.

White-tail deer are unique in that they rely heavily on stored reserves of body fat to survive tough winter conditions similar to what we have recently been experiencing. In order to survive through the harsh portions of winter,  deer will build up and store fat in three primary locations within their bodies to offset the lack of natural food during the peak of winter. OLYMPUS DIGITAL CAMERA

Subcutaneous FatSubcutaneous fat is stored directly under the skin and it easily resorbed and utilized for nutrition. This is the solid white fat that you typically see when skinning a deer. A second type of fat is the mesentery fat that is stored in the gut cavity of deer; and is the fat that you encounter when field dressing a deer. Lastly,  fat is also stored in the bone marrow of some of the larger bones, such as the upper rear leg bones or femur.

Whenever deer are unable to find sufficient natural deer foods, they will draw energy from their stored fat reserves in the order listed above. Subcutaneous fat is more easily resorbed and is utilized first; the mesentery fat within the gut becomes a secondary source of resorbed nutrition. Lastly the fat stored in the bone marrow is  drawn upon when other fat sources dwindle. Unfortunately, once these stored fat reserves have been used up, deer have very limited options for nutrition until new spring growth begins in April. And if spring green-up is late, deer can and will perish as a result of a lack of digestible natural nutrition.

MesentaryFat Adult deer are in many areas are dragging through belly-deep snow, although they are still able to move around. However, last year’s fawns, now about nine months old, are having a considerably more difficult time wallowing  through the snow pack. Expending this much effort uses considerable amounts of energy and will quickly deplete their limited fat reserves. These puffy-faced, hunched over young deer are clearly at risk. As a result of their smaller body size and smaller amounts of stored fat, they have considerably less stored fat to tide them over.

Unfortunately, the snow pack is still with us and any additional significant snowfalls could easily result in the loss of these younger deer. Those of us who can remember the late March blizzard of 1993 will recall that the local deer herd experienced heavy deer losses that continued well into mid April before green-up began. It was no  surprise that deer harvest during the 1993 hunting season declined considerably as a result of late, long snow pack and winterkill.

Fisherman and turkey hunters may encounter dead deer this spring that did not survive the winter of 2014-15. As deer grow progressively weaker from malnutrition, they tend to move downhill in sloped areas and often succumb in gullys and near streams. Typically juvenile deer, less than a year old,  are the first to succumb to winter kill because they never acquire a full complement of body fat. In harsh, drawn-out winters with deep snows older age classes will also succumb if snow pack continues into late spring.Range of marrowfat

If you happen to find the remains of a dead deer in the spring and wish confirm winterkill as the cause of death, it is relatively easy to do. Use a knife to cut through the muscle tissue surrounding the the upper back leg bone (femur) that attaches to the pelvis. Smash the leg bone with a rock or other blunt instrument and note the color of the marrow inside the bone. Refer to the marrow sample pictures below. Normally the color of femur marrow is white. As the last of the fat reserves are utilized by the deer, the marrow changes to a pinkish color and then to a red color and ultimately will have the appearance of red jello. Once their marrow fat reserves drop to about 25% level as shown below, they will in all liklihood not recover.  

Interestingly, fawns typically die first because of their smaller body size and limited amounts of stored fat. Yearling bucks tend to be the next group to die, followed by adult males and females in the toughest of years. When late snow pack gives way to spring green-up, it heralds a return to succulent summer feed and body growth.

Such is a critical part of the cycle of white-tail deer survival.

Mother Nature can be a cruel and heartless mistress.starvemarrow

One Comment

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  1. Jim Zmiyarch 20. Mar, 2014 at 9:16 pm #

    Thank you for this article very informative as always. It is sad to see animals die this way but it is mother natures way of controlling the heard. I have been to wintering yard in the adirondacks in late spring only to see first hand what winter leaves behind in a deer yard. Thanks again Jim

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