Marrow Core Analysis

Photo by Susan Morse.

Thirty years ago, I discovered the carcass of a handsome buck draped awkwardly over sharp-edged rocks. It was May and the grip of winter had loosened. Bright new leaves framed the buck’s face, and trout lilies adorned a warming ground that had recently been covered with snow.

Dozens of deer had endured the winter in a nearby deeryard. While some had survived their months-long ordeal, others had not. How had this buck died? His splendid symmetrical rack had not been cast, which usually occurs by mid-winter. This provided a rough timeframe of death, but no clear cause. The buck’s carcass was too decomposed to determine whether the death was the result of predation or malnutrition.

I decided to examine his upper leg bone “marrow core” to assess what his body condition was at the time of death, a field technique developed by C. W. Severinghaus, E. L. Cheatum, and colleagues of New York State’s Department of Environmental Conservation. Severinghaus had noted that the femur bone marrow is almost solid white with fat when a deer is in top condition. If the deer was malnourished, however, the appearance of its femur marrow is typically red or sometimes yellow and is always gelatinous in consistency. The biologists observed that marrow fat is the last reserve of fat to be depleted in nutritionally stressed deer, and that the progressive reddening color, coupled with a reduction in the waxy feel of the marrow, corresponds to its diminishing fat content. When energy-rich fat deposits under the skin and around the viscera have been exhausted, a deer utilizes marrow fat instead. Such animals catabolize their bodies’ protein and fat reserves to a point of no return.

They chose the upper leg bone for marrow analysis because it enables field researchers to easily extract and examine a relatively large marrow core and because the femur bone more consistently illustrates nutritional decline due to its abundant blood supply. Though it can be an unpleasant task, all one has to do is to expose and cut with a saw the hind leg bone, which attaches to the pelvis, and examine a crosssection of the middle portion of the bone. Lacking a saw, two sharp cracks with the back of a stout knife blade will break the bone. Strike the bone hard in two places roughly 2 inches apart. The fractured bone covering can then be lifted away, revealing the intact marrow core beneath.

My grand buck had been a mature dominate breeder and undoubtedly sired many offspring. Yet his marrow revealed that his health had been severely compromised and that he wasted away before dying. His marrow was watery feeling and dark yellow in color, suggesting that he had been both malnourished and anemic when he died. Serious purulent infections interfere with normal blood-forming functions, causing anemia, which is why his marrow was yellow in color. This proved that the death had not been quick. Perhaps the strain of the rut, coupled with inadequate consumption of nutrient-rich foods had been his downfall. Perhaps he had been wounded by a hunter, or a predator, or in a fight with another buck, an injury that prevented adequate nourishment. In either case, this rut-depleted buck had consumed the very last of his fat, the fat in his bones.

The marrow core analysis technique was developed when many deer herds throughout the Northeast were overpopulated and exceeded the carrying capacity of their habitat. Population crashes were inevitable and hard winters were there to oblige. Winterkill survey biologists and volunteers counted casualties by the thousands. These were also the days before artificial feeding had been banned, and supplemental feeding was commonplace in many areas. Since the microorganisms in a deer’s rumen are adapted to convert natural forage and not artificial food, deer are often unable to digest artificial feed mixes in the winter. This can lead to rumenitis, colon impaction, and acidosis. Deer can also expend lethal amounts of energy moving from feeder to shelter, while increasing their risk of being chased by dogs or killed by cars.

Today, despite laws forbidding it, feeding deer still remains a problem in some areas. And even where humans are doing everything right, winterkill can take a heavy toll on the deer herd. According to Lee Kantar, Maine’s deer and moose biologist, even in good deer wintering habitats, herd losses can range from less than 3 percent in mild years, to more than 35 percent when winters are long and snowy. Jim Farquhar, of New York State’s Department of Environmental Conservation, uses every opportunity to involve deer enthusiasts in winter kill surveys and marrow core analysis. He points out that while public participation is helpful in generating mortality data, it also provides a powerful opportunity for the public to see what really goes on out in the deer woods during a hard winter.

The good news is that many naturalists and biologists are noticing fewer carcasses today than they did in the 60s and 70s. Part of this can be attributed to an increase in scavengers: Coyotes, foxes, fishers, martens, bobcats, eagles, ravens and others consume and scatter carcass remains pretty quickly. But there’s some indication that populations aren’t rising and then plummeting as dramatically as they once did.

There have also been great strides in how we manage our forests for deer yards. Will Staats, wildlife biologist for the New Hampshire Fish and Game Department, explains: “We work closely with land owners, timber owners, and foresters seeking to positively influence timber harvesting practices within major deer wintering areas. We even work hand-in-hand with these folks, helping to mark timber and carefully work towards deer winter habitat improvements, including increased tree size diversity and enhanced recruitment of preferred softwood cover and browse species over time.”

Shawn Haskell, wildlife biologist with the Vermont Department of Fish and Wildlife, is convinced that improved overall herd health in Vermont results in fewer dying animals. He explained that recent data “revealed that yearling fall weights are the best they’ve been since the 40s.” Being bigger is a decided advantage when forced through months of bitter cold with little quality food available. Larger animals with more mass retain heat more efficiently, and hence can conserve vital energy stores better than smaller ones. One “hog fat doe” New Hampshire biologist Helenette Silver studied in the 1960s lost 38 percent of her peak autumn weight by winter’s end, yet she survived.

Photos by Susan Morse.


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