As winter wanes, hunters everywhere worry about deer finding quality nutrition before natural foods and agricultural crops become abundant. Here, courtesy of John J. Ozoga’s insights in Deer & Deer Hunting, is some insight into how the whitetail’s amazing digestive system adapts.
Like other ruminants, whitetails possess a four-compartment stomach (rumen, reticulum, omassum and abomasum). This means they can meet their energy needs from nutrients consumed in food plus those synthesized by the microbial bacteria and protozoa that live in their rumen.
The fermentation process, or breakdown of food by the rumen microbes, is the main difference between ruminants and simple-stomached animals. One advantage of such a digestive system is that it allows an animal to digest cellulose and other complex carbohydrates found in browse and other fibrous foods typically consumed by deer in winter.
The fermentation process is especially important to deer when only low-quality food is available, as is commonly the case during winter. However, deer are not super-ruminants — they cannot use some woody browse species as well as cattle can — and have difficulty surviving on highly lignified (woody) foods.
The rate at which deer can digest food depends upon its cellulose content; succulent food being more rapidly broken down than fibrous foods. The very slow rate at which low-quality browse (such as spruce and balsam) passes through the digestive tract explains why deer “starve” with a full stomach.
This means that foods difficult to digest are retained longer in the rumen-reticulum than are easily digested foods, limiting the amount of food a deer can consume. Such a limitation, if prolonged, will lead to severe malnutrition and, ultimately, death from starvation.
This is another benefit of diet diversity, which normally prevents deer from ingesting too much of any one plant compound that inhibits digestion. Healthy deer tend to avoid eating excessive amounts of certain compounds that inhibit the action of rumen microbes. Conversely, starving deer are less likely to exercise such precautionary measures.
Whitetails have one important advantage over simple-stomached animals: The whitetail’s rumen microbes can produce protein. When a deer’s diet lacks high-quality proteins, for example, the microbes can simply create them using whatever amino acids and other nitrogen are available. Whereas simple-stomached animals must consume a sufficient amount of food containing high-quality proteins, a whitetail only has to be concerned with the “quantity” of protein in its diet. The rumen microbes can compensate for deficiencies in protein “quality.”
Whitetails also have a great capacity to conserve nitrogen when protein intake is restricted, by increasing renal reabsorption and recycling of urea (the end product of dietary protein metabolism), thus reducing their nitrogen loss in the urine.
Generally among ruminants, there is a relationship between body size, stomach size and feeding strategy. That is, unlike cattle and sheep, whitetails have a small stomach relative to their body size and require a fairly high-quality diet.
Whitetails’ feeding habits are extremely variable and opportunistic but highly selective. Their diverse feeding habits change with the seasons, allowing them to choose a wide variety of foods, including grasses, sedges, fruits, nuts, forbs and mushrooms, in addition to portions of those shrubs and trees that best meet their nutritional requirements.
Because a whitetail’s diet changes so dramatically with the seasons, it’s also important to note that the whitetail’s digestive tract can change with its diet, but gradually so. The amount of saliva produced, the lining of the rumen and the rumen’s size, for example, change seasonally to compensate for the shift from succulent summer forage to a more-fibrous winter diet, and back again to more luscious foods with spring green-up. However, it takes two to three weeks for the rumen microbes to completely adjust to a new diet.