Deer often appear in a perpetual state of chewing, a behavior commonly observed whether they are actively foraging or simply resting. This constant jaw movement is not a random habit but a precise biological function, deeply rooted in their specialized digestive system and the type of diet they consume. This behavior is essential for deer to extract nutrients from their fibrous plant diet.
Understanding Deer as Ruminants
Deer are classified as ruminants, a group of mammals distinguished by their unique multi-chambered stomach. This specialized digestive system is adapted for processing tough plant matter, which is rich in cellulose but difficult to break down. Their stomach consists of four distinct compartments: the rumen, reticulum, omasum, and abomasum, each playing a specific role in digestion.
When a deer first consumes vegetation, it chews the food minimally before swallowing it quickly. This partially chewed material, a bolus, travels to the rumen, the largest chamber. The rumen acts as a fermentation vat, housing a diverse community of microorganisms, including bacteria, protozoa, and fungi. These microbes break down complex carbohydrates, such as cellulose, into simpler compounds and volatile fatty acids, which the deer can then absorb for energy. The reticulum, an extension of the rumen, traps larger food particles and foreign objects, and helps form the cud for regurgitation.
The digestive process in the rumen is highly efficient, allowing deer to consume large quantities of forage rapidly, often in open areas where they might be vulnerable to predators. Once the rumen is full, deer can retreat to a more secluded and safe location to complete the digestive process. This adaptation enables them to maximize their food intake while minimizing their exposure to danger. The fermentation also produces gases, such as methane, which the deer must regularly release.
The Two Phases of Chewing
The chewing behavior observed in deer occurs in two distinct phases, both integral to their digestion. The first phase is the initial, rapid chewing that happens during active feeding. During this stage, deer quickly tear off and ingest plant material, chewing it just enough to form a bolus that can be swallowed. This allows them to gather a significant amount of food in a short period.
The second, more prolonged phase is rumination, or “chewing the cud.” This process involves the deer regurgitating small portions of the partially digested food from the rumen back into their mouth. This cud is then thoroughly re-chewed, mixed with additional saliva, and ground into finer particles. This re-chewing increases the food particles’ surface area, making them more accessible to rumen microbes for nutrient extraction. Deer often engage in rumination for many hours a day while resting. This continuous and rhythmic chewing is what gives the impression that deer are always chewing.
Dental Design for Constant Chewing
A deer’s dental structure is specifically adapted to support their fibrous diet and constant chewing. Unlike many other mammals, deer lack upper front teeth (incisors). Instead, they possess a tough, cartilaginous dental pad on their upper jaw. The lower incisors press against this dental pad, allowing the deer to effectively grasp and shear off vegetation like a pair of shears.
Behind the dental pad and a toothless gap called the diastema, deer have broad, flat molars and premolars with ridged surfaces designed for efficient grinding of tough plant material. The jaw muscles and structure allow for a sweeping, side-to-side motion, which facilitates the grinding action between the upper and lower cheek teeth. This continuous grinding helps to break down the cell walls of plants, releasing digestible contents for microbial processing in the stomach. Their high-crowned design ensures they withstand the abrasive nature of their diet, including silica-rich grasses. This specialized dentition is essential for the mechanical breakdown of their food, complementing the chemical and microbial digestion that occurs in their multi-chambered stomach.