Muntjac deer, often called “barking deer” due to their distinctive vocalizations, are a small species native to Asia. These solitary animals are known for a peculiar characteristic on their face: a pair of deep slits or “holes” just below the eyes. This unique facial anatomy is not an injury, but a specialized biological adaptation that plays a significant role in their daily lives.
Identifying the “Holes”: The Preorbital Glands
The noticeable depressions on a muntjac’s face are not open holes but are actually specialized, deeply recessed skin pouches known as preorbital glands. These structures are a type of exocrine gland, similar in origin to the lacrimal gland found in humans, but they are dramatically enlarged in muntjacs. The glands are lined with a combination of sebaceous and sudoriferous tissue, which are responsible for producing a waxy or oily secretion. When relaxed, the glands appear as mere folds of skin or dark, slit-like markings just in front of the animal’s eyes.
However, the surrounding facial musculature allows the muntjac to actively open and even evert, or turn inside out, these glands when needed. This allows the glandular tissue to flare outward, making the feature extremely prominent. The resulting secretion is a complex chemical mixture containing pheromones and semiochemical compounds used for communication. This mechanism of opening the glands makes them appear like prominent, gaping pits to an observer.
The Primary Function: Scent Marking and Communication
The primary purpose of the preorbital gland secretion is to serve as a potent chemical signal for communication within the muntjac’s environment. As solitary animals, especially the adult males, they rely heavily on scent marking to establish and maintain their individual territories. The deer will rub the open glands on low-hanging branches, rocks, and other vegetation, effectively leaving a chemical calling card. This marking behavior delineates boundaries and warns rival males of the territory’s occupancy.
The chemical composition of the secretion conveys detailed information to other muntjacs who investigate the scent mark. This includes data about the deer’s sex, age, overall health, and reproductive status. When a male is excited or preparing for breeding, the scent signal can communicate dominance and readiness to mate. Muntjacs will sometimes flare their glands during other marking behaviors, such as when they defecate or urinate, integrating the facial scent with other body odors.
Gland Usage Across Deer Species
While the preorbital gland is most dramatic in the muntjac, it is a structure found in many species across the deer family, Cervidae. Species like the White-tailed deer and Mule deer also possess these glands, but their size is much smaller and less visible. The muntjac’s glands are unusually large and prominent due to its unique, well-developed facial musculature. This musculature enables the dramatic eversion of the gland.
The degree of visibility and use of the preorbital glands often reflects a species’ social structure and habitat. Muntjacs, which are secretive forest dwellers, rely heavily on scent to communicate in dense vegetation where visibility is limited. Other deer species also use different scent glands, such as the tarsal glands on their legs. However, the muntjac’s facial glands are a dominant tool for chemical signaling, reflecting their solitary and territorial nature.