Scent glands are specialized organs that produce secretions containing volatile chemical compounds (semiochemicals) used for chemical communication, particularly among mammals. In ungulates (deer, elk, and moose), the tarsal glands are a major component of this olfactory system. They function as a personalized chemical signature, playing a central role in social structure and reproductive behaviors.
Anatomical Location and Structure
The tarsal glands are located on the inner surface of the hind legs, positioned directly over the tarsus joint, commonly known as the hock. This gland is externally visible as a distinct tuft of elongated, coarse hair that often appears darker or stained, particularly in mature males during the breeding season. The skin beneath this tuft contains specialized glandular tissues, primarily sebaceous and sudoriferous (sweat) glands, which secrete sebum and other liquids.
The hairs of the tuft are structurally modified with microscopic scales, which significantly increase the surface area for trapping and holding glandular secretions. Underlying the skin are arrector pili muscles, which allow the animal to consciously flare the hair tuft outward, facilitating the rapid dispersal of the accumulated scent into the environment. The gland is defined by the animal’s behavior of “rub-urination,” where it intentionally directs its stream of urine over the tarsal tufts while rubbing the inner sides of its legs together.
This behavior saturates the hair tuft with urine, mixing it with the sebaceous lipids produced by the glands. This constant saturation explains why the hair tuft often develops a dark, stained appearance over time. Even young fawns and does engage in this rub-urination behavior year-round, though it is most pronounced and frequent in bucks during the rutting season.
Role in Chemical Communication
The primary function of the complex scent produced by the tarsal gland is individual recognition. By sniffing the tarsal region of another animal, a deer can determine specific details about the individual, including its sex, age, and overall physical condition.
The characteristic, strong, musky odor associated with the tarsal gland is not solely the product of the sebaceous and sweat secretions alone. Instead, the pungent smell results from a complex biochemical interaction involving the glandular secretions, the urine, and a specialized community of bacteria that colonize the moist, nutrient-rich environment of the hair tuft. These bacteria actively break down the fatty lipids and the urea present in the urine, generating volatile organic compounds.
This microbial decomposition creates a dynamic scent profile that changes based on the animal’s physiological state and social status. For instance, the scent becomes significantly more intense and complex in mature bucks during the breeding season, signaling their dominance and reproductive readiness to other deer. The frequency of rub-urination increases dramatically during this period, reinforcing the chemical signal. The unique bacterial composition on each animal’s tarsal gland ensures that the resulting odor is distinct, allowing for fine-tuned communication within the herd hierarchy.
Distinguishing Tarsal Glands from Other Scent Glands
While the tarsal gland is the most recognized scent organ on the hind leg, it is only one of several glands ungulates use for chemical signaling. It must be differentiated from the metatarsal and interdigital glands, which occupy different anatomical positions and serve separate functions. The metatarsal glands are found lower down on the outside of the hind leg, appearing as a tuft of hair that is often white.
Unlike the tarsal gland, the metatarsal gland in many species of deer, such as the white-tailed deer, does not appear to secrete a strong odor for communication. Some researchers hypothesize that this gland may be involved in thermoregulation, as it is often larger in deer living in colder climates. Its primary role is not centered on social signaling.
The interdigital glands are located between the two toes of each hoof. They secrete a yellowish, cheesy fluid that has an odor often described as rancid or like sour milk. The function of the interdigital gland is trail marking; as the deer walks, the fluid is deposited onto the ground, leaving a physical and chemical marker that allows other deer to follow a path or gauge the direction and timing of travel.
The tarsal gland is distinct because it is the only leg gland whose scent is actively mixed with urine to produce a complex, individually specific social signal. This functional and locational separation highlights the unique importance of the tarsal gland in the animal’s personal and reproductive communication.