The role of skin glands in deer odor communication
June 01, 2010
White-tailed deer are considered to be an individualistic or solitary species, but they are not anti-social. They live much of their life in dense brush or forest cover where communication via visual signs and vocalizations is difficult and often not very effective. Instead, whitetails rely more upon glandular secretions and scent-marking -- referred to as chemical signals or pheromones -- to convey information of social significance.
Chemical signals serve especially well in dense cover, because they can be left on objects in the environment, remain functional in the maker's absence, and are long-lasting.
More scent gland research has probably been conducted with black-tailed deer and mule deer than with whitetails. And although there are many similarities, there are also a number of differences in glandular activity among the three species.
Probably one of the best popular coverage of scent glands in deer is that written by University of Georgia deer researchers Karl Miller and Larry Marchinton, and included in the Stackpole book entitled Deer of the World-Their Evolution, Behavior, and Ecology by Valerius Geist. Much of what follows comes from that text and from more recent research conducted by them and their students.
Here are a few things researchers know, thing they know, and admittedly don't know about whitetail skin glands and their role in the olfactory vocabulary of white-tailed deer.
Odor communication in deer may involve specialized skin glands, urine, vaginal secretions, saliva, and probably some other things we humans don't fully understand. However, skin glands appear to be the primary source of socially important odors among whitetails.
Researchers have identified eight areas of the deer's body with specialized glands that might produce odors of behavioral importance: the forehead, preorbital, and nasal glands on the head; the interdigital, tarsal, and metatarsal glands on the legs; the caudal and preputial areas.
There are two basic types of skin glands: sebaceous and sudoriferous.
Sebaceous glands are usually associated with a hair follicle.
These glands secrete an oily or fatty material. Although the secretions do not produce an odor themselves, they serve to hold other materials that are important scent producers and are easily transferred to objects in the environment.
There are two types of sudoriferous glandular tissue: eccrine and apocrine.
In humans, eccrine glands are responsible for sweating. However, they are not very important in deer.
In contrast, aprocrine glands can produce odors. More importantly, these secretions can be modified by microorganisms to produce distinctive scents.
Some skin glands are associated with areas of special hair that aid in odor transmission. According to Miller and Marchinton, "these hairs can control air circulation to the skin surface, increase the area for evaporation of odors, provide a substrate for the growth of odor-producing bacteria, and serve as a brush for transferring materials to objects in the environment."
The Forehead Gland
Any astute deer hunter knows that the forehead hair of mature bucks differs in color and texture from that of does during the rut, and even during winter after antlers have been cast Microscopic examination of the forehead skin of whitetails reveals a concentration of special sudoriferous glandular tissues, in both males and females. During summer, these glands are quite inactive, but become very active, especially in dominant males, during the breeding season.
Thomas Atkeson and Marchinton found that forehead gland activity increased with buck age and social rank. Generally, older, dominate bucks exhibited greater forehead glandular activity and made more rubs, as compared to younger, subordinate individuals. In the absence of older bucks, my research showed that mature bucks made about twice as many rubs as compared to yearling bucks.
Researchers have identified 57 different volatile compounds associated with buck forehead hair. The tremendous individual variation in occurrence and concentration of these various compounds tend to support the idea that individual animals emit specific odors. This would provide a mechanism by which a dominant buck could be identified by scent left on his signposts.
Since most rubbing is done with the basal portion of the antler and forehead, it's rather obvious that rubs are marked with secretions from the maker's forehead glands. As a result, these highly visible signposts probably carry the maker's distinctive odors and convey information of social significance.
Since these glands are most active in dominant males during the rut, researchers believe the forehead secretions left on signposts permit a buck to signal his dominance and to maintain a high position on the social hierarchy scale.
The Preorbital Gland
The preorbital gland, or lachrymal duct, is a shallow hairless pocket that opens in the front corner of the eye. According to Miller and Marchinton, "the preorbital gland is under muscular control; bucks commonly flare theirs open during dominance displays, does often do so while their fawns are nursing. Inside the preorbital sac there is often an accumulation of material that likely consists of dead skin cells, drainage from the eye, and foreign matter."
Since deer readily react to the odor from this gland, researchers speculate that bacterial action in the preorbital sac produces a scent that conveys some unknown information.
Bucks are known to scent-mark overhanging branches year-round, but the manner and intensity of marking varies seasonally. Some of these sites develop into active scrapes during the breeding season. But keep in mind, in the absence of mature bucks, yearling bucks only make about 15 percent as many scrapes as mature individuals -- indicating that buck age and experience are important factors in scrape-making.
While many hunters believe the preorbital gland is the primary source of scent used in this branch marking, the forehead gland, nasal gland, and saliva may also be involved. Whatever the ingredient, glandular secretions associated with scraping behavior apparently increase with buck age.
Nasal glands are small oval shaped cavities just inside the nostril. Secretions from these glands consist primarily of white, fatty material.
Researchers initially postulated that secretions from the nasal gland might be expelled while snorting and serve a behavioral purpose. However, since this material has very low volatility, and probably would not aerosol well, it's questionable if it could effectively serve in such a manner.
That doesn't mean that nasal gland secretions don't serve some function in signposting, especially of overhanging branches. We just don't know.
Interdigital glands are saclike folds that occur between the toes on all four feet of deer. They tend to enlarge as the deer ages. In whitetails, these glands consist primarily of enlarged sebaceous gland tissue that produces a yellowish, waxy material called sebum.
Although the subject has not been intensively investigated, preliminary work by Atkeson revealed that interdigital glands produce a variety of compounds that evaporate at different rates. This means the odor left in a deer track would change over time, allowing one deer to effectively track another in the proper direction.
According to Leonard Lee Rue III, these glands are functional in fawns less than 2 weeks old, permitting a doe to track and find her fawn if it had moved since the mother's last visit. During our studies at the Cusino Wildlife Research Station, located in Upper Michigan, we observed that a 2 week old radio-tagged fawn, when pursued, would not leave its mothers fawn-rearing territory. Presumably scent-marking by the mother of the territorial boundaries was involved. Whether it involved interdigital scent or some other (possibly urine) is unknown.
The Metatarsal Gland
The metatarsal gland is found on the outside of a deer's hind foot. It consists of a central, oval patch of cornified, hairless skin surrounded by an oval tuft of white hair. The gland tends to be much larger in mule deer and
black-tailed deer than it is in whitetails. In whitetails, the gland is larger in cold climates than it is in southern areas, supporting speculation that it has a sensory function, such as circulatory thermoregulation. Interestingly, the gland is completely absent in some whitetail subspecies in Central and South America.
The metatarsal gland of black-tailed deer is known to produce a "garlic like" odor that serves as an alarm signal. However, no such response has been observed in whitetails. In fact, some investigators suggest that the gland has no specific function in whitetails.
The Tarsal Gland
The tarsal gland, located on the inside of the hind leg at the tarsal joint, consists of a tuft of hair with sebaceous and sudoriferous gland tissues beneath. The primary source of odor comes from urine deposited on the tarsal tuft. But the exact compounds producing tarsal odors in whitetails have not yet been identified. Muscles in the skin beneath the tarsal tuft also permit deer to flare the tarsal gland to release scent in response to injury or some stressful situation.
Whitetails of both sexes and all ages frequently urinate onto their tarsal glands and sometimes rub them together while urinating. Sebaceous glands under the tarsal tuft produce a fatty sebum that coats tarsal hairs. During the so-called rub-urination, this sebum captures certain compounds in the urine.
Investigators at the University of Georgia found that female whitetails urinate in the normal crouched position 8 or 9 times per day and rub-urinated on average 1.2 times per day. After rub-urinating the doe invariably licks excess urine from the tarsal.
Bucks also rub-urinate throughout the year. Although they rub-urinate almost exclusively during the rut, they don't lick excess urine from their tarsals during the rut. This accounts for the unusually heavy staining and strong odor of buck tarsals during the breeding season.
Miller and his coworkers identified 63 compounds from the urine of female whitetails and 55 from that of males. Thirty-six occurred in female urine only and 28 were present only in male urine. During the breeding season, 9 compounds were found only in dominant males, whereas 19 compounds were found only in subordinate males.
As a result, they concluded the following: "Some of these compounds are found only in the urine of the dominant males during the breeding season. Thus there must be differences in the odor of urine among bucks, differences that reflect each animal's dominance and reproductive status. These odors are transferred to the tarsal gland during rub-urination and, along with odors from the bacterial decomposition of other urinary compounds, contribute to the particular odor of a rutting buck."
Hence, the tarsal gland is probably the whitetail's most important gland. Given the available evidence, whitetails can obtain information on individual identity, dominance position, physical condition, and reproductive status based upon odors emanating from the tarsal tuft.
The Preputial Gland
The preputial gland is located on the end of the buck's penis sheath. It consists of enlarged sebaceous gland tissue and long hairs that extend beyond the penis sheath. As in the case of tarsal hairs, these penal hairs become coated with fatty material.
It's possible secretions from this gland carry odors as a buck urinates, maybe in conjunction with tarsal odors while rub-urinating, especially at scrape sites. However, the exact behavioral function of this gland is unknown.
The Caudal Gland
Caudal glands consist of enlarged sebaceous and sudoriferous glandular tissues located at the base of the tail. These glands have only been identified in black-tailed and mule deer. It is unknown if whitetails have these glands. Blacktail fawns reportedly produce an odor from these glands when excited, as do adult males.
This is intriguing, but exceedingly complex stuff. Certainly, odor communication is an important part of whitetail social behavior. However, it appears we are a long ways from fully understanding precisely what chemicals are involved in the whitetails' olfactory vocabulary, where they come from, or how deer react to them.
As noted by Miller and Marchinton, "Even with sophisticated equipment, we are unable to get inside a deer's head to determine exactly what type of information are being relayed by these scents."