Forehead Glands in White-Tailed Deer

1. What they did

In the fall, male white-tailed deer (bucks) rub their antlers on saplings, leaving visible marks called “antler rubs”. Rubbing was long thought to serve only to clean and polish the antlers or as practice for fighting, but rubs keep being made right through the breeding season (the rut), and deer are sometimes seen sniffing and licking them. Earlier work showed that scent is left on rubs, but nobody had found where that scent comes from. Roe deer and muntjac have a scent organ on the forehead; white-tailed deer have no visible one, but the authors’ field observations suggested the forehead is the source.

They set out to describe the structure of the forehead skin and to measure how the activity of its glands changes with season, sex and age:

• Biopsies. From their captive herd of semitame deer, they took a small strip of forehead skin (3.0 by 0.5 cm) from four adult females and four adult males in July, when deer are not breeding, and again from the same animals at the height of the rut in November.
• Wild deer. They took skin samples from 11 live or freshly killed wild deer of various ages, from the forehead, the side of the face, the neck behind the ears, and the back.
• Microscope work. The skin was stained and examined under a microscope without the observer knowing which animal each slide came from. Active sweat glands have a thicker lining and small projections on their cells, so the authors measured the thickness of the lining and counted the proportion of active glands.

2. What they found

The forehead skin

• The forehead had many large hairs, which gave the male’s head a distinctive look in winter coat. The skin was thickened, especially in males during the rut.
• The oil (sebaceous) glands were no different from those on the rest of the body.
• The forehead had many large, well-developed apocrine sweat (sudoriferous) glands, more than elsewhere on the head and neck, though fewer than in the tarsal and metatarsal glands on the legs.

Seasons and sexes (Table 1)

• In summer, the glands of both sexes showed little activity: in seven of the eight deer, 2% to 16% of glands were active.
• In the rut, gland activity rose in both sexes. In females, 8% to 60% of glands were active; in males, 81% to 100%.
• The gland lining roughly doubled or tripled in thickness in males during the rut (26.3 to 37.7 microns, against 14.5 to 17.3 in females).
• Males’ glands were more active than females’ in both summer and rut. Much of the difference between the sexes came from the strong increase in males during the rut.

Age and social rank

• Male fawns showed no gland activity in the fall; their glands were too undeveloped to be scored like those of adults, so ages were not compared statistically.
• Each of the two pens was dominated in the fall by an old male (5+ years), and both had the highest gland development.
• A 2.5-year-old hunter-killed male had glands as active as the old dominant males. He came from a heavily hunted population, so the authors assume he was breeding and likely dominant.

How bucks make rubs

Bucks lick and nuzzle a small tree trunk, then rub it up and down hard, mostly with the base of the antlers and the forehead itself. The authors saw none of the deliberate use of the pre-orbital glands (in front of the eyes) seen in species such as Eld’s deer and muntjac.

3. What they concluded

• The forehead of the white-tailed deer is a scent organ, used to mark antler rubs made by males during the rut. Three lines of evidence support this: the way bucks rub with their foreheads; the larger number and development of apocrine sweat glands on the forehead; and the rise in gland activity in the rut, much greater in males than in females.
• Many people assume the scent on rubs comes from the pre-orbital glands; the authors’ observations contradict this.
• The seasonal change fits the hormone cycle: male hormones (androgens) are low outside the breeding season, rise in the fall as bucks shed antler velvet and start rubbing, and peak at the rut, when the forehead glands are most active.
• Older males, and probably those of higher social rank, have more active forehead glands, although age and rank are linked. An adult male’s place in the social hierarchy appears to be reflected in his gland activity.
• How hormones link social rank to gland development remains unclear, because dominance is not tied to androgen levels in many species.
• The forehead hair of mature bucks differs so much in color and texture from that of does that, during the rut, a perceptive observer can tell the sex from the forehead alone.

6. Other technologies, skills or innovations

• Skin biopsies from anesthetized captive deer, repeated in two seasons on the same animals.
• Standard histology (hematoxylin and eosin staining) and measurement with an ocular micrometer.
• Blind scoring: slides were numbered so the observer did not know which animal they came from.