Water Deer Territoriality - Scent Glands

Chinese water deer are rather modestly equipped with scent glands, the structure, function and biochemical activity of which have, much like the species' ability to detect scent, received scant scientific attention. Sweat, preorbital (sometimes also referred to as “suborbital” or “antorbital”), interdigital, and inguinal (groin) glands have been described in this species, but overall, the variety of glands is lower, and those present are less conspicuous, than observed in many other deer species.

Cross-section of skin from a Siberian roe deer (Capreolus pygargus - A) and water deer (B) illustrating the more basic hair follicle pattern and broader blood vessel diameters of the latter. Scale is 200 µm. Figure 2 from Lee, S-J. et al. (2024). Animals. doi: 10.3390/ani14020185 (CC BY). - Credit: Sang-Joon Lee / Animals / MDPI

In common with all mammals, water deer skin is studded with (apocrine) sweat and sebaceous glands, the latter exuding oils, waxes, and various other organic compounds. In their microscopic analysis, published as part of the 1993 compendium Deer of China, Bing Ni, Hanmin Cao and Helin Sheng found sebaceous glands to be ultrastructurally like the general skin glands, and their morphology in the skin of interdigital and preorbital glands was essentially the same but developed to a greater extent. Sebaceous glands were of a simply branched acinar type (i.e., they have many lobes) and the secretory sac, the gland alveolus, a mass of epithelial cells. Cells in the outer layer of the gland become completely differentiated, the nuclei gradually disappear, and the cells finally disintegrate into sebum that is delivered onto hair and the surface of the skin. More recently, in a paper to Animals in 2024, Sang-Joon Lee and colleagues describe how water deer had very large blood vessels, sebaceous glands, and sweat glands in the skin, significantly larger than those found in roe deer (Capreolus capreolus). We don't know the specific composition of this sebum in water deer, but based on studies on other mammals it would be surprising if it didn't carry potential scent markers.

On the feet

Writing in The Mammals of China and Mongolia, in 1940, Glover Allen noted how there was a gland between the digits of the hind feet, but none in the fore feet, a conclusion that German mammologist Theodore Haltenorth also came to in his chapter on the classification of water deer and muntjac in volume eight of the Handbuch der Zoologie. Both Alfred Garrod, in his 1877 description of the anatomy of a still-born fawn, and Richard Lydekker in The Deer of All Lands, published in 1898, suggested otherwise, however. Indeed, subsequent review by Reginal Pocock in 1923, Stefan Stadler in 1991 and Bing Ni and his colleagues in 1993, confirmed the presence of interdigital glands on the front feet, too, albethey significantly reduced by comparison with those on the rear. Garrod noted how the interdigital skin (i.e., that between the cloves of the hoofs) on the fore feet was studded with minute gland openings, but that a similarly studded though much deeper “pocket” that almost completely separated the toes, was present on the hind feet. Lydekker described something similar; the hind feet having deep glands while those on the front feet are small and shallow. In Pocock's words there was a:

“... tolerably deep and smooth glandular depression [on the front feet] ... [while on the rear feet] the glandular depression is considerably deeper and longer ...”

Despite not possessing a hock gland, on two occasions I have observed hock-licking behaviour in water deer. In this case, based on the white hairs interspersed in the coat and "whiskery" appearance of the recipient, I believe this to be a younger doe licking an older one, possibly the mother licked by a daughter. In the second case, two adult females approached and each licked the other's hock region for several seconds - again, based on the size difference, I considered one was older and the pair were possibly mother and daughter. - Credit: Marc Baldwin

Ni and his co-workers described how interdigital glands are in a saccular concave of the skin, between the toes of both fore and hind feet, with both sets of glands having the same broad structure. They contain sebaceous glands that are spread throughout the dermis and have a short duct opening to a hair follicle, and odoriferous sweat glands opening in or near the epidermal orifice of the hair follicle. Presumably the secretion of these glands is the same creamy-white waxy substance seen in other cervids and, given how bucks can often be seen following ground scent trails to locate females, is deposited as the deer walk around or rest.

Many deer species also have scent glands higher up on their hind legs, one on the ankle (metatarsal) and another around the knee joint (tarsal). These hock glands are particularly potent during the rut and seem to be important elements of deer communication; fawns urinate on them, does use their scent to help identify their fawns, and white-tailed deer (Odocoileus virginianus) bucks often sniff at the tarsal glands of does as a prelude to mating. Interestingly, despite other members of the Capreolinae subfamily (e.g., white-tailed and roe deer) having prominent tarsal and metatarsal glands, and my having observed mutual hind leg licking between water deer at Whipsnade, Garrod, Lydekker, Allen and Pocock all note the absence of hock glands in this species.

In front of the eye

The preorbital (or suborbital) gland of a water deer buck killed on a road in Buckinghamshire. This gland is small and we know very little about the composition of its creamy secretion, although it appears to code for the individual. - Credit: Marc Baldwin

In common with all other cervids, water deer possess a small pit just in front of each eye, surrounded by a naked area of skin, known as the preorbital gland. Unlike most other deer, in which this gland is quite prominent and may even be everted during scent marking (e.g., in some muntjac species), however, the preorbital gland of Hydropotes is rather inconspicuous. In their 2022 Russian paper to Зоологический Журнал (Zoological Journal), Pavel Fomenko and his co-workers note that the preorbital gland of the two-year-old doe found dead in Khasansky District of Primorsky krai during February 2020 was 12 mm (0.5 in.) long, and this aligns with gland opening of the three specimens (a doe and two mature bucks) I have examined.

Interestingly, based on the calliper measurements I have taken from four adult deer skulls (two of each sex), the fossa lacrimalis (the shallow depression in the skull next to the eye socket, in which the gland sits) appears slightly smaller in females than males, although I caution against drawing any conclusions based on this small and informally measured sample. The measurements from the bucks, estimated at around two and three years old, respectively, based on tusk size, were (fossa length, height and depth): 12 mm, 7.5 mm, 5 mm (total skull length was 150 mm / 6 in.); and 15 mm, 9 mm, and 5 mm (skull was 170 mm / 6.7 in.). The does both had a skull length of approximately 160 mm (6.3 in.), and each had a depression of around 9 mm, 5.5 mm, and 2.5 mm. Hence, the length of the fossa was 8-9% of the skull length for the bucks, while those of the females were just under 6%. Were these preliminary observations to be borne out more widely, this might imply a difference in the structure/configuration/anatomy of the glands between males and females for the sexes to have glands with equivalent sized openings but different fossa volumes. (For comparison, the fossa of two Reeves' muntjac (Muntiacus reevesi) buck skulls were 17-18% of total skull length.)

During their 1993 study, Bing Ni and his team reported that preorbital glands open directly onto the skin surface and, while the sebaceous glands in this region are less well developed than those in the interdigital sacs, the odoriferous glands are larger. The only biochemical study covering water deer scent glands of which I'm aware was conducted by the late Ruth Lawson as part of her Ph.D. thesis at the University of Southampton, working alongside Rory Putman and Alan Fielding. Two papers emerged from this work, both published in the Journal of Zoology: one in 2000 and the second in 2001. The researchers sampled the preorbital glands of 53 water deer (26 females, 27 males) caught during the winter at Whipsnade Zoo between 1989 and 1992, heating the samples to environmental and body temperature and analysing the volatiles given off using gas chromatography (GC).

The left-hand orbital area of the skull of a water deer buck, showing the preorbital gland depression (lacrimal fossa - green arrow) immediately posterior to the orbit, below the preorbital vacuity (purple arrow). The holes on the posterior margin of the depression at the border with the orbit (orange arrow) are lacrimal foramina, where the lacrimal duct enters. - Credit: Marc Baldwin

The preorbital secretion, which they described as a creamy liquid, was found to be “extremely complex in form” and, across all the samples, they identified a total of 50 volatiles, although in April 2023 Rory Putman told me that the secretion from the water deer was “perhaps the simplest of all the species we looked at”. Each individual sampled produced a unique secretion with an average of nine or 10 volatiles, the range being from one to 21, suggesting a total of 180 trillion different potential scent combinations in this species. Unfortunately, at the time the cost of mass spectrometry prevented them identifying what these volatiles were, although Putman confessed that he'd love to have known.

Two volatile elements in preorbital gland secretion identified by Lawson and her co-workers varied with age (i.e., youngsters either produced less of them than adults, or they were less volatile in juvenile secretions), and one with sex, secretions from bucks yielding a peak on the chromatograph absent in female samples; but neither did so consistently enough to confidently assign an age or sex to the sample. Only 68% of samples could correctly predict the sex of the subject, although juveniles were more likely to be correctly identified than adults. For comparison female muntjac were correctly identified in 92% of cases. While in their 2000 paper, the researchers caution against over interpreting their dataset:

It is also possible that the deer may not be sensitive to the full range of volatiles recovered by gas chromatography and thus, to their perception, the odour profiles may lack the full complexity resolved by chromatography.

A pair of hand-reared water deer resting together cudding (i.e., chewing the cud) in Raymond Chaplin's garden. The female on the left ("Pippa") licks both preorbital glands of her partner ("Max"). These two were always very affectionate towards each other and Pippa's twins, and licking of scent glands may help reinforce bonds. - Credit: Raymond Chaplin

Conversely, it's also possible that the deer may be able to detect compounds that are either not GC-amenable (i.e., have a high molecular weight) or too volatile to be effectively trapped and stored for analysis.

They nonetheless observed clear species-specific volatile profiles in all the species they studied, suggesting that each might be accurately identified from its scent profile alone and, in the case of the water deer, there is strong coding for individuality, if not age and sex. Overall, preorbitals appear to be perhaps the most important of the scent glands, and bucks will often spend several minutes at a time apparently applying scent from them onto vegetation (see: Scent marking). Additionally, Raymond Chapman has video of one of his captive does licking the preorbital glands of her partner while they're lying together chewing cud.

Incertae statūs

Pocock described glandular tissue around the anus (i.e., circumanal glands), but didn't provide any detail on its structure or secretion, and noted how water deer are apparently unique among the cervids owing to both sexes possessing inguinal (groin) glands, which can only be seen when the groin is inspected. Either side of the mammary tissue lies a sickle-shaped patch of almost hairless skin measuring around 45 mm by 35 mm (1.5 x 2 in.). In her 1991 book Deer, Norma Chapman notes that activity occurs all year around in these glands, as indicated by dark, waxy specks of secretion around them that are easily overlooked as dirt. In his review of the even-toed ungulates in the second volume of Social odours in mammals, published in 1985, Morris Gosling described how, in antelope species, these glands tend to be more active in males than females and produce a yellow waxy substance with a pungent odour, but no specifics are given for water deer. Also in the groin, in their 1996 paper to the Journal of Mammalogy, Stewart Odend'hal and colleagues noted modified sebaceous glands around the hairs at the tip of the prepuce (foreskin) on the penis that replaced the sweat glands. The hairs around the tip of the prepuce are also larger than the adjacent surface hairs that do have associated sweat glands. Just inside the preputial canal, there's a slight increase in keratinization of the epithelium, and there are well vascularized and large sebaceous glands within the canal. This construction presumably provides lubrication between the penis and sheath, but may also transfer onto some of the underbelly fur and be deposited in couches while the buck is resting and/or onto the doe during mating.

The groin area of a water deer buck killed by a car, showing the genital area and the hairless patches on either side belying the presence of the inguinal glands. - Credit: James Whitaker

Intriguingly, despite an abundance of reports of head-rubbing behaviour in water deer, no forehead or frontal glands have been described. Chapman points out, however, that water deer may have enlarged sebaceous glands in their forehead that convey scent, as other members of the Capreolinae do. We know from German zoologist Siegmund Schumacher's (1936) observations, for example, that roe deer (Capreolus capreolus) have concentrations of sudoriferous (sweat) and sebaceous glands on their forehead; the former being most well-developed before, and the latter during, the rut. Similarly, William Quay and Dietland Müller-Schwarze, in their 1970 Journal of Mammalogy paper, report sudoriferous glands in forehead skin of black-tailed deer (Odocoileus hemionus) that were of similar size to those found around the edges of the preorbital sacs and the tarsal glands, although the hair configuration was different. The forehead skin consisted of a thick dermal reticular layer, composed mostly of densely interwoven large collagen fibres. Additionally, Thomas Atkeson and Larry Marchinton describe the forehead skin of both sexes of white-tailed deer (Odocoileus virginianus) having “large numbers of tubular apocrine sudoriferous glands relative to the integument in general” in their 1982 paper to the same journal; those in males were very active during the autumn rut, and most active in dominant bucks.

Finally, with reference to forehead scent, in his letter to Robert Swinhoe in 1872, Howard B. Russell, son of the eminent war correspondent Sir William Howard Russell, described two specimens of water deer he had purchased from a Shanghai market during the winter, on which he noted that the hair on the crown was “short, thick, and close, a yellow scurfy [scaly] substance being often abundant at the roots of the hair”. I know of no other descriptions or analysis of this substance.