Water Deer Senses

We know frustratingly little about the senses of the water deer. Indeed, I know of no behavioural studies on any aspect of water deer olfaction, vision, or hearing, and only a couple of reports in the literature mentioning their sensory anatomy. Consequently, at the time of writing it is only possible to present some basic observations, based largely on their behaviour.

Writing in Shooting in China in 1908, US consul in Kobe Thomas Jernigan noted how:

“The senses of hearing, sight and smell are by no means acutely developed, or the gunner would never obtain the number of close easy shots he gets at these animals.”

While the “stalkability” of a species is at least partly a reflection of how readily it associates humans with danger, early accounts from Robert Swinhoe suggest that Hydropotes was quite heavily persecuted in China during the late 19^th and early 20^th centuries, and it seems likely they did recognise humans as a threat. In my experience, they can be quick to alarm bark and flee when they detect people moving, which they can at tens of metres. Stationary people, particularly those dressed in muted colours or camouflage, are less readily detected, which might indicate their visual sensitivity is tuned most strongly to movement. Nonetheless, while feeding they rarely have their heads down for long, frequently stopping to look around, suggesting that vision is an important sense for them. Similarly, head bobbing is often employed, especially by does, while trying to assess a threat, and they will stare fixedly at intruders.

Hua Shen and colleagues state, in their 2007 Chinese paper to the Journal of Animal Husbandry and Veterinary Medicine on breeding water deer in captivity, that the species “has good hearing and vision”, although they don't quantify this or provide a reference. Based on the paper, the conclusion appears drawn largely from how difficult the authors found getting close to the deer, and how even small noises caused them to flee.

Water deer have relatively large eyes that, based on my trailcam videos, have a tapetum lucidum, Latin for “bright carpet”, akin to other deer. A tapetum is common in nocturnal and scotopic (i.e., active at dawn and dusk) animals, responsible for the eyeshine seen in flash photos or the reflection of a torch beam or car headlights, and their structure differs significantly among different taxonomic groups of vertebrates. I know of no data on the tapetum of the water deer, but all of the deer species studied to date, including the reindeer (Rangifer tarandus) and roe deer (Capreolus capreolus) to which water deer are closely related, have tapetal structures closely aligned with those of other ungulates. Consequently, the water deer tapetum is likely to be a tapetum fibrosum, a “sandwich” of highly ordered, hexagonally packed collagen fibrils that sit behind the retina and reflect light back into the deer's eye. The “recycling” of light in this way enhances vision in low light conditions.

I'm not aware of any data for any deer species, but the tapetum of domestic cats is believed to improve their night vision by about 44% and, given some of the high-speed pursuits that take place at night in (what is to us) complete darkness or moonlight during the rut, their nocturnal vision must be significantly better than ours. Certainly, I have numerous trailcam and thermal videos of water deer chasing at speed in (to my eyes) near pitch blackness. Through a thermal scope I have seen deer watch others chasing several tens of metres away in (again to me) darkness, moving their heads to keep pace with the action, which again suggests water deer have reasonable night vision.

In his 1880 contribution to the natural history of deer, Swedish zooarchaeologist and anatomist Ludwig Rütimeyer noted how water deer had the largest bony auditory sacs of any deer species with which he was familiar, suggesting that they probably have sensitive hearing. More specifically, in 1938, Glover Allen described the auditory bullae, the bony capsule that encloses the middle ear and a few other structures, as “unusually large and inflated”. We generally consider that increased volume of the middle ear cavities results in increased sensitivity to low frequencies, but I know of no behavioural studies assessing this in water deer. In his 1953 article to Country Life, Kenneth Whitehead mentioned a hand-reared doe who enjoyed being taken for walks in the open countryside, her owner having “only to whistle to bring her bounding back to him”. Given that most whistles produced by “untrained” humans typically fall within the 500 Hz to 5kHz range, often between 2 and 4 kHz, this provides some circumstantial evidence that water deer have reasonable hearing in the mid- to high frequency range.

Katharina Mangold-Wirz, in her 1966 German paper to Acta Anatomica, gave the brain weight of the water deer as around 53 grams (1.9 oz.) and, according to Valerius Geist in his 1998 opus, Deer of the World, the brain mass of water deer is relatively low, at 13.6 g/wtkg^0.56. In their chapter on brain size and natural history in the Biology and Management of the Cervidae compendium, published in 1987, Chris Wemmer and Don Wilson gave the average (mean) brain volume for males and females as virtually the same, around 46 and 47 cubic centimetres (just below 3 cubic inches), respectively, although the range was higher for bucks (41-51 cc) than does (45-48 cc). The authors note that this is slightly smaller, on average, than volumes reported from Reeve's muntjac (Muntiacus reevesi), and just below what might be expected for a deer of their size - i.e., Wemmer and Wilson calculated an encephalization quotient of 0.95 for both sexes. Based on Alfred Garrod's dissection of a stillborn fawn in 1877, and Victor Forbes' study of an adult five years later, in which he mentioned that the cerebral organization approached that of the roe deer (Capreolus capreolus) more than any other species known to him, the brain appears relatively convoluted, although we do not know how much of the brain is given over to any particular sense.

Behavioural observations of sniffing and scent marking, as well as the scent glands water deer possess, suggest that many types of scent are important to these deer, particularly bucks, and this seems consistent with their sense of smell being very well developed. In their 1983 booklet, Arnold Cooke and Lynne Farrell imply a keen sense of smell, writing:

“Wild deer are alerted to human scent up to 200m [650 ft.] away if they are downwind. They will raise their necks and sniff in lungfulls of air attempting to confirm their suspicions. Once certain, they will move off even if they have not seen the person; often when they reach a position where they can no longer detect human scent, they will revert to their previous activity.”

I have seen water deer stretch their necks to elevate their heads while scenting in this manner, and they regularly lick their noses during sniffing, which increases molecular dissolution and thereby enhances sensitivity. In August 2023, Sharon Scott captured a lone adult doe making chewing and licking motions on one of her trail cameras that appeared to be associated with scenting behaviour, possibly engaging the vomeronasal organ. The vomeronasal organ (VNO), sometimes also referred to as the Jacobson's organ after the Danish surgeon who wrote about it in 1813, is a small diamond-shaped bulb of tissue that forms part of the nasopalatine duct running along the roof of the mouth. In most species studied to date, the VNO's primary function is the detection of non-volatile chemicals, particularly the steroids found in pheromones, and its use is primarily associated with rutting behaviour in deer.

Sharon has also observed water deer sniffing her cameras, and this is also the deer species that most often spot and sniff intently at my trail cameras. Of the 11 videos of deer sniffing the cameras from one site in Buckinghamshire, six (54%) are water deer, three (27%) muntjac and two roe (18%), despite water deer and roe being captured at roughly similar rates (each about 8% of videos captured thus far) and muntjac being recorded more frequently than either (13% of videos). Smell is obviously an important sense in feeding, too, as Cooke, in Muntjac and Water Deer, noted that from his analysis of 426 feeding trailcam videos, sniffing was twice as common as browsing, while for muntjac browsing was six times more common than sniffing.

Within the nasal cavity is a network of scroll bones, called turbinals, that serve to increase the surface area of the olfactory epithelium, the layer of tissue responsible for conditioning and “scanning” incoming air for odours. Unfortunately, there don't appear to be any data on the area of the olfactory epithelium, nor the percentage that's given over to registering odours versus regulating the temperature and humidity of the inhaled air. As Geist notes, this species lacks a vomer bone, the small, thin strip of bone separating the left and right nasal cavities that most vertebrates have. In humans, this bone appears to be primarily for support, but also helps smooth the passage of air through the nasal passages, assisting with taste, smell and also speech.

Scenting capacity also appears to be linked to structure in the skull called the cribriform plate; a thin “plate” of bone separating the nasal cavity and the brain. The plate is punctuated with tiny holes through which olfactory nerves pass, and we think that the more holes (or the larger the holes) in the plate the more nerves the animal has dedicated to smell. In my experience, the cribriform plate of the water deer is peppered with a great many holes, similar to that of many carnivores, lending weight to the behavioural observations suggesting a keen sense of smell, but this has never been formally assessed to my knowledge.