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, where the head is raised and lowered while trying to assess a threat, and they will also 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 have observed both LED torchlight (wavelength ~400-700 nm) and trailcamera infrared light (850 nm) reflected by the tapetum.

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, the Swedish zooarchaeologist and anatomist Ludwig Rütimeyer noted that water deer possessed the largest bony auditory sacs of any deer species with which he was familiar, suggesting a high degree of auditory sensitivity. Subsequently, in 1938, Glover Allen described the auditory bullae -- the bony capsules enclosing the middle ear and associated structures -- as "unusually large and inflated". It's generally accepted that an increased volume of the middle-ear cavities is associated with enhanced sensitivity to low-frequency sounds; however, to my knowledge, no behavioural studies have directly assessed auditory sensitivity in water deer.

Anecdotal evidence nevertheless suggests that their hearing extends into the mid- to high-frequency range. In a 1953 article in Country Life, Kenneth Whitehead described a hand-reared doe that accompanied her owner on walks in open countryside and could be readily recalled, the owner having "only to whistle to bring her bounding back to him". As most whistles produced by untrained humans typically fall within the 500 Hz to 5 kHz range -- often between 2 and 4 kHz -- this observation, together with the known frequencies of water deer vocalisations, provides circumstantial evidence for effective hearing across these frequencies.

In addition, water deer possess large, highly mobile pinnae that can be rotated independently through approximately 170 degrees. This mobility is likely to facilitate both sound capture and the localisation of sound sources. I have personally observed individuals rotating both ears backwards while fleeing from a disturbance, behaviour perhaps consistent with monitoring for potential pursuit.

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. Dieter Kruska, in his 1970 German paper to Zeitschrift für Säugetierkunde, included Mangold-Wirz's value alongside other studies from other species allowing for some approximate comparison. In this example, the water deer brain represented 0.47% of body weight (BW), which was higher than for red (0.34% BW) and fallow (0.38% BW) but lower than in muntjac (0.79% BW) or roe (0.56% BW). 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, together with the presence of specialised scent glands, indicate that olfactory cues play an important role for water deer, particularly males. This is consistent with the species possessing a highly developed sense of smell and something that Arnold Cooke and Lynne Farrell imply in their 1983 booklet:

"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 frequently observed water deer extending their necks in this manner to elevate the head while engaging in scenting behaviour, both while standing and lying down - extension may continue to the point where the head is nearly vertical elevating the nose to its highest possible position. In addition, they commonly lick their nose during sniffing, particularly when following scent trails; this behaviour likely facilitates the dissolution of odorant molecules and may enhance olfactory sensitivity. Additionally, in August 2023, Sharon Scott recorded a lone adult doe on a trail camera exhibiting chewing and licking movements that appeared to be associated with scent investigation, potentially indicating activation of 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. The VNO's primary function appears to be the detection of non-volatile chemicals, particularly the steroids found in pheromones, hence its use is primarily associated with rutting behaviour in deer (see also: Deer Overview - Senses - Smell and Reproduction - Courtship Behaviour).

Olfaction is a critical sense for detecting potential threats and identifying novel objects within the environment. Both Sharon and I have observed water deer closely investigating remote camera units through prolonged sniffing, sometimes from distances of several metres. Of the 13 instances in which deer were recorded sniffing camera equipment at the time of writing, eight (62%) involved water deer, three (23%) muntjac, and two (15%) roe deer. This distribution is notable given that water deer and roe deer were recorded by the cameras at broadly similar frequencies, while muntjac were captured more frequently than either of the other species. These observations suggest either a particularly strong reliance on olfaction in water deer, or that water deer are more attuned to habitat changes than roe or muntjac, which seems less likely.

Olfaction also plays a significant role in foraging behaviour. Cooke, in Muntjac and Water Deer, reported that in an analysis of 426 trail-camera recordings of feeding behaviour, sniffing occurred twice as frequently as browsing in water deer, whereas in muntjac browsing was six times more common than sniffing.

Within the nasal cavity lies a complex system of scroll-like bones known as turbinates, which function to increase the surface area of the olfactory epithelium. This epithelium is responsible both for conditioning inhaled air and for binding odorant molecules (i.e., moistening the air and catching any scents). Based on my observations, the turbinate folding in water deer is broadly similar to that of Reeves' muntjac (Muntiacus reevesi), but is less elaborate than in roe deer (Capreolus capreolus). At present, there appear to be no published data quantifying the total area of olfactory epithelium in water deer, nor the proportion devoted to olfactory detection as opposed to thermoregulation and humidification of inspired air. Geist has also noted the apparent absence of a vomer bone, the slender median element that separates the left and right nasal cavities in most vertebrates. This observation is difficult to reconcile with the specimens I've examined, however, which clearly exhibit a vomeral crest and a perpendicular ethmoidal bone (see above). In humans, the vomer appears to serve a primarily structural role, but may also facilitate smooth airflow through the nasal passages, thereby influencing taste, olfaction, and speech. If the vomer is indeed absent or reduced in some water deer specimens (?), it's conceivable that this condition could have implications for odour detection, although this remains speculative.

Olfactory capacity also appears to be associated with the structure of the cribriform plate, a thin bony partition separating the nasal cavity from the brain. This "plate" is perforated by numerous small foramina (holes) through which the olfactory nerves pass, and it's generally assumed that a greater number or larger size of these openings corresponds to increased neural investment in olfaction. Based on my observations, the cribriform plate of the water deer exhibits a moderate number of foramina, comparable to those observed in the mesocarnivore skulls I have examined (e.g., red fox, European badger, and domestic cat). This lends some support to behavioural observations suggesting a well-developed sense of smell, although this relationship has not, to my knowledge, been formally quantified. The nasal cavities of water deer possess substantially fewer turbinate folds than those of the carnivores with which I am familiar, however, hinting at a reduced epithelial surface area for olfactory reception and, consequently, a likely overall reduction in olfactory sensitivity.