Among deer in general, while it's always tricky to accurately estimate the age class of females in the field, for males we could often get an idea based on the size and complexity of their antlers, although it's not always a reliable indicator. With water deer, we don't even have antlers to assist with approximating age in bucks, and while the canine size and breadth does tend to be larger in older males, tusks are less conspicuous than antlers and generally require closer observation. Overall, most scientific observers allocate an individual to one of three classes: fawns (0-9 months); yearlings (10-21); and adults (21+ months). Arnold Cooke takes this a little further in his 2019 opus, categorising water deer as: fawns (up to two months, with a spotted coat); first year deer (2-6 months old, around their first rut and sexually mature, but lighter in build than adults); subadults (6-12 months); adults (12+ months).
The only way to get an accurate estimate of age is by closely assessing the teeth. In fawns, the milk dentition is replaced during their first winter and early spring and, hence, the sequence of tooth eruption is a useful tool for estimating the age of juvenile deer. Tusk growth and condition can also help approximate age among bucks. Indeed, in good habitat and with close observation, tusks may be noticeable in bucks as young as five or six months old, although Arnold Cooke and Lynne Farrell, observing bucks in the fenlands of Cambridgeshire, found that they typically couldn't detect tusks until April when the deer were around ten months old. In both sexes, however, tooth wear can give an indication of age, although dental quality and erosion can both be influenced by habitat, diet and population management.
In a 2016 paper to the Journal of Mammalian Evolution, Universität Zürich palaeontologists Kristof Veitschegger and Marcelo Sánchez-Villagra reported the lower jaw tooth eruption sequence as: m1 > m2 > i1 > i2 > i3 > c > m3 > p4 > p3 > p2, based on 12 animals, although they didn't provide age references. (m = molar; i = incisor; c = canine; p = premolar.) Work by Gérard Dubost and colleagues at Branféré Zoological Park in France provided some conflicting information on full molar eruption in this species, but more recent data from a South Korean team, led by Hyejin Seo at the Yonsei University College of Dentistry, has helped clarify the picture.
Based on a sample of 11 animals of both sexes up to 15 months old, killed in traffic accidents in Korea, Seo and her colleagues confirmed that the first molar began erupting in the mandible (lower jaw) at around six weeks (1.5 months) old, and was fully erupted - i.e. the cervical line had emerged from the alveolar bone - at 13 weeks (3.1 months). Overall, the permanent mandibular teeth eruption times were:
- m1 began erupting at 1.5 months and eruption was complete by three months old
- m2 began at five months and was complete by seven months
- p2 and m3 began at 11 months and were complete by 12 and 13 months, respectively
- p3 and p4 each began erupting at 12 months old and were completely erupted a month later
These timings correspond roughly to the durations provided by Dubost and his co-workers in the 2008 paper to Mammalia, in which they also noted how replacement of deciduous (milk) teeth was complete at between 10 and 12 months old.
Interestingly, contrary to most other deer, Seo and her team found that eruption occurred at about the same rate in the top and bottom jaws and the eruption of the deciduous premolars was actually earlier in the top jaw. Ordinarily, we'd expect eruption to be later in the top jaw. Writing in their paper to the Archives of Oral Biology in 2017, the researchers conclude:
“... the deciduous mandibular fourth premolar and the permanent maxillary fourth premolar are key teeth for us to determine whether a water deer is more or less than a year old, when the premolar replacement occurs.”
Understandably, an issue with this technique of ageing is that it can only be done on a dead or heavily sedated animal. Similarly, having ascertained that the individual is an adult, an idea of how old they are is probably only possible via tooth sectioning, although most experienced deer managers will be able to hazard a guess based on the degree of tooth wear.
When a tooth is sectioned, it is often possible to identify alternating light and dark bands in the “inner enamel” (i.e. the inner two-thirds of the enamel that's in contact with the dentine below), known technically as parazones and diazones, respectively. This banding is caused by a change in the arrangement (direction) of the enamel rods. Diazones are arranged parallel to the light source when the tooth is sectioned longitudinally and reflect light away, appearing dark, while parazones reflect the light back and show up more brightly. These are important structural components of the enamel, strengthening the tooth and helping to prevent the spreading of any cracks, and are known as Hunter-Schreger bands (HSBs), named after Scottish anatomist John Hunter and German anatomist Christian Schreger.
In 1953, Japanese anatomist Naobumi Kawai at the Tokyo Medical and Dental University published a paper in the journal Okajimas folia anatomica Japonica describing the HSB thicknesses for a wide variety of different mammals. Based on the sectioning of a molar, Kawai established that the parazone/diazone bands were oriented at 60-70 degrees relative to one another and to be 40-80µm thick, averaging 60 µm. The diazones were each composed of between eight and 14 rods, while parazones had seven or eight. Arnold Cooke and Lynne Farrell, writing in their 1983 booklet on the water deer, note that one band was deposited each year in the molar, but they lacked sufficient animals of a known age to test beyond about four years old.