The water deer is a fast-maturing species, with both sexes attaining puberty early by deer standards, at between four and eight months old. In their Chinese paper to Acta Theriologica during 1984, Helin Sheng and Houji Lu reported that bucks on the Zhoushan Islands reached sexual maturity at five or six months old; does took a little longer to mature, at around eight months. In his chapter on the species in the second edition of The Handbook of British Mammals, published in 1977, Oliver Dansie gave the age at puberty for does as about six months, indicating that both sexes may mature at around the same age. Likewise, in his garden in Needingworth in Cambridgeshire, Raymond Chaplin recorded his five-and-a-half-month-old captive buck successfully mate two does, both also less than six months old - each gave birth the following summer. Additionally, Chaplin observed that the same buck, in adulthood, was relentlessly aggressive towards a juvenile when December arrived, despite the two having previously shared the enclosure without animosity. The latter account, together with the observations by myself and others of mature bucks chasing juveniles, implies that even first winter males are perceived as potential competition by adult males.
Despite reaching sexual maturity in time for their first rut, most bucks stand little chance of breeding until their second winter (but see: Patience is a virtue?), while females seem quite likely to be mated in their first year. Indeed, based on the examination of 85 uteri from deer of all ages, Chaplin concluded that all animals normally bred in their first year and thereafter annually. Similarly, Endi Zhang found that, in May 1993, five of the 15 yearling does in his study at Whipsnade showed signs of late pregnancy.
When young animals reach independence, two options confront them: stay in the area in which they were raised or move somewhere else. Under some circumstances the decision is made for them by their parents or environmental/seasonal factors. Indeed, population density and habitat type combine to dictate dispersal in most species, water deer included. We must also be aware that dispersal is not the same as colonisation, despite being a mechanism for it. Colonisation is the inception of a population in an area, while dispersal refers only to the movement of individuals and implies no such establishment.
Being weaned within five or six months means that well-grown fawns can often be seen accompanying their mothers into autumn, although contact between them is much reduced by this point and some may disperse shortly after this. Indeed, in my experience does may start chasing away offspring from September, sometimes triggering chasing among bucks that can be misinterpreted as early rutting. In his 2019 book Muntjac and Water Deer, Arnold Cooke noted how, on average at Woodwalton Fen in Cambridgeshire, three-quarters of juveniles seemingly dispersed (or died) during their first winter—presumably this is triggered at least in part by competition with adult males during the rut—and, generally, the data collected by Cooke and Lynne Farrell at the fen indicate that any “surplus” young are likely to disperse during the autumn and winter. This may not be the case in all habitats, however, and I have observed fully-grown “tuskless” deer that I have taken, based on their general appearance, to be female, travelling and resting together in groups of two or three on multiple occasions both in captivity and the wild during spring and summer - in two cases, mutual grooming was observed, which suggested animals were familial. Additionally, in parts of Bedfordshire where population densities are high it's common to see large aggregations of deer lying out in fields by day and night, and there are similar observations of loose water deer aggregations resting and feeding in undisturbed areas of the Demilitarized Zone of the Korean peninsula through the summer.
Taken together, the above suggests to me that dispersal can be more of a local displacement or even simply enforcement of inter-personal distance than full-on “upping sticks and leaving home” in some circumstances and/or habitats. Thus, fawns may remain in the vicinity of their birth and/or in a loose association with their mother or sibling(s) until they're at least a year old, possibly for longer if conditions allow. Indeed, it is relatively common to see pairs of well-grown water deer fawns moving together, without any obvious adult affiliation, during the late summer and autumn. Such association is presumably more likely among females (e.g., sisters or mothers and daughters) than bucks, which are more likely to find themselves at odds with mature males in the area, and they're perhaps more likely than does to seek territories elsewhere during the winter and spring, or at least remain in the general area and keep a low profile, depending on the density of territory holding bucks.
Data on roadkill trends for mammals in South Korea suggest that spring and summer is a period of significant fluidity in the water deer population. In their analysis of Korean mammal roadkill data over 15 months between June 2018 and September 2019, for example, Kyungmin Kim and colleagues at the National Institute of Ecology in Seocheon observed peaks for water deer between April and June. In their paper to the Journal of Forest and Environmental Services in 2019, they considered that this reflected yearling dispersal during the spring. Similarly, the National Institute of Ecology Press book Korean Water Deer points to half of all water deer roadkill recorded between May and June being one-year-old bucks and, in their 2021 paper to the International Journal of Environmental Research and Public Health, Minkyung Kim, Hyomin Park and Sangdon Lee suggested that, based on roadkill recorded on Korean expressways between 2004 and 2019, dispersal happens during the spring, although part of their hypothesis is based on roe deer ecology in Spain:
“Born in the previous year, baby deer wander irregularly to find new areas to abandon their family groups and settle in. In the case of Korea, it is also considered that the first-year offspring of water deer, which is the species of the most roadkill occurred, leave their mothers and disperse during the period of May and June.”
Describing the species' colonisation of Cambridgeshire and Peterborough in Muntjac and Water Deer, Cooke illustrates that there is potential to disperse substantial distances over time, but that the colonisation rate seemed relatively low, no more than a kilometre (0.6 miles) per year. We know, that water deer can undertake long range movements, and ten deer imported from Woburn to Studley Royal Park in Yorkshire between 1950 and 1952 provide anecdotal evidence for this. By 1954, only three or four animals remained on the estate, the rest having escaped the park's walls. In 1952, an individual believed to have escaped from the park was found injured near Harrogate, some 14 km (8.7 mi.) away.
Sometimes circumstances enforce migrations on a seasonal timeframe. In China, some populations are forced to make seasonal migrations of several kilometres into hill country, when low-lying grassy deltas flood during the autumn and winter. Similarly, although on a smaller scale, at Woodwalton, Cooke and Farrell have documented water deer forced into floodbanks and farmland in the south of the reserve when the northern section floods during the winter. Equally, in coastal reedbeds, tidal movements may force deer to move on a daily basis, as is observed at Wheatfen Broad in Norfolk, although this is a highly transitory displacement rather than a migration.
We have very few data on the process of dispersal in water deer, and the only study of which I'm aware was that conducted by Xin He at the East China Normal University during his Ph.D. on the spatial behavioural ecology of individuals reintroduced to Shanghai. Between June 2010 and November 2011, He and his colleagues radio-tracked 12 deer released into the Nanhui East Shoal Wildlife Sanctuary in southeast Shanghai during 2010; three males released in June and nine animals, three males and six females, released in October. The data show that bucks dispersed from the release site sooner, travelled for longer and for greater distances than does. Males typically dispersed from the release site between the third and fifth day, moving an average of 785 metres (0.5 miles) over four or five days. Females, by contrast, hung around for eight or nine days, before travelling an average of 462 metres (0.3 miles) in a couple of days. Subadults of both sexes moved shorter distances than adults. The longest dispersal recorded during the study was by a male released during October; he had moved just under four and a half kilometres (2.8 miles) from the release site by late April 2011.
The Nanhui data also show a complex “pulsed” pattern of dispersal, starting and stopping over periods of days, and how all bucks dispersed in different directions, some moving more than a kilometre (0.6 miles) in 24 hours. The suggestion by the researchers, in their 2015 paper to the Italian Journal of Zoology, is that, upon release into a new area, the deer spend some time getting familiar with the environs and food on offer, before dispersing more widely in order to avoid predations.