When it comes to the deer breeding season (or “rut”), it is often the grandiose displays, relentless roaring/barking/groaning/whistling and violent head-to-head clashing of our larger species that comes to mind. The rut of the Chinese water deer is far less ceremonial, even rather subtle; bar some more determined chasing among the bucks and an increase in chittering vocalisations and squeaking, the rut can easily be overlooked. Indeed, writing in 1950, Kenneth Whitehead noted:
“On the whole, the water-deer are very secretive in their breeding habits ... one would hardly realize that the rut was on.”
Ready when you are
Chinese water deer are short-day breeders, which means they rut during the late autumn and winter. Despite one interesting observation from Buckinghamshire showing a buck pursuing a doe with what appeared to be sexual interest on 29th August 2023, the rutting period extends from November to January both here and in China, although the bulk of the activity and copulations appears take place in late November or early December. Territorial marking seems to reach crescendo during December and bucks certainly seem in peak condition during late October and November, losing condition during the rut, particularly in cold winters. In my view, there are several evolutionary drivers for rutting during at this time of year. Firstly, fawn development takes around six months, so, for the young to be born at a felicitous time of year while also allowing the mother to put on sufficient condition to undergo the trauma of birth and produce milk, conception must happen in winter. Secondly, while activity associated with the rut happens around the clock we think the majority takes place during dusk or overnight, and shorter days mean less daylight exposure during which these small deer are likely to be at greater risk from predators. Thirdly, the temperature is low, reducing heat stress associated with repeated patrolling, chasing and fighting, even in winter pelage. Fourthly, water deer bucks fight predominantly with canines, inflicting blows to the head, neck, flank and rump of an opponent; the thick winter coat probably offers protection against these blows. Finally, it also seems viable that, to females, the ability to maintain a territory in cold conditions when many other animals are reducing their activity may signal a strong buck carrying good genes and therefore be attractive.
My experience in Bedfordshire (at Whipsnade, Woburn Abbey and at two agricultural sites just outside Woburn village) has been that much of the rutting and courtship takes place during the first half of December. Both Paul Childerley and Ian Farrington, estate keepers in Bedfordshire, note activity peaking during the second week of December on their grounds, with Childerley adding that the first signs are evident in November. This corresponds with Arnold Cooke's trailcams at Woodwalton Fen in Cambridgeshire, which indicate most activity is in the first half of December, and other reports and photos/video I've seen from the East Anglian marshes. In Norfolk, for example, Elizabeth Dack has a great mating photo from Strumpshaw Fen taken on 3rd December 2018, and Carl Chapman reported water deer copulating at Buckenham Marshes on 19th December 2020.
Sharon Scott tells me she has seen bucks chasing one another in an aggressive manner as early as October and as late as the end of January, with the bulk of the courtship taking place slightly later in her area than elsewhere, during late December or early January. Arnold Cooke has observed signs of rutting during January at Woodwalton, as I have at Whipsnade, and wildlife photographer Mike McKenzie observed two bucks fighting at Claxton in Norfolk on 2nd February 2013.
That the bulk of rutting activity takes place during December may reflect how we often see a cold start to December before things turn milder towards Christmas. Indeed, there is some evidence that the initiation of the rut is actually strongly temperature dependant, delayed by very mild autumns and protracted into late January or early February during cold winters. If the winter remains mild, there may be hardly any activity. Arnold Cooke, for example, noted very little action on his trailcams at the Woodwalton Fen rutting stands during the exceptionally mild winters of 2013/14 and 2015/16, December 2015 being the warmest in central England since records began in 1659. Likewise, at Branféré Zoological Park in Bretagne, France, Gérard Dubost and his co-workers found mating occurred each year during a very short period (typically between 24th November and 4th January), at the first noticeable decrease in temperature and minimum day length. The mean weekly number of matings over the three years was negatively correlated to the mean daily temperature. More recently, Sharon Scott observed the earliest and most intense rut she had seen in 30 years owning the farm during the exceptionally cold start to December 2022.
While the onset of cold weather appears necessary to initiate the rut, my observations suggest that activity will continue, albeit at a reduced pace, if there is a subsequent warm spell. Hence, a cold end to November can result in a “normal” start to the rut even if December is mild, while a mild November and early December may postpone the start until closer to the solstice. Potentially, the rut might be preponed in cold autumns, although data are lacking. This might, though, explain Raymond Chaplin writing, in his 1966 book Deer, that the water deer rut occurred in September and October in Woburn Park. Certainly, Stefan Stadler, during his Ph.D. study at Whipsnade Zoo, recorded some rutting-like behaviour (i.e., a buck approaching a doe and whistling) as early as September, although there was no actual courtship until the winter. Endi Zhang, also at Whipsnade, noted that bucks started showing interest in does during early November, but the does were unreceptive and uninterested at this point.
Observations of rutting and territorial marking behaviour correspond with hormone profile data, suggesting sex hormones are highest during November and December. Based on observations of six adult male water deer kept at Huxia Park in Shanghai, Xiao-Jun Yu and colleagues found rutting behaviour increased significantly from October 2011 to January 2012, with the frequency of fighting, threat displays, chasing and scent-marking (defaecating) highest during November. Around this time, levels of testosterone in the bucks' droppings increased significantly, from around 25 nanograms per gram (ng/g) in October to a peak during November (ca. 41 ng/g) and December (51 ng/g) - it had fallen sharply to around 21 ng/g by January.
Based on faecal analysis of deer at Branféré Zoological Park, Robert Mauget and his team at the Muséum National d'Histoire Naturelle also found that the onset of androgen secretion in males began in October, when the first signs of territoriality appeared, and reached a peak in December (70-80 ng per 100 mg faeces - note that this is combined androgen metabolites and not just testosterone), when most matings happened. Levels dropped off sharply during January and remained low until October, which is curious given the aggression we see in spring. Territorial and non-territorial males were not significantly different in their androgen profiles.
While the female oestrous cycle appears primarily associated with temperature and daylength, in most other cervids studied to date age is also a factor; older females come into season sooner than younger ones. I'm not aware of any data on how much of an influence age has, but in his Ph.D. thesis Stefan Stadler described one instance, recorded on 10th February 1988 (some six to eight weeks after the peak of rut), of a buck mating with a young female, who he suggests may have come into oestrus late owing to her age. Also at Whipsnade, five out of 15 young does Endi Zhang monitored showed signs of late pregnancy. Similarly, Helin Sheng and Houji Lu on the Zhoushan Islands observed that mature does come into oestrus in December (the earliest being 14th December) and had embryos by February, while one-year-old females reached oestrus during January and had embryos come March.
Arnold Cooke's trailcam data from Woodwalton suggest that most of the rutting behaviour happens under the cover of darkness. Similarly, while I have seen chases and copulations during the day, my experience has been that vocalisation and chasing picks up significantly as dusk falls.