The suckling behaviour of water deer has been reasonably well described. Fawns are initially suckled every four hours or so, meaning that the dam returns five or six times over the 24-hour cycle, although Raymond Chaplin noted how there was considerable variation between Pippa, who suckled her twins five to six times per day, and Becky, who suckled her fawn only infrequently. He also observed that does may suckle their fawn if it's distressed, which may result in additional feeds.
Most observers have reported that the primary suckling period (i.e., when suckling occurs most frequently) lasts for about three weeks, the interval between suckling events increasing thereafter and suckling continuing for about eight weeks, sometimes up to 12 or 13 weeks. Chaplin found that Becky suckled her fawn for only four weeks, while Pippa suckled her twins for eight. This represents a comparatively short suckling period for a deer. It's not uncommon for offspring of roe (Capreolus capreolus), fallow (Dama dama) and red deer (Cervus elaphus), for example, to suckle sporadically well into October or November, at nearly six months old. I have never observed or read about such late stage suckling even attempted by water deer adolescents but would be interested to hear from anyone who has.
We don't know what prompts water deer mothers to return to their young to suckle but, in her Ph.D. thesis on red deer (Cervus elaphus) of the Scottish island Rum, completed in 1978, Marion Hall suggested that it might be the sensation of a full udder. Hall explained:
“The mother's approach is possibly stimulated by a full udder, and if male calves do take more milk from her during each visit, so that it takes longer for her udder to become full again, this may explain the tendency for male calves to be visited less frequently than females.”
At Whipsnade, Zhang estimated that over the 24-hour period, dams visited their fawns five or six times, each for a few minutes (averaging two minutes) to suckle and groom them. Zhang described two ways in which suckling was initiated. In the first few days after birth the dam would approach the fawn, sniff and lick it, at which point it would get up and suckle. Once the fawn was a few days old, the dam would approach it to within five or 10 metres (5.5-11 yds) and stop, “staring intensely at the site where the fawn was hidden”, which would prompt the fawn to get up within a few seconds and run or walk to the doe. Sometimes nose-to-nose contact was made before suckling. Zhang noted how, during the first couple of days of life, fawns seemed to be able to suckle for as long as they wanted; beyond this, females terminated the suckling by raising their hind quarters and moving rapidly forwards. At Branféré Zoological Park in southern Brittany, Christiane and Robert Mauget recorded that 63% of suckling bouts were initiated by dams.
Fawns appear to suckle from all four teats, although Chaplin observed that those in the groin were sought preferentially. This is interesting because in other species (e.g., red deer and pigs), it is the anterior teats that produce a higher volume and more concentrated milk and are thus most sought-after, although no similar studies exist for water deer. In his Ph.D. thesis, Zhang described how:
“When very young, fawns searched for the teats at different parts of the dam's body, most often at the front legs, but when they were a few days old, they immediately rushed towards the inguinal region.”
In a 2011 paper to Biological Bulletin Reviews, Vladimir Prikhod'ko and Elena Zvychainaya described a “food stroke” behaviour known from musk deer, whereby the suckling fawn makes “sharp and rhythmic strokes upwards and in the mother's groin with one or both unbent forelegs”, and that the nursing is accompanied by either this behaviour or pushing the mother's udder with the face. They refer to water deer performing this food stroke, citing Hans Frädrich's 1966 paper to Zoologischer Garten on musk deer behaviour, although I have not seen this reference to confirm. Nonetheless, in his Ph.D. thesis, Zhang wrote that “the 'milk step', where fawns make rapid kicks with one front leg, was often observed”, and Dubost and his co-workers described a “Milchtritt” (milk step) involving “blows of young with one foreleg to the female's hind leg”. Likewise, Stefan Stadler reported the milk-step during his study on the same Whipsnade population, and additionally a rapid upward movement of the head. Such “headbutting” by the young is reported to induce milk flow in several ungulate species, but in Stadler's case appeared to cause the mother obvious pain, often resulting in her terminating suckling.
Nursing is occasionally done while the doe is lying down, being perhaps more likely immediately after birth, although typically both the doe and fawn(s) stand. Indeed, in a 1998 paper to Acta Theriologica Sinica, Endi Zhang wrote of how:
“Successful suckling seemed to depend on the dam and the fawn getting into the 'inverse parallel' position described [in] sheep. In this orientation, the fawn's muzzle pushed against the teat while dam stood firmly with her head turned back, nosing at the fawn's hindquarters. Her back was arched and her hind leg extended in a way which raised the teat and made it more prominent.”
Gérard Dubost and his colleagues found that the mean duration of sucking among the free-range deer at Branféré was about a minute and 35 seconds for the first week after birth, increasing to just under two minutes at two or three weeks old and dropping off thereafter. Indeed, at between four and six weeks the fawns sucked for just over a minute, and the average duration was only 42 seconds by seven to nine-weeks-old. At Whipsnade, suckling events were difficult to determine as fawns were often obscured by the vegetation, but Stadler was able to observe 74 bouts each lasting on average about a minute (mean 57 seconds). Sex could be determined in 43 of these cases, and there was a difference, albeit not statistically significant; females suckled for an average of 75 seconds, and males for 55 seconds. As at Branféré, suckling duration at Whipsnade decreased significantly as fawns aged, averaging 102 seconds per bout during first month, 40 seconds during second month, and 27 seconds in fawns more than two months old. Overall, male and female fawns appear to suckle for the same duration and, while two fawns may suckle simultaneously, dams with several fawns may equally feed each individually.
The observation that suckling duration declines with fawn age may reflect the initial need for a high nutrient load to support the substantial growth that this species undergoes. Alternatively, or perhaps additionally, it may also correspond with Zhang's observation that, at a day or two old, fawns suckled weakly and inefficiently, requiring protracted bouts of feeding; declining to around 43 seconds per bout by the time they were a week old. Similarly, while hand-rearing fawns collected from a reserve in China, Hua Shen and colleagues reported that they lacked the drive to suckle and had to be force-fed. They were apparently inefficient sucklers until they were a week or so old; “eats less and spits out more” the team wrote in their 2007 paper to the Journal of Animal Husbandry and Veterinary Medicine. As Raymond Chaplin pointed out in Deer, it's also important to recognise that a fawn's stomach is not adapted to cope with large volumes of fluid, so the amount taken per suckle is small, suckling is infrequent, and the milk is concentrated.
Stadler remarked on how fawns younger than five days old looked exhausted after suckling, which probably explained why they appeared to terminate suckling of their own accord. By two weeks old, the fawns were larger and stronger, and the suckling was almost invariably terminated by the mother, often by simply walking forward a couple of steps.
We know nothing about alloparental care (where a dam suckles or cares for a fawn other than her own) in the wild, but both Gérard Dubost and his colleagues, and Christiane and Robert Mauget, documented the behaviour at Branféré. In their 2009 paper to Current Zoology, Mauget and Mauget reported that just over 10% of the suckling bouts they recorded were non-filial. Females didn't reject unrelated fawns if they were already caring for their own, but did spend significantly less time caring for them than they did their own offspring. On average a female would groom her own fawn about 55 times, versus an alien fawn only about three times, resting with them for a total of around 86 and three minutes, respectively. Dubost and his team, writing in Acta Theriologica in 2011, noted that suckling and cleaning of “alien” young was common in the park, but never replaced true maternal care; all fawns losing their mother within eight days of birth died, despite allosuckling. In August 1987 at Whipsnade, Stadler documented one male fawn suckle from a strange female who initially accepted him; after about 15 seconds she sniffed him and immediately withdrew some 20 metres (66ft.) away to resume feeding. Ordinarily, females were seen to act aggressively to a strange fawn, kicking at them with front feet, although Peter Scherpe, in his 1971 thesis, described an “aunt” in his captive study group that regularly cared for another female's fawn, grooming and suckling it.
Suckling is not always successful, particularly by very young fawns and when the mother and fawn fail to align in the inverse parallel configuration (i.e., fawn's head facing the dam's tail). Attempts are sometimes also rejected by the mother. In one particular example, observed by Endi Zhang at Whipsnade during the afternoon of 6th June 1993, a male fawn tried 10 times to suckle over a 12-minute period while walking behind its dam; all were rejected, although the dam did groom him. The fawn tried again 30 mins later, making another 18 attempts before he was finally allowed to suckle for 37 seconds.
There is a paucity of data on Chinese water deer milk. The Animal Diversity Website states that the milk is high in protein and antibody content, although they don't cite a source. Chaplin describes water deer milk as “rich and nourishing, with the first few feeds being predominantly colostrum”. Indeed, we know that the epitheliochorial placenta of ungulates impedes the transport of immunoglobulins - the large protein molecules associated with antibody production - so young are born in a hypo- or agammaglobulinemic state (i.e., with few, if any, immune proteins) and require a rapid intake of colostrum shortly after birth. Yong Xia and colleagues, in their 2008 paper to Modern Preventative Medicine, note that colostrum of water deer is 39% protein and contains 17 amino acids, seven of which are essential. Inorganic element analysis showed a high calcium content of almost 960 mg per 100g dry powder, plus copper, manganese, zinc and other trace minerals.
Chaplin analysed a small sample of water deer milk and reported it contained 18.4% total solids, 6% fat, 8% protein and no lactose. For comparison, cow's milk is approximately 3.6% fat, 3.3% protein, 4.1% lactose, while human breast milk is 3.5% fat, 1.5% protein and 7% lactose. In some other deer, red deer (Cervus elaphus) for example, the composition of the milk changes during the lactation period, but we have no data for water deer.
Zhipeng Li and his team assayed the milk of eight does kept on a farm in China's Yancheng city for bacterial content. They recorded a high bacterial diversity in the milk (higher than reindeer), with an abundance of Pseudomonas spp. and Acinetobacter spp. (each accounting for about 12% of the bacterial genera present), with lesser contributions from Chryseobacterium spp. and Corynebacterium spp. (each around 5%) and Comamonas spp. (4%). At the taxonomic phylum level, bacteria belonging to the Proteobacteria (60% of bacterial microbiota), Actinobacteria (15%), and Bacteroidetes (14%) were dominant in the milk. We know that the stress of labour increases gut permeability in many mammals, allowing bacterial translocation from the gut into the milk, and in their 2017 paper to Scientific Reports, Li and his colleagues suggest that the penchant for multi-fawn litters in water deer may result in greater stress and greater permeability, thereby allowing for a more diverse milk microbiota.
The rich milk produced by water deer dams allows the fawns to grow rapidly. At Whipsnade, James Kirkwood and colleagues tagged and weighed 79 mother-reared fawns living free-range in the park. Fawns grew at a roughly linear rate during the first month, starting at around a kilogram and being almost three kilos (6 lbs 9 oz.) by 30 days old, with an average growth of 70 grams per day. Hand-rearing of fawns by the authors suggested 50-70 grams (1.7-2.5 oz.) per days, declining to 40 g/day (1.4 oz.) once weaned. In an article to Journal of East China Normal University (Mammalian Ecology Supplement) during September 1990, Li-xing Sun and Helin Sheng reported that fawns in Shanghai gained between 90 and 106 g/day (3-4 oz.) fairly consistently during their first month. Their weight had increased to 12 kg (26 lbs 6 oz.) by the age of six months. It appears, therefore, that the fawns can put on between 50 and 100 grams (up to 3.5 oz.) per day during the first month or so and have reached about 80% of their adult body weight by the time they're six months old.