Water Deer - Evolution

While there are some gaps in the fossil record, deer are believed to have evolved from antlerless, tusked ancestors that resembled modern day pigs, known as the gelocids, that lived during the early Cenozoic. Ling Huang and colleagues at the Kunming Institute of Zoology in China have used chromosome painting studies to show that six chromosomal fissions appear to have transformed the ancestral pecoran (pig-like) karyotype of 58 chromosomes (2n) into the ancestral cervid (deer) karyotype of 70.

The late Eocene, around 46 million years ago (mya), saw widespread extinction, and ruminants similar in appearance to today's duikers (small African antelopes) and chevrotains (small Asian and African "mouse-deer") arose during the Oligocene. In the same period, we see the development large canines for use in combat. In his appraisal of cervid evolution, published in 1966, Valerius Geist notes that the early genera of cervids were antlerless but possessed large upper canines. Most present-day genera, by contrast, have medium to large antlers and have either lost or reduced to insignificance their upper canines.

These first artiodactyls were small, short-legged creatures that probably fed on leaves and succulent plant material. By the end of the Eocene, the three suborders we see today are recognised: the Suina (pigs); the Tylopoda (camels); and the Ruminantia (the goats and cows). We're lacking data on the ancestor of modern ruminants, but it seems probable that they evolved from a rabbit-sized mammal that inhabited middle Eocene forests, some 50 mya, called Diacodexis. Regardless of their precursor, genetic analysis by Clément Gilbert and colleagues, published in 2006, suggests an Asian origin for true deer, with the foundations of the Cervidae running back to some point during the Late Miocene (around 10 mya).

Water deer have 70 chromosomes, as do roe (Capreolus), grey brocket (Mazama gouazoubira) and the white-tailed and mule deer (Odocoileus), while other cervids either have the same or commonly fewer (Cervus have 60-68, for example). In their 1988 publication on deer genetics, Z. Wang and R. Du suggest that chromosome evolution in the Cervidae has been towards fewer chromosomes, suggesting that the Hydropotes/Capreolus/Odocoileus/Mazama karyotype represents the ancestral state and these species appeared early in the Cervidae lineage.

The Capreolinae subfamily, according to the data presented by Gilbert's team, emerged in Central Asia at the Miocene/Pliocene boundary (~5 mya) as Procapreolus and diversified here before migrating into what is now North America*. Similarly, in their 2004 analysis of Old World deer phylogeny, Christine Pitra and colleagues pointed out that while the Asian genus Procapreolus is considered ancestral to Capreolus, it remains unclear (and in their view, unlikely) whether Procapreolus latifrons, a species inhabiting the Late Miocene-Early Pliocene forests of Asia some 5.5 million years ago that appears to be the ancestor of modern day roe, is also ancestral to Hydropotes. Indeed, in a 1998 paper to the Proceedings of the Royal Society, a team led by Ettore Randi at the Istituto Nazionale per la Fauna Selvatica in Italy calculated that, based on cytochrome b nucleotide sequences, water deer separated from Capreolus during the Late Miocene, about 11 mya, well before Procapreolus latifrons appeared on the scene. During the 1960s and '70s, several authors suggested that Dremotherium from the Late Oligocene of Europe and Asia (~28 mya) as the ancestor of both musk and water deer. The difference in soft tissue anatomy between the species and molecular data suggesting that the Cervidae and Bovidae/Moschidae diverged around that time (i.e., about 23 mya) cast some doubt on Dremotherium as an ancestor, however.

Yeong-Seok Jo, John Baccus and John Koprowski note, in their 2018 Mammals of Korea, that the water deer was present in Korea during the first half of the Quaternary, suggesting they've been in Asia for approaching three million years. Wei Dong, in her chapter in the 1993 Deer of China compendium, lists the oldest fossil of Hydropotes from the Middle Pleistocene of northern China (800,000-120,000 ya). I think this is from the Sinanthropus Site at Chouk'outien, although she doesn't specify. In line with Dong, however, in his 1972 paper to the Journal of Mammalogy, Peter Brokx wrote that the water deer was "not known from before the Pleistocene". Brokx cited George Simpson's 1945 work on the principles of mammalian classification for this, although Simpson lists the water deer only as "Recent", which going by his scheme is Holocene.

I'm currently not aware of any fossil evidence of water deer from Korea, but would be interested to hear from any readers who are.

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Antlers answered?

A curious feature of this species, and one that sets it apart from all other members of the deer family, is that the males lack antlers. Bony structures grown from specialised plates on the skull called pedicles, antlers are more than just ornaments evincing fitness; they're an integral component in securing mating opportunities in a family where males must fight one another for access to females (see Antler Development). Consequently, their absence here raises an interesting evolutionary question: does the water deer represent the primitive form (ancestral phenotype) of deer, or is antler loss a derived autapomorphy? In other words, did the first deer to walk the earth look like a water deer, with antlers evolving later in other lineages, or did it sport antlers that were subsequently lost somewhere along the water deer's ancestral line – a secondary loss?

In their deer classification scheme, published in 1987, zoologists Paul Groves and Peter Grubb considered water deer to be the sister group to all other cervids – in other words, they viewed both the presence of well-developed canines and the absence of antlers as primitive traits. On this interpretation, the earliest deer had tusks, and as antlers evolved most species lost their canines in favour of headgear. In 1998, however, renowned deer evolutionary biologist Valerius Geist argued that the morphology and life history traits of water deer don't appear to match what we know about the primitive tropical deer from which modern cervids descended. Geist championed an idea first proposed by Russian zoologist Konstantin Flerov in 1950 – that the lack of antlers in Hydropotes is not a primitive retention but a secondary loss. In the same year, geneticist Ettore Randi and his colleagues at the Université de Montpellier arrived at the same conclusion in a paper published in the Proceedings of the Royal Society of London, this time on the basis of molecular evidence, writing:

"Contrasting with current taxonomy, Hydropotes is not the sister group of all the antlered deers, but it is nested within the [roe deer subfamily]. Therefore, Hydropotes lost the antlers secondarily."

Corroboration followed in 2003, when Ming Li and her team published a cytochrome b dataset in Acta Theriologica that supported the emergence of water deer from an antlered ancestral line. Two years later, in his 2005 revision, Grubb updated his own earlier stance, placing water deer as a sister subfamily to the Capreolinae – an arrangement that, while not widely adopted, more closely reflected the growing consensus that antler loss in this species is indeed secondary.

More recently, a 2026 genomic analysis led by Zhenyu Zhong at the Beijing Milu Ecological Research Center provided further support for secondary loss and shed light on the genetic mechanisms behind it. The researchers found 40 genes associated with antler development have been completely lost in water deer, including ZNF675, a known suppressor of osteoclast-mediated bone resorption. A further 188 genes showed signatures of relaxed selection – evolution is no longer acting to preserve them, having rendered them unnecessary, and they are degrading gradually as a result. Notably, six of the 40 lost genes were among those highly expressed in the antler buds of other deer species, strengthening the link between these losses and antler absence specifically. The analysis, published in Zoological Research, also supported convergent pseudogenization of the RXFP2 gene in both water deer and forest musk deer (Moschus chrysogaster and Moschus leucogaster) – the same gene 'breaking' independently in two separate lineages that both lost antlers is compelling evidence that it plays a genuine role in antler development, rather than its disruption being coincidental. Taken together, these findings suggest that antler loss is still being "written into" the water deer's genome, the genetic signature of an evolutionary transition that may not yet be complete.

The fossil record, though frustratingly sparse as discussed above, is consistent with the phylogenetic view: the oldest known Hydropotes remains date to the Middle Pleistocene of northern China, by which point antlers had been a feature of the deer family for many millions of years. The loss of antlers being secondary, hypertrophication (enlargement) of the canines was most likely a compensatory response, filling the role that antlers play in other cervids. This is consistent with a broader pattern seen across the family -- canine length tends to diminish as antler size and complexity increase -- and supports the idea that, far from being primitive, the water deer is actually a highly specialised member of the Capreolinae subfamily.