Muntjacs were first scientifically described in 1780, from a specimen collected in Java. Muntjacs, of course, live much further afield than this, and, over the years, some of those living elsewhere were split off and given their own species names. As per the standard rules, the one that's found on Java kept the original scientific name, modified only when muntjacs as a whole were given their own genus in 1810. We now call this the red muntjac (Muntiacus muntjak) and it remains one of the best-known species, not least because it's the most widespread.
Red muntjacs are forest-dwelling animals living in Indonesia west of the Wallace Line, throughout southeast Asia and most of India and Sri Lanka. Numerous subspecies live across this range; it has been proposed to split off most of the mainland ones into a separate species, the Indian or "northern red" muntjac (M. vaginalis). This seems to be moving towards more general acceptance, but, for the moment, I'll deal with the two together.
In any event, red muntjacs are typical examples of their kind. They stand about 60 cm (two feet) high at the shoulder, with no significant size difference between the males and the females - something that's rarely true of larger deer. Their reddish coat is one of their few distinctive features, and, like many other muntjacs, they also have a pair of black stripes running down their faces from their antlers. The antlers are simple in shape, with just two points each: the brow tine and an unbranched beam. In most other deer, the brow tine, as its name suggests, branches forward just above the animal's brow, but in muntjacs it seems to be further up. This is because muntjacs have an unusually long pedicle - the knob on the skull to which the antlers are attached. Covered in fur, this is a permanent structure, and it's only the part of the antler above this, brow tine included, that is shed and replaced each year. Even the females have lumps on the skull in this position, although obviously without antlers attached to them.
Other features typical of muntjacs in general, and distinguishing them from other deer include large upper canine teeth in the males, and the fact that they don't have the scent glands on the feet that other deer do.
A more unusual feature of Indian muntjacs is that they are the mammal species with the smallest known number of chromosomes. Females have just three pairs of chromosomes, compared with 23 pairs in humans - and, indeed, in some other closely related muntjacs. This appears to have arisen as consequence of multiple chromosomes fusing together and trimming down on the "junk DNA" between and within genes, so that the number of chromosomes dropped without actually losing any vital genetic material. Significantly, one of the genes that was fused in this manner was the X-chromosome, the material from which is now mixed in with one of the regular chromosomes. While some of the genes from the Y-chromosome suffered a similar fate, a proportion remain in a chromosome of their own so that male red muntjacs have seven chromosomes in total.
Unlike many larger deer, red muntjacs are solitary animals, sometimes gathering together in groups of up to four, some of which are likely fawns, but certainly not forming true herds. They spend much of their time hiding in thick undergrowth, and, as such, prefer areas that are wooded, but not so heavily that the sunlight is blocked from the ground. Having said which, they do seem to prefer more tree cover when they are sleeping, presumably as the shade helps hide them from their many predators. They are active through much of the day, having to freed regularly as many small mammals do, but are especially so around dawn and dusk. Like most larger deer, they are browsers, and they feed mainly on soft leaves, buds, and fruit.
As their alternative name of "barking deer" indicates, they can be vocal animals, barking mainly when they are distressed, but they also communicate by leaving scent marks, and they have large scent glands in front of each eye. They do not seem be especially territorial, but the markings may help to support other social interactions, as well as indicating things such as sexual receptivity. They are probably loosely polygynous, without the mate guarding and general showing-off of larger deer (which is presumably why the bucks aren't larger than the does, having no need to be). They give birth to single fawns, which have the usual spotted coat of other deer, but seem to be weaned unusually early, at around 70 days.
The other common and widespread muntjac is Reeves' muntjac (Muntiacus reevesi) which is found across most of lowland China, and hence is sometimes instead called the "Chinese muntjac". Named for the then Assistant Inspector of Tea for the British Empire (now there's a title!) this is small, even for a muntjac. It stands about 50 cm (1' 8") at the shoulder and is similar in appearance to its southerly counterpart, although the brow tine is typically shorter and the hooves smaller. As in other muntjacs, the males have fang-like upper canine teeth, in their case extending about 4cm (1.6 inches) from the jaw; these are about five times longer than the equivalent teeth in the females.
Because they are one of the species that still retains the full set of 23 pairs of chromosomes, Reeves' muntjacs have traditionally been assumed to be "primitive", having changed the least since the origin of the group. How true that is is debatable - the individual genes may have changed without altering how they happen to be arranged - and the latest studies don't show the species at the base of the muntjac family tree. (Which, to be fair, wouldn't prove anything, either). Indeed, there is evidence that even this number of chromosomes evolved through the fusion of an even larger number in earlier species, since other cervine deer have a whopping 35 pairs.
Reeves' muntjac has similar habitat preferences to the red species, although, perhaps due to living in cooler climes, it is less inclined to travel to high elevations. In 1901, both this and the red muntjac were introduced to the Woburn Abbey deer park in England. The red muntjacs died out within a few years, but the Chinese sort prospered, and have since spread to other parts of the country - mostly as a result of deliberate introductions, although escapes have also happened. This has affected some of the natural habitat where they are found, with a mix of negative and positive consequences.
Reeves' muntjac seems to be more territorial and socially complex than the red species, with males defending territories that encompass those of a number of females, and dominant individuals scent-marking more often than their subordinates. Breeding occurs throughout the year, which means that it must necessarily happen even when the males don't have any antlers. Which is probably what happens when males decide dominance by fighting with their teeth, not in the manner seen in larger deer. Having said which, castrating males does stop them from growing antlers, so there's something going on with the sex hormones there.
Besides Reeves' muntjac, two further species were named in the 19th century that are still recognised today. These are Fea's muntjac (Muntiacus feae) and the black muntjac (Muntiacus crinifrons). The former, which is named for an Italian zoologist, lives in the forested hills of the northern Malay Peninsula, on the border between Thailand and Myanmar. However, even this much isn't entirely certain, since it's easy to confuse with other related species living nearby, being distinguished from them at largely on the basis of having 13 chromosomes in the females and 14 in the males. Which is the sort of thing it's difficult to determine when you've only spotted one in the wild.
The black muntjac is native to eastern China, notably in the Zhejiang and Anhiu provinces. It was originally described from a specimen living near the mouth of the Yangtze River, but it's no longer found in such coastal regions, being restricted to hillier, even mountainous, terrain inland. Until as recently as the 1970s, only five specimens were known to western science, and it remains something of a mystery to this day.
On the bright side, it is fairly distinctive as muntjacs go. Aside from being much darker in colour than most other muntjacs, it also has a large tuft of yellowish hair on its forehead just in front of the antlers, giving rise to its alternative name of 'hairy-fronted muntjac'. It's thought that only a few thousand survive, but they mostly live in nature reserves where there is apparently little threat from illegal hunting, so they are not currently listed as an endangered species.
The closest relative of the black muntjac is the Gongshan muntjac (Muntiacus gongshanensis) with which it's often confused, leading to reports of black muntjacs living further west than they probably do in reality. The Gongshan muntjac also has a dark coat, although it's more dark brown than literally black, and the males don't have the yellow tuft on their forehead... although females do have a small one, just to complicate matters. They are known for certain from Yunnan Province in China and probably also live just across the border in northern Myanmar. They have also been reported from Bhutan and neighbouring regions of India, but there is no hard scientific evidence that the animals seen really were this species, rather than (say) dark-coloured red muntjacs.
In 1932, Wilfred Osgood described a new species of muntjac on the basis of an animal killed on the Kelley-Roosevelt Expedition to Southeast Asia a few years prior. This became known as Roosevelt's muntjac (Muntiacus rooseveltorum)... and then wasn't definitively seen again for over 60 years. Eventually, in 1999, genetic samples from muntjacs in Laos were compared with those of the original specimen and found to be identical, confirming not only that the species still exists, but that it definitely isn't another name for Fea's muntjac, as had been suspected.
Because that's where the confirmed samples come from, we can say that Roosevelt's muntjac is native to northern Laos. It probably lives further afield as well, but most of the evidence that it might do comes from a few blurry photographs that are certainly muntjacs of some kind, but not good enough to be confident they're the same thing. Complicating matters, genetic analysis in 2006 of a specimen appearing to be a Roosevelt's muntjac living in Vietnam showed that it belonged to a different, but very closely related, species, now called the Annamite muntjac (Muntiacus truongsonensis) based on a prior description of what seems to be the same animal in a 1997 newspaper article. A third species said to be related to it, named the Pu Hoat muntjac, is often mentioned in scientific sources, but is apparently based on a bit of skull with attached antlers that have never been genetically analysed; it seems unclear to me that it's really a distinct species although, admittedly, it's also hard to prove it isn't.
Another close relative first named in the 1990s, but for which rather better evidence exists, is the leaf muntjac (Muntiacus putaoensis) which has a confusingly similar scientific name to the putative Pu Hoat species. Here, we do have genetic evidence backing its existence, and it also looks fairly distinctive. It apparently has a reddish coat, and is the smallest of all muntjacs, at just 40cm (16 inches) high when fully grown, and having no brow tine. In fact, the name comes not from any habit of eating leaves (it eats very little but fruit) but from the fact that it is small enough to be wrapped up in a single leaf. A large leaf, one assumes, but there are a number of those in the tropics.
Slightly earlier, in the 1980s, a group of muntjacs living in Borneo were split off from the red muntjacs and similarly identified as a distinct species. There doesn't seem to be any doubt about these, since they look very distinct from the red muntjacs, and both live together on the same island without interbreeding. The Bornean yellow muntjac (Muntiacus atherodes) is paler than the red muntjac and has much smaller antlers, lacking any brow tine. They seem to live in most of the wilder parts of the island, but avoid the mountains (of which there are many). Not much else is known about them.
As if all that isn't enough, in 1994, the giant muntjac (Muntiacus vuquangensis) of Vietnam and eastern Laos was also identified as a separate species. This is, as its name suggests, especially large - for a muntjac. Adults stand a full 70 cm (2' 3") in height at the shoulders, and the antlers are similarly impressive, reaching up to 28 cm (11 inches) in length. It lives in the heavily forested foothills of the mountain range that divides the two countries and there are few studies on it as distinct from its smaller kin. Its current status is, however, thought to be perilous, with very few wildlife studies in the area having been able to find it at all. It is known to be under considerable pressure from hunters and the surviving population is thought to be dropping rapidly, so, while most other muntjacs other than the common two are placed in the "we just don't know" category, this is firmly listed as a critically endangered species.
The variety and richness of tropical habitats has, perhaps, led to greater speciation among the muntjacs than in larger deer. However, it's also clear that the muntjac body plan, of a small browsing deer that can hide in the undergrowth, has been a successful one. Muntjacs belong to the cervine group of deer, most common in the Old World, but it turns out that the capreoline deer of the New World also evolved a similar body plan. It is to those small deer of South America that I will turn next.
[Photos by Suvendu Das, Thomas Kees, and Shizhao from Wikimedia Commons. Painting by Richard Lydekker, in the public domain. Cladogram adapted from Zhang et al. 2021, Srisodsuk et al. 2018, and Heckeberg et al. 2020.]