Java mouse-deer

The **Java mouse-deer** (_Tragulus javanicus_) is a leaf-dwelling, predominantly herbivorous ungulate in the family Tragulidae. When it reaches maturity it is about the size of a rabbit, making it the smallest living ungulate. It is found in forests in Java and perhaps Bali although sightings there have not been verified.

## Taxonomy

The Java mouse-deer’s scientific name is Tragulus javanicus. It belongs to the Tragulidae family, which includes small, hoofed mammals called chevrotains. Scientists once thought it was part of a larger group of chevrotains, but studies of its skull and fur color show it is a unique species found only on Java. Other similar species, like _Tragulus williamsoni_ and _Tragulus kanchil_, live in different places such as Thailand and Borneo.

## Appearance and biology

Mouse-deer has a triangular-shaped head, an arched back, and a round body with elevated rear quarters. The thin, short legs, which support the mouse-deer, are about the diameter of an average pencil. Although Java mouse-deer do not possess antlers or horns like regular deer, male Java mouse-deer have elongated, tusk-like upper canines, which protrude downward from the upper jaw along the sides of their mouth. Males use the “tusks” to defend themselves and their mates against rivals. Females can be distinguished from males because they lack these prominent canines and are slightly smaller than males. Java mouse-deer can furthermore be distinguished by their lack of upper incisors. The coat coloration of the Java mouse-deer is reddish-brown with a white underside. Pale white spots or vertical markings are also present on the animal's neck.

With an average length of  and an average height of , the Java mouse-deer is the smallest extant (living) ungulate or hoofed mammal, as well as the smallest extant even-toed ungulate. The weight of the Java mouse-deer ranges from , with males being heavier than females. It has an average tail length of about . Mouse-deer are thought to be the most primitive ruminants, based on their behaviour and the fossil record, and so  are the living link between ruminants and non-ruminants.

The Java mouse-deer is endothermic and homoeothermic and has an average basal metabolic rate of about 4.883 watts. It also has the smallest red blood cells (erythrocytes) of any mammal, and about 12.8% of the cells have pits on them. The pits range in diameter from 68 to 390 nanometres. Red blood cells with pits are unique and have not been reported before either physiologically or pathologically.

### Geographic range

_Tragulus javanicus_ is usually considered to be endemic to Java, Indonesia. There have been unverified reports of sightings on Bali.

### Habitat

The Java mouse-deer prefers habitats of higher elevations and the tropical forest regions of Java, although it does appear at lower elevations between  above sea level. During the day, Java mouse-deer can be seen roaming in crown-gap areas with dense undergrowth of creeping bamboo through which they make tunnels through the thick vegetation, which leads to resting places and feeding areas. At night, the Java mouse-deer moves to higher and drier ridge areas.

It has been argued that Java mouse-deer are an “edge” species, favoring areas of dense vegetation along riverbanks. Also, Java mouse-deer have been found to be more prevalent in logged areas than in the more mature forests, and their densities tended to decrease proportionately as the logged forests matured.

### Diet

Java mouse-deer are primarily herbivores, but  in captivity. they have been observed to eat insects and foliage. Their diet consists primarily of that which they find on the ground in the dense vegetation they inhabit, and they prefer the plants of the faster-growing gap species over the closed forest understory species, likely because of the increased richness of secondary protective compounds that the gap species provide. They are often classified as folivores, eating primarily leaves, shrubs, shoots, buds, and fungi, in addition to fruits which have fallen from trees. The fruits which Java mouse-deer commonly consume range from , while the seeds range from .

### Social behavior

Groups of Java mouse-deer are commonly referred to as “herds,” and  females are termed “does,” “hinds,” or “cows.” Males are referred to as either “bucks,” “stags,” or “bulls,” and their young are commonly called “fawns,” or “asses”.
It was previously believed that Java mouse-deer were nocturnal, but more recent studies have shown that they are neither truly nocturnal nor diurnal but are instead crepuscular (they prefer to be active during the dim light at dawn and at dusk). That behavior has been observed in both wild and captive Java mouse-deer. Although Java mouse-deer form monogamous family groups, they are usually shy, solitary animals. They are also usually silent; their only  is a shrill cry when they are frightened.

Male Java mouse-deer are territorial and mark their territory and their mates with secretions from an intermandibular scent gland under their chin. The territorial marking usually includes urinating or defecating to mark their area. To protect themselves and their mates or to defend their territory, mouse-deer slash rivals with their sharp, protruding canine “tusks.” It has also been observed that when threatened, Java mouse-deer beat its hooves quickly against the ground, reaching speeds of up to 7 beats per second, creating a “drum roll” sound.

The territories of _Tragulus javanicus_ males and females have been observed to overlap considerably, but  individuals of the same sex do not share their territories. When giving birth, however, females tend to establish a new home range. Female Java mouse-deer have an estimated home range of , while males inhabit, on average, . Additionally, male Java mouse-deer, in nature, were observed to travel distances of  daily on average, while females average  daily.

## Reproduction

Java mouse-deer can breed at any time during the year, which has been observed during captivity. However, some sources have observed that the breeding season for the Java mouse-deer in nature occurs from November to December. Additionally, female mouse-deer have the potential to be pregnant throughout most of their adult life, and they are capable of conceiving 85–155 minutes after giving birth. The Java mouse-deer's gestation period usually lasts 4.5 months, or 144 days.

Typical litters consist of a single fawn, which resembles a miniature adult, although the tusk-like incisors prevalent in males are not visible in the young mouse-deer. The average mass of a newborn fawn is , and these precocial young are capable of standing within 30 minutes after birth. Fawns can eat solid food within two weeks, but it takes around 12 weeks to completely wean the fawns. On average, it takes the young, both male and female, 167 days (about 5 months) to reach sexual maturity. Mouse-deer have been observed to live up to 14 years in captivity, but their lifespan in nature is still an open question.

## Predators

Humans are one of the main predators that the Java mouse-deer face. Humans' destruction of their habitat, as well as from hunting and trapping the mouse-deer for food, their pelts, and for pets, have caused considerably reduced the Java mouse-deer population. Mouse-deer are particularly vulnerable to being hunted by humans at night because of their tendency to freeze when illuminated by having a spotlight shone on them. Because of the small size of the Java mouse-deer, dogs are also a common predator for them, as well as crocodiles, big cats, birds of prey, and snakes.

## Diseases

Although research into the diseases and parasites that  affect the Java mouse-deer are still starting, bovine viral diarrhea virus (BVDV 1), a pestivirus of the family flaviviridae has been detected in Java mouse-deer. Mouse-deer acquire this virus through fetal infection during early pregnancy. Once acquired, individuals with BVDV can gain lifelong immune tolerance.

## Conservation status

Java mouse-deer is currently categorized as “Data Deficient” on the International Union for Conservation of Nature's Red List. The data deficiency is caused by the uncertainty  regarding the distinct separation of _Tragulus_ species, in addition to the lack of information on _Tragulus javanicus_. Even comparison of past observed numbers of Java mouse-deer with those presently observed does not greatly aid researchers because of the high likelihood of inaccuracy in past observations. Although listed as “Data Deficient,” it is highly probable that a decline in the numbers of Java mouse-deer is occurring, and upon further investigation of this issue, the Red List status of _Tragulus javanicus_ could easily change to “Vulnerable”.

Some conservation actions have been implemented. including legally protecting the species, but although that has been in effect since 1931, it has made no significant difference since hunting of Java mouse-deer still occurs. Additionally, some areas of Java which the Java mouse-deer frequents have been protected, yet enforcement of these regulations is still needed. One of the greatest conservation efforts needed is simply more information about the species: a more complete definition of its taxonomy, as well as more information on its habitat and behavior.

## Indonesian folklore

Historically, the mouse-deer has featured prominently in Malaysian and Indonesian folklore, where it is considered a wise creature. This character, Sang Kancil (pronounced “Kahn-cheel”), is a diminutive but wise mouse-deer. Sang Kancil is a tiny and cunning hero who, through his intelligence, is able to prevail over his larger tyrants and foes.