The Java mouse-deer is the smallest living ungulate; when it reaches maturity it is about the size of a rabbit. These tiny animals have a triangular-shaped head, arched back, and round body with elevated rear quarters. Their thin, short legs are about the diameter of an average pencil. Although Java mouse-deer do not possess antlers or horns, males have elongated, tusk-like upper canines that protrude downward from the upper jaw along the sides of their mouth. Males use these “tusks” to defend themselves and their mates against rivals. Females can be distinguished from males because they lack these prominent canines, and they are slightly smaller than the males. Java mouse-deer can furthermore be distinguished by their lack of upper incisors. The coloration of their coat is reddish-brown with a white underside and there are pale white spots or vertical markings on their necks.
Java mouse-deer are native to Java, Indonesia, and perhaps Bali, although sightings there have not been verified. They live in tropical moist forests and it has been argued that they favor areas of dense vegetation along riverbanks.
Java mouse-deer are crepuscular, meaning they prefer to be active during the dim light of dawn and dusk. During the day, they roam in crown-gap areas with a dense undergrowth of creeping bamboo, through which they make tunnels through the thick vegetation which lead to resting places and feeding areas. At night, they typically move to higher and drier ridge areas. Although Java mouse-deer form family groups, they are usually shy, solitary animals. They are also usually silent and the only noise they make is a shrill cry when they are frightened. Males are territorial, marking their territory and their mates with secretions from a scent gland under their chin. This 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.” When threatened, Java mouse-deer will also beat their hooves quickly against the ground, reaching speeds of up to 7 beats per second, creating a “drum roll” sound. The territories of males and females overlap considerably, yet individuals of the same sex do not share their territories. When giving birth, however, females usually establish a new home range. Additionally, males may travel distances of 519 meters (1,703 ft) daily on average, while females average 574 meters (1,883 ft) daily.
Java mouse-deer are monogamous and form pairs. According to some sources, in the wild, they breed from November to December. In captivity, they may breed yer-round. The 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 370 grams (13 oz), and it is capable of standing within 30 minutes after birth. The fawn will start eating solid food within 2 weeks but will be completely weaned at around 12 weeks old. On average, it takes the young, both male and female, 167 days ( around 5 months) to reach reproductive maturity.
The main threat to the Java mouse-deer comes from humans. Through the destruction of their habitat, as well as from hunting and trapping the mouse-deer for food, their pelts, and for pets, humans have considerably reduced the Java mouse-deer population. These animals 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.
The IUCN Red List and other sources don’t provide the number of the Java mouse-deer total population size. Currently, this species is classified as Data Deficient (DD) on the IUCN Red List.