Dian’s Tarsier, Tarsius dentatus
DIAN'S TARSIER
Tarsius dentatus
Geographic Distribution and Habitat
The Dian’s tarsier is found only in central and eastern Sulawesi, an Indonesian island east of Borneo. The region is recognized as an important biodiversity hotspot. The northern boundary of their range is the Isthmus of Palu, although their southern boundary is unclear beyond Lore Lindu National Park. Their eastern boundary extends to the eastern peninsula past Ampana and Luwuk, and their western boundary extends to the Palu-Koru fault (where there is a displacement in plates of the Earth’s crust).
The Dian’s tarsier can be found in habitats that have experienced various levels of human disturbance, but is found most abundantly in undisturbed or slightly disturbed habitats. They can be found anywhere from mangroves to forest gardens, lowland forests to montane regions, as long as there is some type of shrubby cover for them to sleep and forage in.
Tarsiers have been causing researchers headaches since they were first described by Western scientists in 1705. Not only can scientists not agree on where tarsiers fit within the primate order, but also on the differences between tarsier species. In the 90s, researchers named a new species of tarsier T. dianae, which happened to have a very similar range as a subspecies of the Makassar tarsier, T. fuscus dentatus. After further examination of the species, including comparisons of their calls, researchers began to realize T. fuscus dentatus was likely a separate species from the Makassar tarsier, but not a separate species from T. dianae; thus, we now have Dian’s tarsier as T. dentatus with T. dianae recognized as a junior synonym of the species (meaning a previously used name for the same species).
Size, Weight, and Lifespan
Both males and females have an average head-to-body length of 4.72 inches (120 mm). They have long, skinny tails that are on average 10.62 inches (270 mm) in length. While close in size, some sources report the Dian’s tarsier as being sexually dimorphic, with males weighing 0.21–0.22 pounds (95–110 g) and females weighing 0.23–0.29 pounds (104–135 g).
Currently, the lifespan of Dian’s tarsier is not known.
Appearance
The Dian’s tarsier has a uniform gray coat, contrasted only by their tail, which is darker in color and tipped with fuzzy-looking fur. Like other tarsiers, their long hind legs, built for leaping between branches and small body size, make them look almost more like furry frogs rather than a primate. They have long fingers and toes for gripping the sides of trees. Their large eyes allow them to detect any bit of light emitted in the night, which likely makes up for their lacking a tapetum lucidum, which is a reflective layer behind the retina of prosimian primates and other nocturnal mammals that allows them to see better in the dark and gives them a distinctive nighttime eyeshine. Perhaps the only thing proportional to tarsier’s eyes is their ears, perched like two satellite dishes on either side of their head, ready to pick up the slightest sound of a bug trying to sneak by.
Diet
Tarsiers, including Dian’s tarsier, are the only primates to be strictly carnivorous. There have been no reports of any tarsier species eating any kind of plant material. Their diet consists mostly of insects, but can include small vertebrates such as frogs and lizards.
Behavior and Lifestyle
Because researchers have been so caught up in trying to figure out tarsiers’ taxonomy, they have only recently begun to look into details regarding their behavior. Tarsiers are also difficult to study, not only because of their small size, but also because they’re nocturnal, meaning they’re only active at night. On top of that, tarsier researchers don’t even have the luxury of being able to locate their study species by the eyeshine reflected by other nocturnal mammals’ tapetum lucidum. Because of this, there’s still a lot to learn about Dian’s tarsier.
Further adding to the difficulties of studying tarsiers, they are arboreal and spend almost all of their time in trees, although typically staying only up to six feet from the ground. Similar to many prosimian primates, tarsiers often move through the forest understory by vertical clinging and leaping (or VCL). Now imagine trying to observe the behaviors and lifestyle of a tiny primate, small enough to fit in the palm of your hand, at nighttime while they bounce between trees like a bouncy ball, and they don’t even have reflective eyes to help you locate them. Tarsier researchers certainly have their work cut out for them.
Dian’s tarsiers form small groups of 2–7 individuals. Researchers determine which individuals are in a group based on their sleeping associations. While groups of Dian’s tarsiers will sleep together during the day, they don’t always spend much of their active time together during the night. After finishing patrolling their range and foraging for bugs and lizards all night, Dian’s tarsiers will rush back to their sleeping site. They move fastest right before sunrise to reach their sleeping site quickly (players of Stardew Valley will likely relate to this need to rush to bed in time). Groups will perform a territorial symphony before tucking in just before dawn. Researchers think this reverse lullaby serves to strengthen the bond between group members and to let outsiders know where not to traverse. Groups typically have one preferred sleeping site (although some populations have been noted to change sleeping sites each day) made of dense vegetation such as clumps of fig, shrubs, bamboo, tree cavities, or fallen logs. Sleeping sites are often on the edge of their home range, allowing them to be prepared to defend their territory boundaries at all hours.
Their home ranges range from 0.003–0.006 square miles (1–1.8 hectares). In regions with high disturbance, home ranges are larger, likely due to the need to travel farther to locate food and other resources. Pesticide and herbicide use in heavily farmed areas reduces the number of insects, making food scarcer for tarsiers. However, areas that have experienced only slight disturbances tend to have denser understory growth, creating a better environment for the Dian’s tarsier’s favorite prey. Because food is more available, home ranges don’t need to be as large in these habitats, even compared to areas that have experienced no disturbance at all. The amount of home range traveled in a night also differs by habitat type. Dian’s tarsiers living in slightly disturbed habitats will often cover less than half of their total home range in a night, while those living in undisturbed and heavily disturbed habitats travel only about a third of their home range in a night. Population densities (the number of individuals of a species found in an area) are also higher in slightly disturbed or undisturbed habitats, indicating these are ideal habitat types for Dian’s tarsier.
Predators of Dian’s tarsier have not been confirmed, but given their convenient snack size, it is likely they are hunted by snakes, raptors, monitor lizards, and civets. Feral cats are also capable of hunting tarsiers.
The most well-known form of communication used by tarsiers is their loud call, specifically the morning duet performed by groups before tucking in for the day. To add to the difficulty of studying tarsiers, some species have also been found to use ultrasonic calls that are outside of the human observer’s hearing ability. While this form of communication hasn’t been recorded for Dian’s tarsier, it is likely they also use calls outside of our hearing range.
Tarsiers are also known to scent mark, either by rubbing their scent glands on surfaces or by urinating. This likely serves to advertise their sexual state to other group members, and/or to warn other tarsiers that they’ve strayed into another’s territory.
Along with most of their behaviors, not much is known about tarsiers’ reproduction. There have been some dedicated studies on a few species of tarsier, but the Dian’s tarsier is not one of them. However, from these studies, we can make some guesses on how Dian’s tarsiers might be similar.
From what we currently know, tarsiers have a gestation length of about 6 months, after which they give birth to a single infant. There has never been any record of a tarsier giving birth to twins, although they have the anatomy to be capable of it. More similar to other prosimian primates, tarsiers have a bicornuate uterus, where the top of the uterus forms two separate chambers that fuse together at the bottom, allowing for two embryos to develop. Tarsiers also have 2–3 pairs of mammary glands, which are 2–3 times more than necessary for only a single baby to nurse from. That said, it is understandable how tarsiers are only able to produce one offspring at a time, as infants are about 25% of the adult’s body weight.
Having such a large baby also makes it difficult for tarsier mothers to travel and forage while caring for their young. Instead of carrying them on the mother’s chest or back like many other primates do, tarsiers carry their baby by the scruff of the neck in their mouth and “park” them nearby while they quickly find some bugs to eat.
While the relationship between the Dian’s tarsier and their environment has not been studied as of yet, we can make some educated guesses on what their ecological role might be. Being the only fully carnivorous primate, they likely serve as the forest’s pest control. While the insects and small vertebrates that tarsiers prey on also have their own important ecological roles, keeping their populations in check is necessary to maintain the area’s biodiversity, and tarsiers likely serve the important role of managing these populations.
The Dian’s tarsier is classified as Vulnerable by the International Union for Conservation of Nature (IUCN, 2015), appearing on the IUCN Red List of Threatened Species. Their population is decreasing.
The primary threat to the Dian’s tarsier is habitat loss, mostly from illegal logging. Agricultural pesticides also harm tarsier populations by depleting their food source. They are sometimes preyed on by domestic dogs and cats, and also receive retaliation from farmers due to misinformation about them being crop pests. Tarsiers are sometimes caught and sold in the pet trade.
While the Dian’s tarsier is found in some protected areas, these protections are not well upheld and the area will still experience illegal habitat degradation, mostly due to slash-and-burn agriculture to convert forest habitat into alang-alang grassland, which is harvested for use in making materials such as paper and roofing for houses, as well as in traditional medicine as it is believed to aid in everything from fevers to kidney stones.
Dian’s tarsiers are listed in Appendix II of the Convention on International Trade in Endangered Species (CITES), an international agreement between governments whose goal is to ensure that international trade in specimens of wild animals and plants does not threaten their survival.
In addition to further research to further understand the Dian’s tarsier and better tailor conservation efforts for them, conservationists are also calling for local education to not see tarsiers as crop pests, and instead as beneficial, as they’ll often eat insects that are actually harmful to crops. Conservationists are also calling for the protection of all of Sulawesi due to its exceptional biodiversity.
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- https://www.iucnredlist.org/species/21489/17977790
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Written by Lina Rademacher, April 2025