Geographic Distribution and Habitat
The spectral tarsier, also known as the Sulawesi tarsier and the eastern tarsier, and formerly known scientifically as Tarsius spectrum, is taxonomically distinct from the Philippine tarsier (Carlito syrichta) and is endemic to Sulawesi and other small surrounding islands in Indonesia.
Sulawesi sits east of Borneo, west of the Maluku islands, and south of Sulu Archipelago. It is the eleventh largest island in the world and includes four peninsulas. The spectral tarsier is found everywhere on the island, at altitudes ranging from sea level to over 6,500 ft (2,000 m). It inhabits tropical moist lowlands, submontane regions, and mossy cloud forests, where ferns and orchids thrive, and also plantations, rural gardens, and degraded forested areas.
The tarsier family (Tarsiidae) includes 3 genera, and at least 14 species and 7 subspecies. However, the taxonomy of the species continues to be debated.
Tarsiers are prosimians who belong to the suborder Haplorrhini, or “dry-nosed” primates, along with the true simians (monkeys, apes, and humans). Haplorrhines are considered to be less primitive than those belonging to the suborder Strepsirrhini, or “wet-nosed” primates. Strepsirrhines include lemurs, aye-ayes, lorises, and galagos (bush babies).
Haplorrhines diverged from Strepsirrhines 63 million years ago.
While strepsirrhines retained their ability to make Vitamin C, haplorrhines (including tarsiers) did not. Another distinction between the two is the “disconnected” upper lip that characterizes haplorrhines, allowing for their facial expressions.
Some scientists believe that tarsiers deserve a narrower taxonomic classification, asserting that tarsiers occupy a small evolutionary branch between haplorrhine simians and strepsirrhine prosimians.4
Size, Weight, and Lifespan
Males are larger than females and weigh, on average, 4 oz (120 g). Males are about 14 in (36 cm) long from nose to tail, with a 4 in (11 cm) tail. Females, on the other hand, weigh about 3.5 oz (100 g) and are approximately 13.5 in (35 cm) long, including tail.
Their lifespan is between 8 and 12 years.
The alien-looking spectral tarsier is a small primate with very large round eyes—so large, in fact, that they do not rotate in their eye sockets. Fortunately, they can turn their heads 180 degrees in each direction (similar to owls who can turn theirs even further). Although spectral tarsiers are nocturnal (active at night), their eyes do not have a tapetum lucidum, which is a reflective layer behind the eye that enhances light and therefore helps with night vision. Interestingly, their eyes have a fovea, which is typically only found in diurnal species that are active during daylight hours. The fovea is a small depression in the retina of the eye where visual acuity is highest. The center of the field of vision is focused in this region, where retinal cones are particularly concentrated. For this reason, it is thought that originally they may have been diurnal and evolved to become the nocturnal creatures we know today.
Their large ridged ears move independently to produce an acute sense of hearing to facilitate nighttime hunting. They have a well-developed sense of smell. Their upper lip is not fastened to the gum beneath, as is the case with most other prosimians, allowing them a range of facial expressions unique to tarsiers.
Their thin long tail is covered in light hair and their body is fully covered in elaborate tactile hairs of a brownish/gray color. Their five fingers and toes end with large pads. Their hind legs are powerful and elongated, but of all tarsiers, the spectral tarsier has the shortest limbs, feet, and hands. Their tail, feet, and hands have friction ridges (fingerprints) on the inside. Even though they never have twins, females have two or three pairs of mammary glands.
Exclusively carnivorous (meat-eating), spectral tarsiers hunt for insects and arthropods by spotting them visually or by listening for them. Noisy insects, like cicadas and rhinoceros beetles, commonly appear on their dinner menu. They also consume grasshoppers, locusts, crickets, butterflies, and moths, which they prefer over termites, spiders, and beetles. Insects are most abundant between November and April, when it rains the most. During the dry season, tarsiers forage more frequently on the ground, as air and leaf insects are not as easily found.
Cuscuses (or possums) and small bats are competitors since they, too, are nocturnal and feed on the same insects.
Behavior and Lifestyle
Spectral tarsiers are nocturnal and live in groups of two to eight individuals, usually one adult male and one adult female (occasionally two), and their offspring. Mating pairs stay together for several years.
They move through the trees by clinging vertically and leaping. The large pads at the end of their fingers and toes help them get a firm grip on smooth surfaces like bamboo. They use their tail for support when climbing up or going down a tree, and to control momentum and direction when they leap. Their leaps are pretty impressive for such tiny creatures—up to even 16 ft (5 m) when they are chased by a predator.
They spend more time on the ground than other tarsiers do. While on the ground, they move by hopping or running. When hunting for flying insects, they also “canter”—they stretch their body while holding on to a branch with their hind limbs as they catch their prey with their hands and mouth.
Males and females have an average home range of 4–7 acres (2–3 ha) and groups are territorial. Territorial encounters are frequent while foraging, especially during the dry season when insects are not as abundant, and groups travel farther and longer for food. Intergroup encounters are more frequent during the mating season. At such times, the resident adult male vocalizes and lunges at the intruding adult male. Resident females may be more lenient in their response to a male intruder—especially if they find him attractive—but they are harsh when the intruder is female.
James Petiver, a London apothecary, was the first to mention tarsiers in 1705 in a publication called The Philosophical Transactions of the Royal Society.
In 1765, French naturalist Comte de Buffon described tarsiers as being related to opossums. A few years later, seeing similarities between tarsiers and prosimians, German naturalist Johann Christian Polycarp Erxleben, called them lemur tarsiers.
Finally, in 1780, German naturalist,
Gottlieb Conrad Christian Storr, classified tarsiers as their own species.
Tarsiers have the largest eyes in relation to their body size of any other mammal.
Unlike lemurs and lorises, tarsiers share some physical characteristics with monkeys, apes, and humans. For instance, their upper lip is not attached, they have a placenta, and their eyes lack a tapetum lucidum. They also share characteristics with prosimians—they are small and nocturnal, transport their infant in their mouth and park them, and have a grooming claw and multiple pairs of mammary glands. No wonder there is no definite consensus on their classification as a species.
Spectral tarsiers do not recognize the vocal duets that Diana tarsiers sing, even though they share the same territory.
Spectral tarsiers are most active from sunset till just before sunrise. During that time, they travel away from their sleeping site and scan for insects on the ground or in the air, with periods of rest, grooming, and socializing in between. When foraging alone—more than 32 ft (10 m) away from each other—adult females seem to capture about a third more insects than males do, but when they forage close to one another, both males and females capture about the same number of insects. Adult males and females do not forage more than 80 ft (25 m) away when the female is lactating; they are further apart—130 ft (40 m) or so—when females are pregnant or non-reproductive.
Surprisingly, and contrary to other nocturnal species, spectral tarsiers increase their foraging time, the number of calls they produce, and scent marking during full-, gibbous-, and quarter-moon periods. It could be that on clear nights it is easier for them to capture insects, although one might hypothesize that they themselves would be more vulnerable to predators and that is why they travel as groups. Indeed, spectral tarsiers have to remain vigilant at all times. Adult males never stop scanning for predators. They give alarm calls to warn the group; even more so when infants need protection, and it is not unusual for a group of five or more adult male and/or female tarsiers to band together and mob a predator, such as a snake, forcing it to retreat.
When the night is over, they retire to their sleeping site—preferably a large hollow in a fig tree. They keep the same tree, usually located in the center of their home range, for several years, and every day, as they get back together to rest, all members of the group, including juveniles, join into a chorus before going to sleep.
Spectral tarsiers communicate through gestures and postures—like when an individual sits next to another to offer or solicit grooming—but also through vocalizations, which differ between males and females. Successive squeaks that rise in pitch and end in a trill are typical of females; series of squeal barks are typical of males.
Alarm calls are specific to the type of predator encountered. Even parked infants, as young as one week old, use alarm calls—sometimes correctly, but other times because they’re scared and cannot make sense of phenomena in their environment (like falling leaves or things they’ve never seen before). Twittering alarm calls announce the threat of birds; harsh loud calls are for the threat of snakes. When infants erroneously produce alarm calls, mothers generally do not respond. However, when the threat is real, mothers produce the same alarm call and move away from their offspring if the predator is a bird of prey, or closer to the infant if the predator is a snake. This seems counterintuitive, but it is the best way for them to protect the little one—birds are fast and sense movements, so they’ll go toward the mother and not the baby. Snakes, however, hunt by sensing body heat and smell, so removing both herself and the baby is safer. In other situations, tarsiers’ natural reaction is to freeze until the danger is out of sight.
Since some groups include one adult male and two adult females, it is safe to conclude that spectral tarsiers are not strictly monogamous.
Females give birth every 12 to 19 months to one offspring (never twins), born after a gestation period of approximately 193 days, the longest among all prosimians.
For a few days after she’s given birth and while she is lactating, the female moves to a new sleeping tree, a short distance from the group tree. This may be to avoid predation and possibly reduce the risk of infanticide as she forbids the adult male or any other group member, including older offspring, to enter her new sleeping site. The new mother returns to the group sleeping tree when she is done lactating.
Infants are almost a quarter of an adult’s size at birth and are born with hairy tails. They are nursed for 2 to 3 months, although they start eating solid food as early as three weeks old, when they also start moving about on their own. They are transported in their mother’s mouth and parked (i.e., left on a branch) while she leaves to forage for food. She is never far and keeps moving her baby as she goes to different trees.
When her offspring is six weeks of age, the mother encourages her to follow and carries and parks her less often. Infants occasionally fall, but it’s rarely because of the mother’s neglect; rather, it’s mostly because the little one loses grasp of the support she is on. Infants are visited often by juvenile members of both sexes who play with and groom them. In some cases, females even let them eat prey they’ve captured. Adult males are not aggressive towards their infants. Although they occasionally cuddle and groom them and only while in the sleeping nest, they never share food with them.
Once they’re grown, spectral tarsiers disperse from their natal sleeping site. Males go about 2,300 ft (700 m) away, while females travel half that distance.
Because they are found in areas where owls are absent and because they share similar physical characteristics with them (large eyes, movable ears, head rotation) and have the same taste in food, it is thought they likely play a similar role as owls do in controlling insect populations. Although they are often seen as pests by locals, when they live near human population areas, they probably play a beneficial role in protecting crops from devastating insects like grasshoppers.
Although not in imminent danger of extinction, the International Union for Conservation of Nature Red List of Threatened Species classifies the spectral tarsier as Vulnerable (IUCN, 2015). They are primarily threatened by habitat loss. The spectral tarsier population density has steadily decreased over the years, from over 150 individuals per 0.6 sq mile (1 sq km) in the 1990s to over 80 in the mid-2000s.
Human activities have drastically shaped the island. For example, where land was cleared by natural or managed fires, there are now open areas of grassland. Minahasa is the most populated area on the island and is highly deforested—in fact, only 1% of the original forest remains. Most of the land is used for taro, cassava, banana, papaya, mango, and coconut plantations.
People living close to protected forests illegally clear land for their survival needs, including buffer zones specifically designed to shelter natural reserves from human activities. To compound the problem, local authorities are often not in a position to enforce the regulations in place and nature reserves that by law should be restricted to researchers are now open for tourism. Some tarsiers can also be victims of the pet trade; most die after a few days of being captured.
The Malaysian palm civet and the Sulawesi palm civet, monitor lizards, snakes, and nocturnal birds are natural predators of tarsiers.
International treaties are in place for the protection of tarsiers; however, better management of the protected areas where they live is needed. Education of local population is also needed.
No Sulawesi tarsiers have been maintained in captivity, so there are no captive conversation efforts like there are for other tarsier species. Even if there were, it is not certain captive breeding would be successful.
- The Spectral Tarsier – Sharon L. Gursky
- Primate Ecology & Social Structure – Volume 1 – Lorises, Lemurs and Tarsiers – Robert W. Susman – Department of Anthropology, Washington University
- Primate Societies – Lorises, Bushbabies, and Tarsiers: Diverse Societies in Solitary Foragers – Simon K. Beader
Written by Sylvie Abrams, January 2019