SIAU ISLAND TARSIER
Geographic Distribution and Habitat
The tiny volcanic island of Siau, in Indonesia’s North Sulawesi province, is home to this tiny, enigmatic primate. Siau Island tarsiers (Tarsius tumpara) are found nowhere else in the world. (Some scientists speculate that these Critically Endangered primates may also inhabit some of the smaller islands in close proximity to Siau Island, separated only by shallow ocean.) The tarsiers’ potential habitat is diminished by Mt. Karangetang, one of Indonesia’s most active volcanoes, which dominates more than 50 percent of Siau Island’s geography. Although there are rumored reports of Siau Island tarsiers residing near the volcano’s caldera, surveys from 2009 cite evidence of dwellings in only two places: on the shores of a small freshwater pond at the extreme southern end of the of the island, and on a steep cliff face along the east coast road that runs parallel to the ocean.
In 2008, the Siau Island tarsier was classified as a distinct tarsier species. When they were first surveyed in 2002, Siau Island tarsiers were thought to be a population of Sangihe tarsiers (Tarsius sangirensis), who inhabit Sangihe Island (also known as Sangir Island). Both Sangihe and Siau islands are part of the Sangihe Islands archipelago, a volcanic arc island chain in the Sulawesi Sea stretching 124 mi (200 km) north from the northern tip of Sulawesi. Sangihe Island is 37 mi (60 km) to the north of Siau Island, across ocean depths greater than 3,281 ft (1,000 m).
Conclusions regarding the Siau Island tarsier’s habitat are largely drawn from extensive studies of other tarsier species. Primary forests are preferred (though no tracts of primary forest have been identified on Siau Island); tarsiers are also found in secondary and mangrove forests, forest gardens, and in a variety of other habitats that have been altered, to varying degrees, by human disturbance but that still provide adequate shrub cover.
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.44
Size, Weight, and Lifespan
Nature has created a miniature primate. Siau Island tarsiers measure just 4 to 6 in (about 10–15 cm) in head-to-body length. A rope-like, nearly hairless tail adds another 8 in (20 cm). A skeletal wonder of unusually long, articulating tarsus bones give these tarsiers their excessively long rear feet (and “tarsus” happens to be the word origin of the name “tarsier.”)
Weight has not been recorded for the Siau Island tarsier; however, its “sister species,” the Sangihe tarsier, weighs between 3.5 and 4 oz (0.1–0.12 kg).
The lifespan of the Siau Island tarsier, and of the Sangihe tarsier, is unknown. However, for other members of the genus Tarsius, such as the spectral tarsier, also known as the Sulawesi tarsier (Tarsius tarsier), lifespan in the wild is between 8 and 12 years; for the Philippine tarsier (Tarsius syrichta), lifespan in the wild is up to 24 years, but only 12–15 years in captivity. In the wild, Horsfield’s tarsier (Tarsius bancanus) lives 15 years.
Siau Island tarsiers bear a resemblance to Sangihe tarsiers, with slight variations in coat color. Instead of a golden-brown coat, like that of Sangihe tarsiers, Siau Island tarsiers are cloaked in a brownish-gray pelage (common to other tarsiers). And instead of the nearly white underside that distinguishes their sister species, Nature has given Siau Island tarsiers a gray underside. A dark gray ring outlines its gray, furry face, and a prominent brown line encircles each eye—as if the eyes needed further accenting (they don’t!). Sangihe tarsiers are similarly marked, but the line encircling each eye is fainter.
As prosimians, tarsiers are predisposed to their prehistoric appearance. They are among the smallest prosimians and are among Nature’s most curious-looking primates; Siau Island tarsiers are no exception.
Imp-like, cartoonish, less-kind folk might say “ghoulish”—are all apt descriptors of this diminutive being. Some might be reminded of Yoda, the tiny goblin-like character and legendary Jedi master from the Star Wars films. Others might think of a Troll doll (a 1960s U.S. toy fad) with muted, brownish-gray hair with flecks of white—rather than the typically iridescent hair color of a Troll—and with much bigger eyes.
The tarsier’s eyes are exceptionally large; each eye is as big as the animal’s brain and, together, give the skull an unusual shape. Its pupils sit small, centered, and overwhelmed by the irises (which are varying shades of goldish-brown). Because a tarsier’s eyes do not move, it may appear that he is looking at you with a wide, fixed gaze. So, if you allow your imagination to be ignited, you might think of a mythical spellcaster or forest wizard whose eyes have the power to enchant. Or maybe not. But the tarsier’s enormous eyes do assist this nocturnal creature with enhanced night vision. (Unlike many nocturnal animals, tarsiers’ eyes lack the light-reflecting layer known as the tapetum lucidum. Instead, as in human primates, their eyes are equipped with closely packed cones, or photoreceptor cells, an adaption that gives them sharp central vision, also known as “foveal” vision.) The upper lip is not directly connected to the nose, allowing for a wide range of facial expressions. Bat-like ears, spindly fingers, unusually long feet, and its long, skinny tail complete the Siau Island tarsier’s primeval good looks.
Siau Island tarsiers are insectivores, a fancy way of saying that they eat mostly insects. They are fond of spiders, too. Frogs, lizards, and small birds are also on their menu. Their wide mouth, relative to their small size, accommodates larger prey. Strong jaws and sharp teeth are adaptations to their strictly carnivorous diet: tarsiers are the only completely carnivorous primates in the world!
Behavior and Lifestyle
Phylogenetically linked to other Sulawesi tarsiers (foremost, perhaps, to Sangihe tarsiers)—which is just a super “science-y” way of saying that these species are related to one another through evolutionary development—Siau Island tarsiers, as members of this primordial clan, live in small, monogamous or polygamous groups of two to six individuals. Much of the scientific speculation about the little-studied Siau Island tarsier is based on other tarsier species, whose members have been the subjects of more intense field research.
A lone Siau Island tarsier skull, taken from the island by an explorer in 1897, is in the collections of Dresden Museum, Germany.
Arboreal creatures, Siau Island tarsiers spend virtually all their time in trees. They are excellent climbers and even more impressive as jumpers; an adult can jump nearly 10 ft (3 m) high! Their agility is a great asset when capturing prey, and their ability to rotate their head 180 degrees in each direction spells doom for nearby bugs. They have keen hearing. Rather than forage, like most insectivores, tarsiers patiently wait on a branch for an unsuspecting bug to appear, then jump toward it. Their large rear feet assist them in this feat. They might also reach out, and with great vertical strength, grab flying prey with their long, spindly fingers; Siau Island tarsiers are known to snatch a bird in flight!
Prominent pads on their fingers and toes allow tarsiers to “stick their landing”—like gymnasts do—as they jump from tree to tree during their nightly travels. Like all tarsiers, Siau Island tarsiers are fitted with two “grooming” claws on the second and third digits of their feet designed to comb through their velvety fur.
Siau Island tarsiers while away the daylight hours sleeping in tree holes, high up off the ground. Wildlife researchers suspect that individuals sleep in separate sites, particularly in disturbed habitat. This solitary practice, a contrast to their nightly social activities, appears to be deliberately intended to reduce the risk of predator attack on an entire family group.
Humans are the tarsiers’ most lethal predators. Because they have been so systematically preyed upon by humans, tarsiers have made further behavioral modifications. As example, they limit their vocalizations and never (according to researchers) engage in more than a duet—no loud group choruses. Furthermore, the scent mark left by Siau Island tarsiers fades much more rapidly than other Sulawesi tarsiers; within an hour it has dramatically diminished, and within a day the scent is nearly undetectable to the human nose.
Researchers are intrigued that Siau Island tarsiers, and to a lesser degree, Sangihe tarsiers, have evolved adaptations to avoid detection by human hunters. Feral cats and dogs also prey upon the tarsiers.
Like members of their sister species, the Sangihe tarsier, Siau Island tarsiers are known to engage in morning duets. Unlike Sangihe tarsiers, whose calls are characterized by two-note phrases, morning duet calls of Siau Island tarsiers are typically one-note, isolated phrases. One occurrence of a multi-note phrase was captured in a field recording, however.
With Sangihe tarsiers, males emit a chirping note, while females emit a whistle note. These “sister” tarsiers also use vocalizations when they find themselves in a threatening situation.
Not a lot of further information is available specific to the communication methods of Siau Island tarsiers. But it is a reasonable conjecture that these tiny primates make use of their grooming claws and engage in social grooming (as do other tarsier and most primate species), an activity that helps to establish familial bonds with one another. The extent of this social grooming cannot be certain, however; as an example, social grooming in Horsfield’s tarsiers occurs only between mothers and their infants.
Scent marks are important in demarcating territory and conveying various messages to one another. While Siau Island tarsiers have adapted a “dissipating” scent mark—to avoid detection and predation by humans, Sangihe tarsiers leave a lingering scent to send messages to one another. Sangihe tarsiers’ scent is unique to each individual and helps members to recognize one another; unfortunately, scientific documentation is lacking as to whether the same is true of Siau Island tarsiers.
Siau Island tarsiers are nearly two years old when they reach sexual maturity (capable of siring and bearing offspring). Mating rituals are unreported in the species.
After a gestation period of about six months, a female gives birth to a single infant whom she nurses for eight weeks, until weaned. Infant Siau Island tarsiers are born furred and with their eyes open. Amazingly, they are able to climb a tree and jump within one day of being born!
Parental care is not described in the species.
As insectivores, it is plausible that Siau Island tarsiers help to keep the structure of insect communities in balance within their environment. Of course, their true role within the web of life that we share with them, as earth citizens, is unto themselves—designed by Nature and not scripted by humans or for human benefit.
The Siau Island tarsier is listed as Critically Endangered—just one step away from extinction—by the International Union for Conservation of Nature (IUCN, 2010), appearing on the IUCN Red List of Threatened Species. The species also has the ignoble distinction of continually appearing on “Primates in Peril: The World’s 25 Most Endangered Primates,” a list compiled by the IUCN Species Survival Commission, Primate Specialist Group (2006–2008, 2008–2010).
Habitat loss, hunting, volcanic eruption . . . each conspires to threaten Siau Island tarsiers’ survival.
Their position in the world is made precarious foremost by humans, who have razed tarsier habitat and repurposed it for human use. Virtually no tracts of primary forest remain on Siau Island and few notable tracts of secondary forests remain, resulting in fragmented tarsier populations. The tarsiers are already restricted to a small area because of Mt. Karangetang, which looms over the island, threatening to erupt. (Mt. Karangetang last erupted on August 19, 2007, causing avalanches that reportedly reached the coast.) According to a spokesperson for the IUCN, “Depending on the magnitude of the eruption and the path of the lava flows, the population [of Siau Island tarsiers] could be severely affected or even possibly disappear.”
But humans are most responsible for the disappearance of Siau Island tarsiers. Besides razing and transforming tarsier habitat for their own use, the relatively large human populace of Siau Island hunt and eat these tiny primates. During Sunday afternoon social gatherings, as many as five to ten Siau Island tarsiers are eaten at one time, after being roasted on a spit. Locals think of the roasted tarsiers as a snack food and refer to them as “tola-tola.”
Between habitat degradation and hunting, more than 80 percent of the Siau Island tarsier population has been wiped out (as reported by National Geographic). Their numbers continue to decline. A 2009 study using geographic information systems (GIS) to remotely sense remaining habitat placed the population as being 1,358–12,470 individuals. The wide range is attributed to a large number of unknown pixels (obscured by clouds) in the GIS dataset.
The Siau Island tarsier is 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. However, as is often the case with endangered species, laws created to protect them are largely ignored and difficult to enforce.
Currently, no specific conservation programs exist for Siau Island tarsiers. And Siau Island offers no wildlife reserves where these Critically Endangered primates might be afforded a modicum of protection. Furthermore, virtually no ecotourism exists on the island. Tourism, however, has the potential to negatively impact this species. Tarsiers are shy, nervous animals and become stressed with human contact. (The stress level with Siau Island tarsiers is possibly even more heightened, due to their already negative relationship with humans.) Activities such as camera flashes, being touched, or being kept inside an enclosure overstress the animals, causing them to react with self-harm: they bang their head against the walls of their confinements, thereby cracking open their thin skull and killing themselves (this behavior is referred to as “tarsier suicide”).
Captive breeding programs have failed with tarsiers. “Ex-situ” conservation (occurring outside a species’s ecological niche) do not appear to work with this genus. Researchers report on 146 tarsiers taken from their natural habitat and brought to North America or Europe to establish breeding programs—all the tarsiers died. (A record of a lone Siau Island tarsier who was captured alive in April 2002 reveals that the kidnapped individual died, in transit to a zoological museum in the Netherlands, in April 2005.)
Given the fragility of Siau Island tarsiers and their Critically Endangered classification, conservationists say that it is crucial that emphasis be placed on “in situ” conservation; that is, conservation within the animals’ natural environment.
Conservationists also call for continued taxonomic research; improved and intensified tarsier-tracking activities like those being conducted near Tangkoko Nature Reserve, North Sulawesi, Indonesia; and the creation of community-based programs. Helping locals to better manage their lands without destroying tarsier habitat and educating and instilling within local people a sense of pride for these large-eyed, diminutive primates are imperatives in saving Siau Island tarsiers from extinction.
The creation of reputable sanctuaries is another key imperative. Conservationists/advocates are particular about how these sanctuaries should be created and how they should operate. Sanctuaries should be modeled after the Philippine Tarsier and Wildlife Sanctuary (also known as the Tarsier Research and Development Center), located in Corella, Bohol, in the Philippines and operated by the Philippine Tarsier Foundation, the main non-profit private organization in the Philippines that seeks to protect the Philippine tarsier. (Contrarily, The Loboc Tarsier Conservation Area in the Philippines is not an official sanctuary area, and it is not supported nor run by the Philippine Tarsier Foundation. Tarsiers here are reported to be enclosed, at least some of the time, put on display for tourists, and are prone to tarsier suicide.)
Some conservationists have asserted that sanctuaries and tarsier tracking programs should be developed side-by-side to promote more effective in-situ and, possibly, successful ex-situ conservation.
If no conservation programs are developed, however, the future of Siau Island tarsiers (as well as other endangered species within the Sangihe Island chain) is bleak.
Written by Kathleen Downey, March 2019