TARSIERS: THEIR CHARACTERISTICS, SOCIAL BEHAVIOR AND MATING

TARSIERS

The tarsier is a bizarre, lemur-like primate that looks like a squirrel from Mars on an LSD trip and about the size and shape of a African bush baby. Found only in the highland jungles of Borneo, Indonesia and the Philippines, it has enormous eyes and a gremlin-like smile and can rotate its head nearly 360̊. It forages for food at night and is particularly fond of eating cockroaches Tarsiers are found only in some rainforest in some parts of the Philippines and Indonesia..

The Dyaks call the tarsier “hantu”, an ancestral spirit. Some scientist argue that they are prosimians like lemurs and lorises but others say they more closely related to monkeys and apes. Tarsiers are similar to lemurs but have different nose structures (a dry nose) and lack the reflective material behind the iris that lemurs have. Monkeys also share these traits. The olfactory and sight-related structures of tarsier skulls is similar to that of monkeys. Some scientist view them as missing links between lemurs and monkeys. The name tarsier refers to the tarsier’s elongated tarsal, or ankle, region.

Sharon Gursky-Doyen wrote in Natural History magazine, “Tarsiers stand out in numerous ways. Their saucerlike eyes are larger, relative to the head, than those of any other mammal. The animals boast two or three pairs of nipples, even though a female gives birth to only one infant at a time (apparently not all of the nipples are functional). And they are the most carnivorous of the primates, with a diet consisting entirely of insects and, in some cases, small vertebrates. They are extreme leapers — indeed, they are named for an unusually long tarsal (ankle) bone that acts as their launcher. They are reportedly capable of leaping as far as eighteen feet. [Source: Sharon Gursky-Doyen, Natural History magazine, October 2010]

There are at least six species of tarsier. One is found on several islands of the Philippines. Another is found in southern Sumatra and Borneo and some nearby islands. Sulawesi is one of the best places to see them. Three species are found there. The prefer secondary forests and mangrove with dense vegetation rather than canopied primary forests. For most species there are healthy numbers. One species is listed as threatened. The biggest threats to tarsiers are deforestation, habitat loss and capture by humans.

Tarsier Expert: Sharon Gursky-Doyen is an associate professor of anthropology at Texas A&M University. She received her PhD from the State University of New York at Stony Brook and has been studying tarsiers throughout Sulawesi, Indonesia, since 1990. While continuing her work on spectral tarsiers, she is also investigating the effects of alti- tude on the recently rediscovered pygmy tarsier {Tarsiiis piiiiiihis). She is the author of The Spectral Tarsier f(Prentice Hall, 2007) and coeditor (with K.A.I. Nekaris) o{ Primate Aiiti-Piedator Strategics (Springer, 2007).

Tarsier Characteristics

Adults weigh between 80 and 165 grams and have a head and body length of 85 to 165 millimeters, with tail between 135 and 275 millimeters long. Their fur is usually gray or brown in color. The are built a little bit like a kangaroo in that they have short forelimbs and long hind legs and a long mostly naked tail. They also have relatively large hands and feet, long fingers and toes, rounded pads and highly developed first toes on their hands and feet allow them to grip almost any surface and effectively grasp prey. Tarsiers also have a muscular upper lip, which is associated with facial expressions.

Tarsiers are quite mobile in the trees. When they are awake their eras are in nearly constant motion. Their disproportionally long legs and elongated tarsal bones give them great leaping ability. They can make quick, acrobatic leaps of up to 20 feet — more than 20 times their body length — with great ease. Their leaps have been comparted to those of a tree frog because they can leap and land on near flat surfaces.

Tarsiers are one of 26 species regarded as a singer. To make this list and animal has to be able to repeat several series of notes in a recognizable temporal pattern. Other singers include titi monkeys of South America, a lemur from Madagascar called the indris, and all 12 species of gibbon.

Tarsiers can make sounds beyond the range of human hearing. Josie Garthwaite wrote in Discover: “A saucer-eyed Philippine tarsier opens its mouth wide, squints, and lets out a great burst of . . . silence. The gape-mouthed expression of these primates has long been considered an act of yawning or stretching. But Sharon Gursky-Doyen, a biological anthropologist at Texas A & M University, became convinced the pint-size creatures were actually making ultrasonic screeches. She set up microphones at the Philippine Tarsier Sanctuary and found the animals were emitting sounds up to 75 kilohertz; humans cannot hear past 20 kilohertz. “It was mind-boggling,” she says. “It makes you reevaluate everything you’ve done, heard, and observed.” The shrill call may serve as an alarm signal that is undetectable to approaching predators. [Source:Josie Garthwaite, Discover, October 29, 2012]

Tarsier Senses

Tarsiers have acute hearing and night vision. Their large, close-set eyes are 150 times larger in relation to their body than human eyes and, relative to body size, are largest eyes of any animal in the world. Each eye is larger than its brain and both eyes together are larger than its stomach. If human eyes were as big in proportion to their bodies they would be the size of softballs. Their eyes are so big the tarsiers can’t rotate them in their sockets, which is why tarsiers have developed their ability to rotate their heads.

The tarsier’s ability to rotate it head like an owl, almost 360 degrees, gives them an extraordinary field of vision. Headhunters in Borneo viewed the tarsier’s head turning ability as an omen. A tarsier turning it head 360̊ was viewed a sign that a head was going to be lost soon.

Tarsiers have excellent hearing. Their ears are very thin, mobile and somewhat similar to those of bats. They can be unfurled and retracted as needed; twisted and crinkled to focus sound; and can easily pick of the sounds of moving insects. When a tarsier hears a beetle it swivels it head and the ears bend forwards to get a better read on the sound. A tarsier’s sense of smell is not developed in part because it eyes take up so much space in its head there isn’t any room left for a sophisticated nose. Consequently smell is not a very important sense and is sued mainly to mark territory.

David Attenborough wrote, “Uniquely among prosimians, the retina at the back of the eye has a groove down the middle which gives extra definition to its sight. Its neck is so mobile it can be twisted through 180 degrees in either direction. When the animal’s ears pick up the sound of an insect moving behind it, its head swivels round immediately and its sight is so good and its reactions so swift that it can pluck a fast-flying insect from the air with the skill of a cricketer making a difficult catch in the slips.”

“The tarsier’s nostrils, which project sideways, are circular. Fur grows almost to the edge of them and surrounds them, separating them from the upper lip. The nostrils of other prosimians, in contrast, are shaped like commas, permanently moist and linked to the upper lip by a strip of naked skin. The difference in the structure of the nose is a reflection of the smallest extent to which the tarsier relies on its sense of smell compared with other prosimians. The only other clambering tree-dwellers that have noses like the tarsier are the monkeys. Those scientists concerned with working out the ancient lineages of animals regard this as being of great significance. The tarsier, they believe, is the only prosimian that truly deserves the title, It, unlike the others, really is a “pre-monkey”, an ancient twig on the great branch that was to lead to that group. It was also, of course, the branch that lead to ourselves.”

Trouble Classifying Tarsiers

Horsefield tarsius

Sharon Gursky-Doyen wrote in Natural History magazine, “All living tarsier species are classified in the genus Tarsius. Beyond that, their taxonomic position has been a source of dispute. The eighteenth-century French naturalist Buffon, who, upon examining a juvenile tarsier, thought it might be a kind of opossum, was not the first to find them a bundle of contradictions. While other living primates fall fairly neatly into two main groups — the Strepsirrhini (the suborder that embraces lemurs, lorises, and galagos) and the Haplorrhini (the one that includes monkeys, apes, and humans) — tarsiers seem to belong to both at once. [Source: Sharon Gursky-Doyen, Natural History magazine, October 2010]

“A variety of characteristics mark tarsiers as Strepsirrhini: their small body size, grooming claws, nocturnal habits, and two-horned (as opposed to single-chambered) uterus, as well as aspects of their parental care (a mother will park her infant in a tree while she forages, and infants are transported by mouth, the way a dog carries a puppy). On the other hand, tarsiers possess numerous features linking them with the Haplorrhini, including a dry nose, a mobile upper lip that is not attached to the nose, a fovea centralis (a depression in the middle of the retina that increases visual acuity), and a hemochorial placenta, which provides close contact between the mother's blood and the fetal circulatory system. Certain skeletal traits, most notably an eye socket backed by bone, also seem to favor a haplorrhine connection, but they may have evolved independently.

“Most taxonomists today assign tarsiers to their own infraorder within the suborder Haplorrhini, but their unusual combination of traits shows that their lineage branched off long ago from the rest of the suborder. Fos- sils representing Tarsius and closely related genera, found in North America, Africa, and Asia, date as far back as 45 million years, and their lineage is believed to have separated from all the other Haplorrhini as early as 71 million years ago. Strepsirrhini and Haplorrhini diverged perhaps 78 million years ago, not long after the origin of all primates. Consequently, modern tarsiers are pivotal in understanding the roots of primate evolution.

Tarsier Behavior

Tarsiers spend nearly all of their time in trees and are primarily nocturnal. During the day they usually sleep on vertical tree branches or in the hollow of an old tree. They don’t build nests. Vocalizations include loud shrieks in battles over territory and loud duets between separated males and females.

Tarsiers feed mostly on insects such as beetles, ciadas, moths and grasshoppers but also eats small lizards, nestling birds and even snakes. Other primates eat mostly fruits, vegetables and leaves. When it sees prey it adjust its position and suddenly leaps. Lorises and tarsiers are the only primates that eat mostly animal prey. They drink water by lapping.

Tarsiers are found mostly in pairs or groupings of up to eight animals. A tarsier pair occupies a territory of around one to two hectares. They mark their territories with urine and scents from various glands. Even so there seems to be a great deal of overlapping of territories.

Sharon Gursky-Doyen wrote in Natural History magazine, Spectral tarsiers “use their urine and various body glands to scent-mark along the boundaries of their home range; they announce their claim with early family choruses; and they vocally confront and chase any members of neighboring groups that threaten to intrude. They exhibit tremendous attachment to a particular site, with individuals and sometimes family groups continually using the same sleeping tree for years. Like many territorial primates, spectral tarsiers return to the same tree when it is time to sleep. They prefer hollowed-out fig trees with multiple entrances and exits. These typically form when a "strangler" fig tree grows around another tree, kills it, and the dead supporting tree rots away, leaving an empty space. In measuring the diameters (at "breast height," 4.5 feet aboveground) and heights of their sleeping trees, I was able to demonstrate that individuals residing in larger sleeping trees were more likely to be found at the same site in later years, while individuals residing in smaller trees were more likely to move. I also discovered that po- lygynous groups were more likely to have the larger sleeping trees. [Source: Sharon Gursky-Doyen, Natural History magazine, October 2010]

Tarsier Family and Social Behavior

Tarsier have a gestation period of around six months, unusually long for such a small animal.Young are born with their eyes open and are capable of climbing and making short leaps. Female young often stay with their parents until they are adults. Males usually leave when they are juveniles.

Sharon Gursky-Doyen wrote in Natural History magazine, “A majority of the sexually mature adults are monogamous, and mates often stay together for most of their lives, which average seven years. With their immature offspring (as many as two per female) they occupy home territories in small family groups. Although direct paternal care is rare, it is common for a member of the group other than the mother — typically an adolescent sister of the infant — to help with the caretaking. Examples of such "allomothering" include sharing food, babysitting, grooming, and playing. An adolescent female will also transport a young infant by mouth if, say, it falls out of a tree where the mother parked it. [Source: Sharon Gursky-Doyen, Natural History magazine, October 2010]

“How might those patterns of behavior have evolved? When I surveyed the primate literature, I found that three main factors had been hypothesized to lead to sociality, or gregariousness, in primates. One is intanticide: if outsiders of their own species pose a threat, relatives stick together to defend their offspring. Another is food abundance: the patchier the distribution of food in the habitat, the more a group may need to come together to share and defend their resources. And finally there is pre dation pressure: members of a group cooperate to warn and defend against common enemies.

“Infanticide has been observed in captive tarsiers, but does infanticide — or the threat of it — play a role in wild tarsier sociality? I kept track of how much time males spent near females, noting whether or not the females were pregnant or lactating. When a female was lactating — that is, had an infant — the average distance between the male and female of a pair was significantly less than when the female was pregnant or at some other point in her cycle: 85 feet versus 135 feet. By remaining near and traveling with the female and the new infant, her mate could prevent neighboring males from getting too close and killing the infant.

“Given the exceptionally large prenatal investment tarsier females must make, it is not surprising that males inust help protect the infant. Newborn infants weigh about a third of the mother's weight — imagine a 120-pound woman producing a 40- pound baby! However, the presence of an infant only explained a small proportion of the gregariousness exhibited by spectral tarsiers, since the majority of social interactions did not involve infants.

“I therefore began examining the role of food abundance. I found that individual tarsiers were more likely to remain near other group members when insect abundance was high rather than low: the average distance between group members was 87 feet compared with 175 feet. Although the level of sociality was increased by food availability, as it was by the presence of an infant, it did not even come close to the coordinated mobbing behavior” when a predator is present.

Tarsier Mobbing Behavior

Reporting from a mountainside in Tangkoko Nature Reserve on the Indonesian island of Sulawesi, Sharon Gursky-Doyen wrote in Natural History magazine, “All of a sudden I hear high-pitched shrieks from higher up the mountain.... Somewhere a group of tarsiers is upset, and I want to know why. As I get closer to the commotion, I slow down, not knowing what awaits me. Cautiously, I scan the foliage for tarsiers and for whatever threat has caused them to call with such urgency. Then I see it: a large python coiled up in a tight ball. Four, five, no, six spectral tarsiers — each no bigger than my hand — are sounding the alarm. And they are all leaping toward the python. [Source: Sharon Gursky-Doyen, Natural History magazine, October 2010]

“The tiny tarsiers repeatedly lunge so close to the intruder that I think they are about to become snake dinner, and then they leap out of reach. One individual is truly brazen: he jumps onto the python's back and bites it! The snake's muscles ripple as it tries to capture and strangle the animal on its back. But the daring tarsier is too quick, and darts away. For nearly thirty minutes, the tarsiers lunge and retreat; even the individual I was following earlier arrives to join in the mobbing. Finally the python uncurls — it must be twelve feet long — and slithers away. After calling for another twenty minutes the tarsiers move off But they remain skittish throughout the night, breaking out into alarm calls and frequently returning to the scene of the face-off.

“The spectral tarsiers' mobbing of a predator is a total surprise. What might have prompted such brazen — and coordinated — behavior? I know that a male-female pair and two offspring, a juvenile female and an infant, sleep during the day near the site of the incident. But on my nightly "focal follows," the excursions in which I track the activity of one individual, I rarely encounter more than one or two tarsiers in any one place. Yet I've just seen at least six adults join together in attacking a python. Maybe the species is more gregarious than anyone has realized. And the incident is significant in another way: the vast majority of species known to mob predators are diurnal, not nocturnal. [Ibid]

Tarsier Mating Behavior

Tarsier females go into estrus for one to three days every 18 to 27 days. An unusual characteristic about tarsier mating behavior is why some individuals choose to be monogamous and others polygynous (one male mating with several females). Few species have such a variable mating system.

Sharon Gursky-Doyen wrote in Natural History magazine, “While there are a few primate species that vary in their mating patterns, rarely has the variation been observed within a single population, such as that inhabiting the Tangkoko Nature Reserve. Consequently, I wanted to know what led individual tarsiers to choose monogamy or, much less frequently (about 15 percent of the time), polygyny. In some species, the male's help is required in order to successfully rear offspring, and that favors what Devra G. Kleiman, an ethologist and conservation biologist affiliated with the Smithsonian National Zoological Park and the Uni- versity of Maryland, has called "obligate monogamy." But spectral tarsiers don't provide much direct paternal care, so that is not a factor. Why, then, were polygynous groups not more common? [Source: Sharon Gursky-Doyen, Natural History magazine, October 2010]

“Groups that were fortunate enough to possess territories with large fig trees for sleeping sites were significantly more likely to be polygynous than were groups whose sleeping trees were smaller or of another species. While monogamous groups consistently used only one sleeping site, polygynous groups tended to have multiple sites, giving them more options if something were to happen to one of their sleeping trees. That is a significant issue, because tree falls are frequent, owing to the high winds at Tangkoko Nature Reserve and the diffuse root structure of the fig trees. Although fig trees are fairly common within the reserve, those making the best sleeping sites are relatively rare, which is why polygyny is so much less common than monogamy.

“In choosing a mate, a female spectral tarsier apparently looks not only for a male whose mobbing displays demonstrate his readiness to defend her and her offspring against predators, but also, where possible, for one whose territory includes at least one high-quality sleeping site. Why would such a sleeping site — namely a large, hol- low fig tree — be so important? One possible ex- planation is that its numerous entrances and exits provide more avenues of escape if a predator invades.

“My observations of spectral tarsiers suggest that polygyny — one form of sociality — may have arisen in primates when females chose to be with a male that controlled the best territory. A safe sleeping site could be one measure of the "best" territory, but that is only an example. And the tarsiers' mobbing behavior may be comparable to the way their ancestors and other early primates responded to predators. Snakes are persistent predators of modern placental mammals, and according to Lynne Isbell, an anthropologist at the University of California, Davis, they may have been major driving forces of evolutionary change in mammals. Their ability to hunt, moving silently even in the trees , was and remains a major threat to primates. Mobbing behavior may have evolved as a survival tactic in the face of that threat and, in turn, been a major leap toward group living.”

Tarsier Species

Phillippine tarsier

The western tarsier lives in primary and secondary forests of hilly lowlands and swampy plains in Borneo and southern Sumatra. It weighs 86 to 135 grams (3 to 4.8 ounces) and measures 12.1 to 15.4 centimeters (4.8 to 6.1 inches) from head to foot. Its tail is 18.1 to 22.4 centimeters (7.1 to 8.8 inches). Its numbers are unknown. Its habitat is threatened by logging and palm oil plantations. [Source: Canon advertisement in November 2011 National Geographic]

The western tarsier has huge eyeballs and is nocturnal. A single eyeball is larger than its brain. A determined hunter, it consumes one-tenth of its body weight each night and uses sound to locate prey such as insects, spiders and other arachnids and small animals. It captures many kinds of prey by leaping on them and is capable of covering 40 times its body length in a single pounce. It can also turn its head 180 degrees like an owl.

Philippine Tarsier

One rare animal that is found in Bohol that is found in few other places in world is the tarsier, the smallest primate in the world. It is a nocturnal primate with a body measuring from 10 to 13 centimeters and tail that is longer than its body. The pint-size primated can be seen in their natural habitat in the hills around the town of Corella.

The Philippine tarsier is the world’s second smallest primate after the pygmy tarsier in Sulawesi. Found on the Philippines island of Bohol, the Philippine tarsier has a long tail and large eyes and is about the size of a kitten. They are nocturnal creatures and live primarily in second growth forests. The Philippine tarsier is seriously endangered. They are hunted and sold as pets. Their habitat is shrinking quickly. The Philippine Tarsier Foundation has bred and released several dozen tarsiers.

The Philippine tarsier is just 10 centimeters (four inches) tall, weighing 120 grams (four ounces), with a rat-like tail, bat-like ears, and giant eyeballs, each one as big as its brain. The tarsier is nocturnal, lives in the forest, and is highly sensitive to daylight, noise and human contact. [Source: AFP, December 12, 2011]

Different species of tarsier are found in the Philippines, Brunei, Indonesia and Malaysia. And populations in all these countries are listed by the International Union for Conservation of Nature (IUCN) as “decreasing”. The Philippine tarsier, or tarsius syrichta, is categorized as “near threatened”, while species in other countries are already “vulnerable”, “endangered” and “critically endangered”. There are only several hundred tarsiers left living in the wild on Bohol, according to the Philippine Tarsier Foundation.

Suicidal Philippine Tarsiers and Tarsier Tourism

AFP reported from Bohol: “The tiny creature turns its head slowly through 180 degrees and stares, boggle-eyed as another group of noisy tourists takes its picture from just inches away. Its strange appearance is obvious, but what these tourists may not realize is that their very presence is putting the animal at risk. People go near and they’re loud, or make a picture with the flash, or they’re touching them” and that stresses them out. [Source: AFP, December 12, 2011 \=]

“According to conservationists, if a tarsier becomes stressed it will kill itself by bashing its head against a tree or the bars of its cage. “Most of those tarsiers, when they become stressed they commit suicide," says Carlito Pizarras, known as The Tarsier Man. "They don’t breathe and slowly die. If you put them in a cage they want to go out. That’s why they bump their heads on the cage, and it will crack because the cranium is so thin." \=\

At one tourist place on Bohol, “a guide calls to a group of tourists and points to a tarsier clinging to a tree branch. "Now there you are, I think you are hiding," he says in a loud sing-song voice, before encouraging the group to move closer and take a picture. A sign warns visitors not to use their camera flash, and the guide tells them not to touch. But the tarsiers here live in a thinly forested area, with light seeping through. Their ultra-sensitive eyes are murky, and their movements slow. At this time of day they should be asleep. \=\

“"Before we put them in our hands," the guide says. "But if you touch, they die. They are so very sensitive." These ones have been “domesticated” using cages, he adds, and now they are “tame”. "Try to go closer," the guide says. "They won’t bite." Pizarras argues that tarsiers in the wild are very defensive and do bite — and that these docile animals are not tame but weak.” \=\

Threatened, Stressed Out and Suicidal Philippine Tarsiers

"The tarsier is a superstar but unfortunately it’s suffering because of its fame," Joannie Mary Cabillo, the programme manager at the Tarsier and Wildlife Sanctuary, told AFP. "The government is backing up but not that much. We have a presidential proclamation and laws to protect the tarsiers but unfortunately nobody is sanctioned." The government declared the tarsier a “specially protected” species in 1997, outlawing hunting of the animal, and effectively banning restaurants and souvenir shops from keeping them on display. [Source: AFP, December 12, 2011 \=]

Theresa Mundita Lim, director of the Protected Areas and Wildlife Bureau at the Department of Environment and Natural Resources (DENR), told AFP the indications are that the tarsier population has gone up since then. But she acknowledges that tarsier tourism is a double-edged sword, and more needs to be done to protect the animal. "We can still do more through education and stricter enforcement," she says. "There has to be stricter monitoring, also for tourists. "It’s not just up to us. We issue the policy but the policy needs to be implemented at the field level." \=\

The DENR’s tarsier conservation programme has an annual budget of five million pesos ($115,000). "It’s not enough. But we also rely on social mobilisation," says Lim, adding that it is sometimes concerned tourists who report centres where the tarsiers are not being well treated. People caught breaching the wildlife act can be fined or even jailed, but tarsiers are still found on the black market in Manila for sale as pets, fetching about 6,000 pesos each. \=\

Philippine’s Tarsier Man and His Tarsier Sanctuary

Carlito Pizarras is the field manager at the Philippine Tarsier and Wildlife Sanctuary in Corella on the island of Bohol, one of the country’s most popular tourist destinations. Unlike other tarsier attractions on the island, visitors at the sanctuary are allowed to look, but not to get too close or touch the animals. [Source: AFP, December 12, 2011 \=]

AFP reported: “As a child Pizarras would hunt tarsiers with his father, a taxidermist. The stuffed creatures were best sellers, fetching 250 pesos ($6) a time. Aged 12, he decided to start keeping the animals in captivity, venturing into the forests at night to collect crickets for them to eat and learn about their habits in the wild. He realized how sensitive they were to human contact. By the time he reached his 20s the tarsiers living around his village were becoming scarce. Hunting, habitat destruction and predatory house cats were causing numbers to dwindle. \=\

“Pizarras knew he was also partly to blame, so he stopped hunting them and became a pioneer of conservation. The tarsier became Bohol’s logo, and a big tourist draw. And the Tarsier Man, once derided for his strange behaviour, became a national treasure, even presenting a pair of the animals to Britain’s Prince Charles in 1997. But Pizarras, now in his 50s, believes not enough is being done to protect the creature. It may be a tourist symbol, but more should be done to make it a symbol of conservation. "In other areas they are still putting tarsiers in cages for the tourists," he says. "I don’t know why but the government gives them permits." \=\

“Pizarras says most of these live in the 167 hectares (413 acres) of forest around the sanctuary — elsewhere it is much harder for them to thrive. "In the 70s the population was going down so I decided on my own to stop hunting tarsiers," he says. "My father got angry with me because this was our means of livelihood. "But I said maybe someday my kids and my grandchildren can see them no more, and he understood." \=\

Studying Tarsiers

Sharon Gursky-Doyen wrote in Natural History magazine, “Tarsiers remain shrouded in mystery, because studying their behavior is no easy feat. Not only are they small nocturnal forest-dwellers (bad enough!), but, as I learned early on, some of their most peculiar features make them hard to track. Then, tarsiers have the owl-like ability — shared with no other mammal — to rotate their heads backward 180 degrees. Often when I am out in the jungle tracking a tarsier, it will look in one direc- tion, but then leap the opposite way! That makes it very easy to lose the individual I am following. And unlike the majority of nocturnal mammals — but like all haplorrhines — tarsiers lack the light-reflecting layer of tissue behind the retina known as the tapctum lueiihitii. In low light, that "bright carpet" improves vision and, as a byproduct, renders an animal's pupils visible as "eyeshine." Absent any eyeshine, the strik- ingly large eyes of tarsiers do not broadcast their location as one might hope. [Source: Sharon Gursky-Doyen, Natural History magazine, October 2010]

“When I first began studying tarsiers in the 1990s, they were considered solitary creatures, like most other nocturnal foragers. But when I started tracking them using radio telemetry, I learned that sometimes other tarsiers w^ere not so far away. The conventional approach is to put a radio collar on an individual and track it over the course of one night, picking different nights to watch different individuals. To determine whether tarsiers might be more social than they were reputed to be, I tried a new technique. I would radio-collar a pair of tarsiers and perform "simultaneous focal follows" with an assistant: the two of us would synchronize our watches, each take a radio receiver, and then note our respective tarsier's location every five minutes over the course of twelve hours. So, for example, I might observe a mother while my assistant would simultaneously track her offspring; or we might track two mates this way. Then we would compare our notes. Once we started watching pairs rather than individuals, we , discovered that spectral tarsiers are far from solitary.

“To record insect distribution on the ground and in the air, I began collecting insects by means of pittall traps (holes in the ground), sweep nets (similar to butterfly nets), and Malaise traps (stationary nets named for the Swedish entomologist Rene Malaise).

Studying Tarsier Mobbing

Sharon Gursky-Doyen wrote in Natural History magazine, “As you might imagine, interactions between tarsiers and their predators are relatively rare and difficult to observe. I thus looked for ways to mimic the presence of predators. First, I used physical models of predators, such as carved wooden civets, rubber snakes, and plastic birds of prey; and second, I recorded the vocalizations of predators at zoos and then played them back in tarsier territory. [Source: Sharon Gursky-Doyen, Natural History magazine, October 2010]

“In 74 percent of encounters with a rubber python, the tarsiers alarm-called, and in 42 percent of the incidents, once joined by other individuals, they also mobbed the snake. Such an encounter had a measurable effect on their social behavior throughout the night. The average distance between group members when no rubber snakes were present was 135 feet, but when rubber snakes were planted within the group's territory, that distance shrank by about half, to 67 feet. Upon encountering the model bird of prey (a falcon), their response a little more than half the time was to freeze, on average tor twenty-one minutes. On other occasions they both mobbed and alarm-called. And when raptor vocalizations were played back, the tarsiers responded in 42 percent of the experiments by alarm-calling, and in 38 percent by both alarm-calling and mobbing the speakers.

“The model civet often elicited harsh alarm calls, but it was mobbed in only about 10 percent of the encounters. Thinking the experimental setup might be overlooking the tarsiers' well-developed sense of smell, I organized a new set of tests. I observed the reactions of twenty different adults as each was exposed to four different situations: a wooden civet model covered in civet as their bodies, provide balance when the animals leap in their characteristic upright posture. Below: Asian palm civet is a skillful climber and dangerous to the tree-dwelling tarsiers. The author's experiments show the predator's scent, sight, or call may independently elicit tarsier alarm calls and mobbing responses.

“The results were revealing. The tarsiers never ignored the wooden model with civet urine: it provoked alarm calling every time, and they mobbed it in 77 percent of the encounters. In contrast, when exposed to the wooden civet model without urine, the tarsiers responded with alarm calls 39 percent of the time and with both alarm calls and mobbing 15 percent of the time; during 46 per- cent of the trials, they ignored it. In response to the stick with urine, the tarsiers alarm-called during 93 percent of the trials, but never mobbed; they ignored it in 7 percent of the trials. Unsurprisingly, the stick without urine provoked no response at all.

Explanations for Tarsier Mobbing

Sharon Gursky-Doyen wrote in Natural History magazine, “Mobbing is obviously a risky tactic, yet in both sets of experiments, more adult tarsiers mobbed the ostensible predator than resided in the local territory. What drove other adults to get involved? I observed that adult females regularly attended mobbings, but they were usually pas- sive participants, alarm-calling nearby and watching from a safe distance. The aggressive participants, those lunging at the predator and then retreating, were usually adult and adolescent males. That was an important clue. [Source: Sharon Gursky-Doyen, Natural History magazine, October 2010]

“To my knowledge there were a number of common hypotheses to explain mobbing behavior, all some practical, protective outcome, such as driving away the predator.One rationale, known as "infant silencing," suggests that mobbing distracts predators from young offspring, which learn to remain silent during the exchanges. That hypothesis predicts that mobbing will be restricted to groups with young infants. However, I found that mobbing occurred just as often in groups without immature offspring, thus knocking a hole in that explanation. Another idea is that mobbing instills "site avoidance." That is, individuals will avoid a locale where a predator was previously en- countered and mobbed. But tarsier mothers apparently had no qualms about parking an infant in or near a tree where they were previously exposed to a rubber snake. The data did not support that hypothesis.

“According to the "perception advertisement" hypothesis, the potential prey animals, by openly identifying themselves (in this case through mobbing), inform the predator that it has lost the advantage of surprise. Discouraged, the predator then leaves. Naturally, the opportunity to test that was limited to when an actual snake appeared. But based on preliminary observations, the hypothesis fell flat: there was little evidence that the snake spent significantly less time in the area after being mobbed than when the tarsiers only emitted alarm calls or just ignored it. The same set of observations also rejected the "move-on" hypothesis, which states that because it is discomforted by harassment, a predator entering an area will leave sooner the more intensely it is mobbed.

“Finally, the "cultural transmission" hypothesis states that an individual learns to fear an object when it witnesses other animals mobbing it, and thus learns to avoid it or mob it in the future. However, when studying the response of infants, I found that nursing infants, even in their first week of life, alarm-called when exposed to a model snake, despite never having seen a snake previously. Their awareness of danger from snakes was not culturally transmitted, undermining that hypothesis.

“Because none of the above hypotheses seemed satisfactory, I proposed a new one: spectral tarsiers inob predators as a "costly sig- nal." In effect, the signaler advertises that it can afford to perform an otherwise detrimental act — something that a weaker competitor cannot do as effectively. The classic costly signal is the peacock's tail. The tail makes the bird more vulnerable to predators, but the message to the potential mate is, "I have survived in spite of this huge tail, hence I am fitter." Similarly, while aggregating around a dangerous snake, tarsier males may demonstrate their current physical condition, agility, and speed — and therefore suitability as a mate. According to this hypothesis, the trait of mobbing behavior has evolved in males because it is attractive to females, thus increasing a male's chances of procreating. The driving force is a type of natural selection known as sexual selection.

“Because spectral tarsier groups contain only one adult male, any additional males that show up at a "mob scene" must come from other groups. But I observed that males did not show up at all such events. In 80 percent of the cases, including both experimentally elicited and naturally occurring events, males preferred to join groups that contained adolescent females — they came to impress the gals! By observing the males mobbing, young females can evaluate the ability and willingness of males to pro- tect them and their future offspring against predators.

“Mobbing, then, seems to be a way for a male to get an adolescent female to leave her group and form a new pair. That conclusion naturally made me curious about why tarsiers join groups, leave groups, or remain in their parental group. To this end, I started to explore dispersal — the permanent departure of an animal from its original home.

“Because dispersal involves leaving the protection of a familiar group and territory, an animal that takes the plunge increases its risk of predation and takes a gamble on finding food resources. Therefore the payoff needs to be significant. In mammals, males tend to be the ones to seek new territories. One of the most widely accepted explanations for that is the preponderance of polygynous mating systems — one male siring the offspring of several females in a group. In a polygynous group, the females invest more time and energy in their offspring than the male does. Consequently, they usually have a greater stake in a home range proven to have sufficient resources for successful reproduction, and the males are the ones likely to strike out on their own.

Studying Tarsier Mating Behavior

Sharon Gursky-Doyen wrote in Natural History magazine, “Starting in 1994 and continuing through 2008, 1 tracked seventy-four banded individuals, noting their location relative to their initial sleeping trees. Both sexes proved equally likely to disperse from their natal territories, but males dispersed significantly farther than females, an average of 2,165 feet away, compared with 873 feet for females. One possible explanation for the difference in distance may be that it reduces the chances of inbreeding. [Source: Sharon Gursky-Doyen, Natural History magazine, October 2010]

“I decided to examine possible ecological factors, specifically variation in insect abundance, size of home territory, sleeping trees (size and species), and habitat quality (number of trees, number of tree species, number of large trees). After a field assistant and I spent more than a thousand hours following the movements of adult individuals in ten groups, we were able to conclude that polygyny is not limited by insect biomass, insect abundance, or territory size, but primarily by ac- cess to high-quality sleeping sites — that is, tall, wide fig trees. Real estate ruled!

Image Sources: Wikimedia Commons

Text Sources: National Geographic, Natural History magazine, Smithsonian magazine, Wikipedia, New York Times, Washington Post, Los Angeles Times, Times of London, The Guardian, Top Secret Animal Attack Files website, The New Yorker, Time, Newsweek, Reuters, AP, AFP, The Economist, BBC, and various books and other publications.

Last updated April 2014