Geographic Distribution and Habitat
The Siberut macaque (Macaca siberu) is endemic to the island of Siberut, located about 100 miles (160 km) off the western coast of Sumatra. Siberut is the largest of the four islands in the Mentawaian archipelago and part of the biodiversity hotspot known as Sundaland, which is composed of Malaysia, Brunei, Singapore, and the western half of Indonesia. Of all the biodiversity hotspots in the world, Sundaland has the most endemic mammal species—a whopping 173—of which 81% are considered threatened according to the International Union for Conservation of Nature. The large number of endemic species is likely due to the fact that the islands drifted from Sumatra 500,000 years ago and are now separated by the Mentawei basin. Surprisingly, studies have shown that the mammal communities on these islands are more genetically similar to those of Borneo, Java, and Malaysia than to nearby Sumatra. Siberut macaques reside over nearly the entire island, with an estimated range of about 1,700 square miles (4,440 square km), and they are found up to an elevation of 980 feet (300 m). They make their homes in almost all of the forest types found on the island: from swamp forests, to mangrove forests, to palm groves, and almost everywhere in between. They require large home ranges as they spend a very large part of their day traveling—the most of any macaque—and they strongly favor continuous, dense forest to live in—an increasing rarity on Siberut.
TAXONOMY IN TRANSITION
Originally believed to be subspecies of the southern pigtail macaque (Macaca nemestrina) and conspecific with the Mentawai macaque (Macaca pagensis), Siberut macaques were elevated to full species status based on genetic and morphological studies in 2003. This research confirmed that, while indeed deserving of their status as a full species, they are closely related to southern pigtail macaques, more so than to the Mentawai macaques.
Size, Weight, and Lifespan
Siberut macaques are about the same size as other macaques. They can weigh up to about 32 lbs (14.5 kg) and are up to 22 inches (56 cm) long, from head to rear. Males are larger than females. In the wild, Siberut macaques are expected to live about 26 years. Captive macaques have been known to live up to 35 years.
The soft or spongy tissue of a plant or fruit, which is usually white or pale in color (i.e., the white part between the skin and fruit of an orange).
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Siberut macaques have a similar body and face shape to other macaques. They have rather large, forward-facing eyes, a long narrow nose bridge, and a mouth that can open wide to reveal their formidable canines. Their hair is a mosaic of shades from light tan, to deep brown, to black. They sometimes have two prominent white tufts of hair on either side of their face. Their long arms and legs are well adapted to a life spent in the forest, allowing them to agilely clamber up a tree or reach up high to pluck a piece of fruit from a branch.
Siberut macaques are primarily frugivorous. Fruit consumption generally accounts for about 75% of their diet, although depending on the season and the availability of fruit, this number can vary from as low as 43% to as high as 96% of their diet. Important fruit trees include rattan and palm tree fruit, as they have long fruiting seasons that can nourish Siberut macaques for a long period of time. They supplement this fruit-heavy diet with other food items such as invertebrates, mushrooms, leaves, and even crabs and crayfish. They may sometimes also consume small amounts of pith, sap, and flowers. Interestingly, in one study, males were observed eating pith much more frequently than females. This may be because only males are strong enough to break palm trees open to access the pith. Females only consume it if they find a trunk that’s already open.
Behavior and Lifestyle
Siberut macaques are diurnal and semi-terrestrial, sleeping in tall trees at night. They are almost always found in dense, continuous forest as opposed to forest edges or clearings. They spend about a third of their time on the ground, which is on the high side for macaques. Time on the ground is usually spent foraging, which they do in small groups of between two and six individuals. They may also raid farms for food, with one “guard” keeping watch to warn the others of danger. One study found that Siberut macaques spend about 67% of their waking time traveling, 15% resting, 12% foraging, 10% eating, and 6% involved in social activities, although this is an average over the year and the activity budget varies significantly depending on the season. Females spend 2.5 times longer foraging than males, who spend more time resting. They have an extremely long daily traveling distance, and they spend more time traveling than any other macaque species.
Because they are usually silent when fleeing, many people have been led to believe that Siberut macaques are very quiet. On the contrary, they are considered highly vocal primates and make their fair share of noise (perhaps more than their fair share!) during the course of their day.
Siberut macaque lives within large groups, up to about 30 individuals, which oftentimes split up into smaller foraging groups during the day. The groups consist of multiple adult males and females and their offspring. Their home ranges are believed to be about 334 acres (135 ha) in size. Both males and females have dominance hierarchies. The highest-ranking females are usually sisters. Males leave their natal group when they reach maturity and start as the lowest-ranking male upon finding a new group. They have to work their way up the hierarchy through competition. Males are dominant over females, although groups of females have been known to attack a male as a group. Females may also be helped by close relatives to attack a low-ranking male due to competition over food.
Aggression is common among males, particularly when a solitary male attempts to join a new group. After aggressive encounters between groupmates, they often display reconciliation behaviors. Dominant females mount subordinate females, and they may also display their tolerance by kissing the subordinate females. In males, the subordinate one mounts his dominant counterpart.
Intergroup dynamics are largely peaceful. Home ranges are large and they usually overlap with other groups. These territories are not defended, although if two groups are occupying the same space at once, one of them, usually the larger, may drive the other out.
Siberut macaques are very vocal, and are known for their early morning vocalizations, which are believed to play a role in social cohesion. They “coo” regularly when foraging, which can be either a long or short call. When frightened or in agonistic encounters, they may squeal, scream, growl, bark, or screech. They also use body language to communicate. Males pucker their lips both to attract females and as an agonistic sign to other males. Branch shaking is also used to achieve this. Low-ranking males often bare their teeth towards higher-ranking males. Touch and olfactory communication is also likely used extensively by Siberut macaques.
There is little information available on the reproductive habits of the Siberut macaque; however, they are likely very similar to their close relatives, southern pig-tailed macaques, of which Siberut macaques were previously considered a subspecies. Siberut macaques are very likely not monogamous, with females instead mating with multiple males throughout their life. If only a few females are receptive to breeding at a time, the highest-ranking male can often monopolize them and prevent other males from breeding with them. When more than a few females are in estrus at a time, the highest-ranking male cannot maintain control over all of them, and lower-ranking males get their chance to breed. A sexually receptive female presents her behind to males to display the anogential swelling that signal that she’s in estrus. The male draws back his ears and juts out his lips before copulation.
After a gestation period of about 170 days, the mother gives birth to a single offspring. Mothers are the primary caregivers, nursing, carrying, and protecting their babies. During their first month of life, the baby is almost never separated from her. When they are about five weeks old, they begin to independently explore their surroundings, which can be dangerous. Not only are young macaques entirely dependent on their mother for food and protection from predators, they are also at risk of being “kidnapped” by other adult females, which is usually done by a higher ranking female to a lower ranking female’s baby. When they reach adolescence at about a year old, they cease nursing but their mother still provides care such as grooming and social support, especially to their daughters. This care can last for the rest of the mother’s life. Females become sexually mature at about 3 years of age, and males at 4.5 years.
Some studies of southern pig-tailed macaques suggest that female rank may be a more important predictor of reproductive success than male rank. Interestingly, higher-ranking females produce more female offspring. From an evolutionary perspective, this may be because female offspring are more energetically expensive than males. They stay in the group longer and nurse more often. But because they typically remain in the group into adulthood, their mother gains a blood-related ally. Lower-ranked females, on the other hand, more often produce male offspring. They are less taxing to raise because they don’t nurse as often and in general require less attention. When the male offspring mature, they leave to find another group, where they can try to increase their rank through competition.
Three other primate species live on the island of Siberut and are sympatric with the Siberut macaque: the Siberut langur (Presbytis siberu), the pig-tailed langur (Simias concolor), and Kloss’s gibbon (Hylobates klossii). Of the four primates, only the Siberut macaque is endemic solely to Siberut—the other species are also found on other islands of the archipelago. Siberut macaques are likely predated by eagles and pythons.
The International Union for Conservation of Nature lists Siberut macaques as Endangered (IUCN, 2015), appearing on the IUCN Red List of Threatened Species. They were previously classed as Vulnerable in 2008, showing that their situation has become more dire. This assessment is based on a population decline estimated at more than 75% over the past four decades (about three generation lengths). In 1980, their population was estimated at 39,000 individuals. Twenty-five years later, it was estimated between 17,000 and 30,000 individuals. In 2015, the population within Siberut’s only protected area, which covers about 40% of the island, was estimated at about 9,000 individuals. Their population densities have varied wildly at the different locations where they have been studied. It has ranged as high as 92.7 individuals/square mile (35.8 individuals/square km) in relatively undisturbed peat swamp forest down to just 14.5 individuals/square mi (5.6 individuals/square km) in Siberut National Park.
Siberut macaques face a variety of threats. They are vulnerable to hunting as well as habitat loss due to commercial logging, forest clearing, and conversion of forests into oil palm plantations. Although Siberut macaques are not a primary food source for the local people, hunting pressure has increased recently. This is likely due to several factors. A more robust road network, built for logging, has provided access for hunters into previously remote and inaccessible areas of the island. The widespread adoption of air rifles replacing bows and arrows has increased the lethality of hunters. And finally, local beliefs and rituals that regulated hunting have slowly dissipated over time, likely due to missionary and government influences. Interviews with local people in 50 villages throughout the island have revealed some shocking statistics: about a quarter of residents actively hunt, and 71% of those target Siberut macaques. On average, each hunter kills two Siberut macaques every three months, equating to an estimated 1,500 to 3,000 killed each year—that’s between 5 and 18% of the entire population! In addition to being hunted for meat, Siberut macaques are also killed as pests. Because the large trees that they need to survive are often logged, they are forced to wander into gardens and coconut groves around human settlements, where they are trapped.
If hunting weren’t enough, approximately 60% of the forest cover on Siberut has been lost, putting the Siberut macaque and countless other endemic species even more at risk. Once considered one of the most important ecological assets of Indonesia, Siberut has unfortunately lost so much forest and habitat that many of its species are on the brink of extinction. The habitat loss is largely due to conversion to oil palm plantations. Palm oil is found in many of the products we consume, from food, to cosmetics, to detergent. It is the cheapest vegetable oil available and nets companies a whopping profit. The downside is that valuable tropical habitat, largely in biodiversity hotspots like Indonesia and Africa, is completely destroyed to create room for oil palm plantations. Despite the existence of much less ecologically damaging alternatives to palm oil, the industry still thrives and habitat continues to be destroyed.
Sibert macaques 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. They do not have strong legal protection in Indonesia because they are a relatively newly described species. Unlike the other three primates on Siberut, they are not protected under Indonesia’s National Species Conservation Strategy. About 40% of their range is covered by the Siberut National Park. Population surveys are needed, particularly in unprotected habitat, to better understand the current status of Siberut macaques.
Other strategies that have been proposed include protecting the Peleonan Forest in northern Siberut, which lacks protection and is home to unusually large populations of primates, education about hunting, the development of economic alternatives for local people so they have options besides selling their land to logging companies, the regulation of air rifles in Indonesia, a campaign to emphasis traditional beliefs surrounding hunting, improving farming techniques to reduce the dependence on primates for protein, and the inclusion of local people in conservation projects and ecotourism on Siberut.
Specific recommendations for Siberut National Park, which curiously has a lower population density of Siberut macaques than even unprotected forests such as the Peleonan Forest, include increasing funding to the park to improve enforcement and train staff in conservation management, promoting outreach activity to engage the local community in the park and raise awareness about the value of the protected habitat, conduct land cover assessments to determine the rate of habitat loss within the park, and perform long-term monitoring of Siberut macaque population trends inside and outside of the park.
- Richter, C., A. Taufiq, K. Hodges, J. Ostner, O. Schülke. 2013. Ecology of an endemic primate species (Macaca siberu) on Siberut Island, Indonesia. SpringerPlus 2:137.
Written by K. Clare Quinlan, July 2022