Etymology
The name "sifaka" is a reference to a common vocalization given by western dry forest sifakas in which they give an explosive, hiss-like "shee-faak" call several times in succession. On the east coast, local residents refer to the larger bodied diademed sifakas as "simpona", a name which resembles their sneeze-like "Zzuss!" vocalizations.
Physical description
The Silky Sifaka is a large sifaka from northeastern Madagascar. It has a head-body length of 48-54 cm, a tail length of 45-51 cm, a total length of 93-105 cm, and a weight of 5 – 6.5 kg. The pelage is long, silky and white, which gives this species its common English name. In some individuals, silver-gray or black tints may appear on the crown, back and limbs, and the pygal region (at the base of the tail) is sometimes yellow. The muzzle and face are bare, the skin a mix of pink and black, with some individuals having all pink or all black faces. The tips of the naked black ears protrude just beyond the white fur of the head and cheeks. This species does not occur with any other sifakas and cannot be confused with any lemurs within its range.
Silky Sifakas exhibit extreme individual variation in partial skin pigmentation loss or leucism. Although all infants are believed to be born with predominantly black faces, with age some individuals lose their pigmentation and exhibit pink faces to varying degrees. The first western explorer to observe silky sifakas (Alfred Grandidier in 1871) believed that silky sifakas were an albino subspecies of diademed sifakas. It is now know that silky sifakas are not albinos.
All individuals possess some skin pigment and no photo-phobic individuals have been observed.
Unlike Perrier's Sifaka and Milne-Edwards' Sifaka, where adult males and females are difficult to distinguish, adult male and female Silky Sifaka can be readily distinguished from one another by the pelage coloration of the upper chest. Adult males possess a large brown “chest patch” that results from chest scent marking with the sternal gular gland. As rates of male chest scent marking increase during the mating season, male chest patches become far larger in size and can cover the entire front torso to the abdomen.
Behavior and natural history
A number of lemur surveys first documented the presence of Silky Sifakas within Marojejy National Park, Anjanaharibe-Sud Special Reserve the Makira Forest Protected Area, the Betaolana Corridor and the Tsaratanana Corridor. More recently, a 14 month study and two short-term studies have examined the behavioral biology, communication, and feeding ecology of silky sifakas in Marojejy National Park. Silky sifakas exhibit the greatest elevational range of any sifaka species and can be found as low as 300m in altitude in the Makira (Andaparaty) and as high as 1875 m in Marojejy. Thus, they inhabit several types of elevation-specific habitats including primary montane rainforest, sclerophyllous forest, and even low ericoid bush at their highest elevations. The silky sifaka exhibits variable social structure living in male-female pairs, one-male groups, and multi-male/multi-female groups. Groups range in size from 2 to 9 individuals. 95% kernel home ranges vary by site from 34 to 47 hectares.
Approximately 25% of the day is spent feeding, 44.4% resting, and the remainder of day is devoted to social behavior (16.8%), traveling, and sleeping. Long bouts of terrestrial play involving adults are not uncommon. Rates of aggression are low, and mainly occur during feeding. Females exhibit feeding priority over males. Like other eastern sifakas, silky sifakas are folivorous seed predators that consume a huge variety of plant species. A recent 2 month study, documented feeding from 76 species across 42 families (mainly trees, but many lianas as well). During this short-term study, the most important plant families in the diet of Propithecus candidus were Moraceae (20.30%), Fabaceae (12.87%), Myrtaceae (12.65%), Clusiaceae (10.13%) and Apocynaceae (9.49%). These four most preferred foods accounted for 37.06% of total feeding time: fruit from Pachytrophe dimepate (16.09%), seeds from Senna sp. (8.43%), young leaves from Plectaneia thouarsii (6.52%), and fruit from Eugenia sp.3 (6.02%). Overall, 52% of feeding time was spent consuming leaves, 34% fruit, and 11% seeds. Flowers and soil were rarely consumed.
Mating is believed to occur on a single day each year in December or January with infants born in June or July. Generally, females give birth to a single infant every two years. Occasionally however, births in consecutive years by the same female have been observed. Infants initially grasp the fur on their mother’s belly and about four weeks later begin to ride “jockey style” on their mothers back. As is typical of Propithecus, all group members interact affiliatively with infants. Grooming is the most frequent form of non-maternal infant care, followed by playing, occasional carrying, as well as nursing in a few remarkable instances. Dispersal has been observed only once when a young adult male immigrated in 2007 aggressively forcing the older resident male out of the group he had been a member of for at least 7 years. Although eastern sifakas generally exhibit male and female group transfer, female transfer has not been observed.
The fossa is the only documented predator of the Silky Sifaka, other than human beings.
No aerial predation attempts by raptors have ever been observed, although these sifakas sometimes stare skyward and emit loud “aerial disturbance” roars in the presence of the large Madagascar buzzard (Buteo brachypterus), which does not eat lemurs, and other small birds. Loud sneeze-like “zzuss!” vocalizations are their second type of alarm call, and are emitted to terrestrial disturbances, in response to lost calls by other group members, and after receiving aggression. Acoustic analyses have revealed sex and individual differences in the acoustic structure of the silky sifaka zzuss vocalization. In sum, adult eastern sifakas have a moderately sized vocal repertoire of about 7 call types. Infants have several specialized vocalizations as well. Despite the relatively small size of their vocal repertoire, some eastern sifakas are highly vocal with high call rates averaging 7 calls per hour per individual in silky sifakas. The most frequently emitted vocalizations are low amplitude, low frequency, tonal "hum" and "mum" vocalizations. These contact calls are used in a variety of circumstances including group movement, affiliation, foraging, and while resting.
As in all prosimians, olfactory communication is well developed in sifakas. Eastern sifakas possess several specialized scent-marking glands that include a sebaceous chest gland only found in males and mixed apocrine-sebaceous genital glands in both sexes. Sifakas do not allomark, as in true lemurs, by directly scent-marking conspecifics. Females scent-mark trees by rubbing their genital glands against trees in a rhythmic vertical motion. Males scent-mark trees in several ways, by rubbing them with their chest gland, genital glands, or a combination of the two. Males routinely gouge trees with their toothcombs just prior to chest marking which leaves long lasting visible marks. Silky sifakas do not eat bark or gum, so such non-nutritive male tree gouging is likely communicative in function. Both sexes often urinate while scent-marking. Although males scent-mark two or three times as often females, female scent-marks are responded to far more often and more quickly than male marks. A one year study found that only 17% of male P. candidus marks are responded to by other group members but 71% of female marks received a response on average within 61 seconds. In both P. edwardsi and P. candidus, male overmarking of a female's mark is the most common response, followed by males overmarking the scent-marks of other males. Male eastern sifakas preferentially use one type of scent-marking, combined chest-ano-genital marking, when depositing an overmark. The high rates of overmarking practiced by male eastern sifakas lead to totem-tree marking in which certain trees are covered with male scent-marks and gouge marks. Extensive scent-marking of the home range border has not been observed in P. candidus or P. edwardsi.
Conservation
According to the most recent IUCN Red List assessment, the Silky Sifaka is "Critically Endangered". It is one of the rarest and most critically endangered lemurs. Silky sifakas are one of four lemurs listed as one of the “World’s Top 25 Most Critically Endangered Primates”. Global population size is estimated between 100 and 1000. Habitat disturbance, such as slash-and-burn agriculture (‘tavy’), logging of precious woods (e.g., rosewood) and fuel-wood, also occurs within the protected areas where they are found. Unlike Tattersalli's sifaka, there is no local taboo or fady against eating this species. Lemur traps are routinely encountered during surveys within silky sifaka habitat.
Illegal logging of precious wood, such as rosewood and ebony, has emerged as one of the most severe threats to Madagascar's northeastern rainforests. Thousands of logs, worth millions of dollars, have recently been confiscated at ports of Vohémar, Antalaha, and Toamasina. Most of these logs were harvested from the two largest protected areas in the region, Masoala National Park and Marojejy National Park. Harvesting these extremely heavy hardwoods is a labor intensive activity requiring coordination between local residents who manually cut the trees, but receive little profit, and a criminal network of exporters, domestic transporters, and corrupt officials who initiate the process and reap most of the profits. The impacts of such selective logging include violating local taboos as well as ecological consequences such as increased likelihood of fire, invasive species, impaired habitat, and loss in genetic diversity.
Local villages adjacent to its remaining protected areas adopted a two-pronged strategy towards Silky Sifaka conservation education. A "cognitive component" was implemented to increase knowledge and awareness through radio interviews, slide presentations, and the disbursement of literature in twelve primary and secondary schools, and an "emotional component" was begun to link Silky Sifaka conservation with positive emotional experiences, with the goal of establishing a psychological connection between the children and the lemur. To do this, groups of children were taken on three-day educational eco-tours in Marojejy National Park. The children's reactions to their up-close observations of the sifakas were "overwhelmingly positive and empathic".
Efforts are underway to expand Anjananharibe-Sud Special Reserve and to link existing parks and reserves through protection of continuous forest "corridors". These include Betaolana between Marojejy and Anjananharibe-Sud, and Makira, linking Anjananharibe-Sud with Masoala National Park to the south. The corridors will provide much-needed habitat for forest-dependent species like the Silky Sifaka. In addition, the corridors would provide the possibility of increasing genetic exchange between populations.
In May 2006 the WWF launched a project to use community-based natural resource management (CBNRM) to save this sifaka and its habitat. The project seeks to increase the education and awareness of local populations and to make them an active part in the management and habitat conservation of the lemur. It focuses on the corridor of Marojejy to southern Anjananharibe in the northern section and the Marojejy-Tsaratanana corridor in the western section. Ideally, the project hopes to transfer management of the lemurs and their habitat to local populations.
Range and distribution
The Silky Sifaka has a very restricted range in Northeastern Madagascar that includes the humid forest belt extending from Maroantsetra to the Andapa Basin and the Marojejy Massif. Marojejy National Park represents the northern limit of its known distribution, although at one time it may have occurred as far north as Sambava. The Androranga River may represent the northwestern range limit within the Tsaratanana Corridor. The Antainambalana River, within the Makira Conservation Site, is currently regarded as the southern limit. Northeastern Makira (Amparihibe, Bezavona) may contain Silky Sifakas, though none have yet been observed there.
This sifaka, rare and localized as it is, can be found relatively easily at Camp 2 (Marojejia) of Marojejy National Park where numerous comfortable bungalows, flush toilets, and covered dining areas replete with full cooking supplies have recently been built at three camps along the established trail to the remarkable Marojejy summit. Two days and nights should be set aside at Camp 2 to ensure finding the main tourist group. It is steep and a reasonable level of fitness is required. Visitors should visit the Information Kiosk in Manantenina, in the morning, to organize their trip. Trips can be arranged in advance through the ANGAP office in Andapa or through several private tour companies. With the recent completion of several bridges, visitors can now drive through the villages of Manantenina and Mandena, reducing the hike to Camp 2 to about four or five hours. Additionally, silky sifakas can be seen in the northwest portion of the park near Doany, but this site remains far off the traditional tourist circuit and requires expedition-level preparations to visit.
It is also possible to see this species in Anjanaharibe-Sud Special Reserve in the Befingotra Forest and within the Makira Forest Protected Area (managed by the Wildlife Conservation Society) in the Andaparaty Forest to the north-west of Maroantsetra along the Antainambalana River. Trips to Anjanaharibe-Sud are best organized through the ANGAP office in Andapa. Neither of these sites have bungalows or any other tourist accommodations, and it could take several days or a week to find P. candidus. It is also possible to find this species in other parts of Anjanaharibe-Sud and Makira, but this is difficult and unreliable.
