The silky sifaka (Propithecus candidus) is a large lemur characterized by long, silky, white fur. It has a very restricted range in northeastern Madagascar, where it is known locally as the simpona. It is one of the rarest mammals on Earth. The silky sifaka is one of nine sifaka species (genus Propithecus), and one of four former subspecies of diademed sifaka (P. diadema). Studies in 2004 and 2007 compared external proportions, genetics, and craniodental anatomy supporting full species status, which has generally been accepted.
The silky sifaka has a variable social structure, and lives in groups of two to nine individuals. It spends most of its day feeding and resting, though it also devotes a considerable amount of time to social behaviors, such as playing and grooming, as well as travelling. Females occasionally take priority over males during feeding. Like other eastern sifakas, it consumes mainly leaves and seeds, but also fruit, flowers, and even soil on occasion. It is a seasonal breeder and only mates one day a year during the start of the rainy season. As with other sifaka species, nonmaternal infant care is common. Group members of all ages and both sexes often groom, play with, occasionally carry, and even nurse infants that are not their own. The silky sifaka vocalizes frequently despite its moderately sized vocal repertoire consisting of seven adult calls. Like all other lemurs, it relies strongly on scent for communication. Males frequently scent-mark on top of scent marks made by other group members, particularly females. Males also gouge trees with their toothcomb (a special arrangement of the bottom, front teeth) prior to chest scent-marking. This chest marking results in males having brown-stained chests, the only visible trait that can be used to distinguish between adult males and adult females.
The species is only found within a few protected areas in the rainforests of northeastern Madagascar, with the majority of the remaining population in Marojejy National Park and Anjanaharibe-Sud Special Reserve. A few groups have also been found in the Makira Forest Protected Area, the Betaolana Corridor, and some unprotected forest fragments. The silky sifaka is hunted throughout its range as no local fady (taboo) exists against eating this species. Habitat disturbance, such as slash-and-burn agriculture (tavy), illegal logging of precious woods (particularly rosewood) and fuel-wood, also occurs within the protected areas where it is found.
Etymology The name "sifaka" is a reference to a common general alarm vocalization given by western dry forest sifakas in which they emit an explosive, hiss-like "shee-faak" call several times in succession. On the east coast, local residents refer to the larger-bodied diademed sifaka as simpona, a name which refers to the species' sneeze-like "zzuss" alarm vocalizations. The calls are emitted in a variety of stressful circumstances such as presence of humans, falling trees, terrestrial predators, and after aggression between group members. The specific name, candidus, is Latin for "white", while the specific name of the taxonomic synonym sericeus is derived from the Greek word for "silk".
Taxonomy The silky sifaka was initially described in 1871 by French naturalist Alfred Grandidier in a formally published letter to French zoologist Alphonse Milne-Edwards. Grandidier's description was based on his own observations north of Antongil Bay in the last few months of 1870. He then named the species Propithecus candidus due to its white color, which he likened to that of Verreaux's sifaka (Propithecus verreauxi), but without the dark fur on its head or the ash-colored spot on the back. The first specimen was obtained in 1872 and provided by a "Monsieur Guinet", a planter from Sambava. The specimen allowed both Grandidier and Milne-Edwards to more thoroughly describe the species based on its skin, coat, and skull. Upon those findings, they changed the name to P. sericeus. Upon further review in 1875, Grandidier demoted the silky sifaka to a variety or "race" of the diademed sifaka. By the time German zoologist Ernst Schwarz standardized lemur taxonomy in 1931, P. sericeus had become a taxonomic synonym for the species, with the original name, Propithecus candidus, taking priority.
Schwarz placed all sifakas into two species, the larger diademed sifaka from the eastern rainforests and the smaller Verreaux's sifaka from the dry forests and spiny forests of the west and south. At the time, both species comprised four subspecies, and the silky sifaka was listed as P. diadema candidus, a subspecies of the diademed sifaka. In his 1982 book Primates of Madagascar (book)|Primates of Madagascar, anthropologist Ian Tattersall upheld this classification. When anthropologist Colin Groves reviewed the taxonomy in his book Primate Taxonomy (book)|Primate Taxonomy in 2001, he also upheld the subspecies status of the silky sifaka because variations in fur coloration between the available specimens suggested converging similarities with the diademed sifaka's coloration. Groves later noted that the coloration of the two species did not overlap, suggesting that the populations were distinct.
In 2004, Mayor et al. showed that despite having a similar karyotype (the number and appearance of chromosomes) of 42 chromosomes (2n=42), the silky sifaka was distinct from the diademed sifaka. This was shown through genetic tests (D-loop sequencing) and by comparing external proportions. For example, the silky sifaka has a shorter tail. Their analysis indicated a closer relationship with Perrier's sifaka (Propithecus perrieri). Russell Mittermeier and colleagues followed by adopting the full species status of the silky sifaka for the second edition of Lemurs of Madagascar in 2006. Although Groves maintained the silky sifaka as a subspecies in the 3rd edition of Mammal Species of the World in 2005, he recognized it as a distinct species in 2007 by acknowledging the work of Mayor et al. and also noting the additional distinction that the silky sifaka has relatively long molar teeth compared to the length of its toothrow. Despite the promotion to full species status, the silky sifaka is still considered to be a member of the P. diadema group of four closely related, large-bodied, eastern-rainforest sifakas. The other three members of this group are the diademed sifaka, Perrier's sifaka, and Milne-Edwards' sifaka (Propithecus edwardsi). The species status of the silky sifaka, as well as other sifakas, does not have universal support: in 2007, Tattersall argued against species distinctions within Propithecus, claiming the decisions were made prematurely.
In 1974, Tattersall spotted what he thought was a color variant of the silky sifaka north of Vohemar in northeast Madagascar. Describing it as such eight years later in The Primates of Madagascar, he not only cited its mostly white fur, but also uncharacteristic traits such as a patch of orange on its crown and tufted ears. It was not observed again until 1986, when a team led by paleoanthropologist Elwyn L. Simons captured specimens for captive breeding and identified it as a new species, named as the golden-crowned sifaka (Propithecus tattersalli) in 1988.
Geographic range and habitat The silky sifaka is confined to a small region of northeastern Madagascar within a strip of humid forest stretching from Maroantsetra in the south to the Andapa Basin and the Marojejy Massif in the north. Marojejy National Park represents the northern limit of its current distribution, although historical sifaka range maps created by Grandidier and Milne-Edwards in the late 19th century show the silky sifaka as far north as the Bemarivo River, north of Sambava. The Androranga River may represent the northwestern range limit within the Tsaratanana Corridor. The southern limit of its range appears to be the Antainambalana River, within the Makira Conservation Site. It is not known if the silky sifaka has ever ranged as far south as the Masoala Peninsula. As of 2009, new observations of a few groups of the silky sifaka in unprotected forest fragments adjacent to northeastern Makira (Antohaka Lava and Maherivaratra) may slightly enlarge the known geographic range of this species. The presence of the silky sifaka has been documented within Marojejy National Park, Anjanaharibe-Sud Special Reserve, the Makira Forest Protected Area, the Betaolana Corridor, and the Tsaratanana Corridor. In 2008, 16 groups were discovered in western Marojejy near Antsahaberoaka.
The silky sifaka tends to be found at higher elevations than any of the other sifaka species and also occupies the greatest range of elevations for the group. In Marojejy National Park and Anjanaharibe-Sud Special Reserve, where most of the remaining groups exist, it is not found below of elevation and not above . However, at its southernmost location in Makira (Andaparaty), several groups inhabit forest fragments at an unusually low elevation of . The silky sifaka inhabits three types of elevation-specific habitats: primary montane rainforest, sclerophyllous forest, and the most elevated portions of low ericoid bush. How sensitive the silky sifaka is to disturbance or whether it avoids habitat edges ("edge-intolerant") or is more edge-tolerant like the diademed sifakas is unknown. Like other rainforest sifaka species, it seldom crosses unforested regions between forest fragments.
In the Anjanaharibe-Sud Special Reserve, the silky sifaka is shares the same geographic range as the white-fronted brown lemur (Eulemur albifrons) and an all-black population of indri (Indri indri). In Marojejy National Park, it is sympatric with the white-fronted brown lemur. In 2008, the silky sifaka was suggested to be sympatric with the red ruffed lemur (Varecia rubra) near Maherivaratra and Andaparaty.
Description
The silky sifaka is one of the largest sifaka species, with a head-body length of , a tail length of , a total length of , and a weight of . As its common English name suggests, its long, white fur has a silky texture. Not all individuals are completely white: some have silver-gray or black tints on the crown, back, and limbs. The base of the tail ("pygal region") can be yellow. The ears and face are hairless, and the skin may be a mix of pink and black, completely black, or completely pink. The tips of the ears protrude slightly above the fur on the rest of the head. Its eyes have a deep orange-red coloration. Its appearance is distinctive, and since no other sifakas share its range, it is not easily confused with other lemur species.
While it is difficult to distinguish adult males and females in other eastern rainforest sifakas, such as Perrier's sifaka and Milne-Edwards' sifaka, gender in the silky sifaka is easily discerned due to differences in fur coloration of the upper chest. Females have white fur, while males have a large brown patch of fur that results from scent marking with a gland on the chest and throat (the sternal gular gland). During mating season, the size of the "chest patch" increases to cover both the chest and abdomen as a result of increased scent marking.
Behavior Until the 21st century, brief observations and lemur surveys had merely documented the presence of the silky sifaka in special reserves and national parks. More recently, a 14-month study and two short-term studies in Marojejy National Park have revealed previously unknown details about its behavioral biology, communication, and feeding ecology. The silky sifaka has a variable social structure and is known to live in male-female pairs, one-male groups, and multiple-male/multiple-female groups. Group sizes range from two to nine individuals, while the home ranges are estimated to range from , varying in size by location.