The diademed sifaka (Propithecus diadema), or diademed simpona, is a critically endangered sifaka endemic to rainforests in eastern Madagascar. Along with the indri, it is one of the two largest living lemurs, with an average weight of and an average length of , half of which is its tail. It has a striking appearance that features a silky coat of brightly colored limbs that contrast with black, white, and gray shades on its torso, head, and tail. Russell Mittermeier, one of the contemporary authorities on lemurs, describes the diademed sifaka as "one of the most colorful and attractive of all the lemurs."
The diademed sifaka is a diurnal, arboreal folivore that seasonally augments its diet with a variety of foods, including seeds, fruit, and flowers. It lives in small, territorial, female-dominated troops of two to nine individuals, which consist of infants, juveniles, and adults of both sexes. Every day, a diademed sifaka patrols the boundaries of its home range with its fellows, depositing scent markings at contested trees along the way. Group confrontations are rare due to its low population density and well-defined territories. Females are seasonal breeders, being receptive to mating for only a few days a year. The diademed sifaka has an abnormally slow life history for a folivorous primate of its size, as adults can live for up to 25 years in the wild, with sexual maturity typically occurring at five years of age. It was previously considered to be the titular subspecies of a broader species that included the other eastern sifakas of the Propithecus genus. As such, "Propithecus diadema" can also refer to a taxonomic group consisting of the diademed sifaka and three other species.
The diademed sifaka is classified as critically endangered by the IUCN Red list and faces existential threats from habitat loss due to illegal logging and slash and burn agriculture. Poaching is another threat to the survival of the species, as many of the taboos that prohibit killing or eating other sifaka species in Malagasy culture do not apply to the diademed sifaka. It is threatened by these issues both within and outside of protected areas. Population recovery is made difficult by its slow reproductive rate, high infant mortality, and low population density, which increase its susceptibility to inbreeding depression.
Etymology The name "sifaka" is derived from the terrestrial alarm vocalization produced by some species of western sifaka. Eastern sifakas instead produce a sneeze-like "vouiff" vocalization in the same context. As such, some scientists instead refer to the species as the diademed simpona, which is adapted from local Malagasy names derived from the sneeze vocalization, including , , and . Early spellings from French sources include and . An alternative etymology relates to , which is the Kadazan name for Hose's langur (Presbytis hosei).Blench, Roger & Martin Walsh 2009. Faunal names in Malagasy: their etymologies and implications for the prehistory of the East African coast. The diademed sifaka is known as the in the Tsinjoarivo Classified Forest, which means "multicolored" and "big" in Malagasy. Its binomial name, Propithecus diadema, was coined by Edward Turner Bennett in 1832 in reference to the white frontal band that encircles its face, which resembles a diadem. Synonyms that have been applied to the species include Macromerus typicus Smith, 1833 and Indris albus Vinson, 1862.
Taxonomic classification The diademed sifaka is a member of the genus Propithecus. It is the titular member of the P. diadema group, which includes the three species of eastern rainforest sifakas and the eastern dry forest Perrier's sifaka (P. perrieri) to the exclusion of the five members of the P. verreauxi group of western sifakas. The diademed sifaka's somatic number is 2n = 42. The X chromosome and 18 of its autosomal chromosomes are metacentric, 14 are submetacentric, and eight chromosomes and the Y chromosome are acrocentric. One of its metacentric chromosomes could have formed from a Robertsonian translocation between two acrocentric chromosomes, resulting in a reduced somatic number compared to the 2n = 44 seen in Milne-Edwards's sifaka (P. edwardsi). Like other lemurs, the diademed sifaka evolved on Madagascar.
Changes in taxonomy The diademed sifaka was first described by Edward Turner Bennett in 1832 based on a skin and skull presented to the Zoological Society of London by Charles Telfair. In his description, Bennett proposed the genus Propithecus in addition to the species diadema, as he believed that the specimen sufficiently diverged from Lemur to warrant such a designation. The following year, Andrew Smith published a description of a diademed sifaka specimen owned by Jules Verreaux in the South African Quarterly Journal, which he named Macromerus typicus on account of its long legs. Auguste Vinson later described the species as Indris albus in an 1862 edition of the Comptes rendus de l'Académie des Sciences using a specimen that was shot by a member of his entourage in what would later become the Analamazaotra National Park. Vinson divided the modern Indriidae family into three genera; a short-tailed group containing his Indris genus and a long tailed group containing a woolly lemur genus and a western sifaka genus. Indris contained two species, there being the modern Indri indri as Indris niger alongside the diademed sifaka. Vinson referred to his I. albus by the local term for the species, ascribing it the common name .
Although its early describers considered it to be a specific taxon, the diademed sifaka would later be subsumed as the Propithecus diadema diadema subspecies of a broader P. diadema species. This classification became widely accepted after it was supported by Ernst Schwarz in his 1931 revision of lemur taxonomy. Schwarz's P. diadema also included the silky sifaka (P. candidus) and Milne-Edwards's sifaka as subspecies. An additional subspecies, the black sifaka (P. d. holomelas), was included in this taxonomy but was later described as synonymous with Milne-Edwards's sifaka after Ian Tattersall found the two subspecies to be sympatric. Schwarz's revision was challenged in the 1980s after karyotypic analysis by Yves Rumpler et al. found genetic differences between P. d. edwardsi and other members of the former P. diadema species. Mireya Mayor et al. proposed elevating the eastern subspecies to specific statuses in 2004 due to their allopatric distribution and high degree of genetic distance. This rearrangement was not wholly accepted by the scientific community; Tattersall argued that pelage variation in the Tsinjoarivo Classified Forest population was indicative of insufficient reproductive isolation between the diademed sifaka and Milne-Edwards's sifaka to qualify as separate species without the phylogenetic species concept, and attributed the revision by Mayor et al. to an inflationary trend in lemur taxonomy. In light of these concerns, Mittermeier et al. defended the elevation of the eastern sifakas to specific statuses in the third edition of Lemurs of Madagascar.
The Tsinjoarivo Classified Forest diademed sifaka population possesses anomalous morphological traits, e.g. varying degrees of melanism in some individuals. Some scientists have considered designating this population as a subspecies of P. diadema. Others have described it as a unique species; the Duke Lemur Center named the Tsinjoarivo population P. marshi in 2001. Another hypothesis posited that the Tsinjoarivo population is a spatiogenetically isolated clade of P. diadema that is clinally related to P. edwardsi that bred with an immigrant population of P. diadema males from the north. Subsequent nuclear genome analysis and field observations indicated that this population exhibits a pelage mutation that is not the result of significant genetic isolation, as individuals with standard pelage colorations have been observed to produce melanistic offspring.
Anatomy and physiology
The diademed sifaka is the largest species of sifaka. It exhibits very little sexual dimorphism, though females tend to be slightly larger and heavier. Males are, on average, long, with females averaging in length. Tails are typically long. The species has rather long legs, with an intermembral index of 64. On average, males weigh , whereas females weigh .
The adult brain weighs 37 g (1.3 oz) and has a disproportionately enlarged motor cortex compared to lemurs outside of the Indriidae family. The skull is positioned at a right angle atop the spinal column, which helps the diademed sifaka maintain an upright posture. As a strepsirrhine, it possesses a tapetum lucidum. Its retinas mostly contain rod cells, and the area centralis of the eye is flat. The mandible is robust for a lemur, such that it resembles that of a simian when viewed from the side. Its teeth have high crowns and the molars have long shearing blades, which allow it to efficiently chew its leafy diet. The molars are also bilophodont, a trait that, among primates, is only seen in Indriidae and Old World monkeys. It has a deciduous dental formula of × 2 = 20, with many of the deciduous teeth erupting before birth. Scholars such as Madeleine Friant and Adolf Remane have constructed the permanent dental formula as × 2 = 30, which is a reduction of typical lemuroid dentition inasmuch as it lacks permanent lower canines (C−). This has been challenged by William Warwick James and Daris Swindler, who each posited that the deciduous lower canines (dc−) are replaced by incisiform teeth that fulfill the function of two of the lower incisors (I2) in the toothcomb. By this model the dental formula is × 2 = 30. The toothcomb consists of four teeth, which differs from the typical six-toothed toothcomb that is seen in lemurs not belonging to the Indriidae and Daubentoniidae families. The diademed sifaka's possession of two, rather than three, premolars is a uniquely indrisine characteristic among lemurs.
Like all other indriids, the diademed sifaka is a hindgut fermenter. It has a long small intestine that allows it to efficiently digest its fibrous, fat-rich diet while also accommodating for simple carbohydrates. The breakdown of the cell walls of ingested leaves occurs in the cecum, which is both enlarged and highly sacculated to facilitate this process. It lacks any adjacent appendix-like structures like those found in some other strepsirrhines. The diademed sifaka has two colons; the first is arranged as an ovoid spiral known as a tortillon. This shape prevents blockages that could result from the diademed sifaka's voluminous, leafy diet. Beyond the tortillon is a second, dorsal colon that connects to the rectum.
The diademed sifaka is an arboreal vertical clinger and leaper. It has well-developed erector spinae, which are attached to the sacrum via an ossified crest. This grants the diademed sifaka additional control of its vertebrae while leaping. While at rest, it holds its knees to its chest as it clings to a tree for support. It can suspend itself by its feet while foraging but otherwise keeps itself upright. It has an enlarged brachioradialis muscle, which stabilizes the elbow joint during suspensory movements at the cost of a restricted range of motion. The diademed sifaka sometimes descends to the ground to eat, play, or cross gaps in forests, wherein it terrestrially locomotes by bipedally leaping. The hands and feet are long and narrow. The digital formula is 4>3>5>2>1 and all digits except the index finger are webbed about the proximal phalanges. The digital webbing is more pronounced in the feet than in the hands.