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 the Bay of Antongil 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 the 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 "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, anthropologist Ian Tattersall upheld this classification. When anthropologist Colin Groves reviewed the taxonomy in his 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 2nd 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, a 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 Simons captured specimens for captive breeding and identified it as a new species, named as the golden-crowned sifaka (Propithecus tattersalli) in 1988.
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