Taxonomy
The taxon was first formally identified in 1951 by Jean Théodore Delacour where he described siki as a subspecies of N. concolor (a species placed in Hylobates at that point of time). Since its description it has been considered to be a subspecies of variously N. leucogenys, N. gabriellae or N. concolor. The assignation of N. siki as a subspecies of N. gabriellae was due to the interpretation of a single baculum, but later research indicated that the specimen was part of a different species. It was then assigned to N. leucogenys due to similarities of its song, as well as strong visual similarities between the females of the two species. However, it has been considered a separate species since 2001.
In the north of the range of N. siki, it overlaps or intergrades with N. leucogenys, and based on mtDNA and voices these species are closer to each other than to the remaining Nomascus; some maintain that siki should be regarded as a subspecies of N. leucogenys. A southern population formerly associated with N. siki was described as a new species, N. annamensis in 2010.
Read more about this topic: Southern White-cheeked Gibbon