Members of the species are not a uniform colour; unweaned juveniles are a light brown, turning to black after weaning.

[2] Within its original distribution, the species is still common in the remaining large forest blocks in Laos, but in Vietnam the populations are scattered due to human encroachment on their habitat for logging and farming.

[2] Hunting for food, traditional medicine and the pet trade is a serious threat to this species in both Laos and Vietnam.

[2] 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 time).

[2] 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.