Dichotomous Key/Hylobatidae

1

 * (a) Completely black, without light pattern - 3
 * (b) Not black or not completely black - 2

2

 * (a) Almost completely black, but with light (whitish or yellowish) elements (facial pattern, genital tuft, hands and feet) - 5
 * (b) Not dominantly black - 10

3

 * (a) Body weight 8–15 kg; large, inflatable throat sac; males with long genital tuft (ca 13.5 cm) - Symphalangus syndactylus
 * (b) Body weight lower than 11 kg; throat sac very small or absent; genital tuft in males short and inconspicuous (less than 6 cm) - 4

4

 * (a) Body weight 5–7 kg; no throat sac; crown fur without crest - Hylobates klossii
 * (b) Body weight 6–10 kg; throat sac very small and visible only when inflated during certain vocalisations; crown fur stands straight up, with prolonged hairs forming a crest - Nomascus concolor (male)

5

 * (a) Crown without crest; body weight 4–9 kg; light cheek beard present or absent - 6
 * (b) Crown fur stands straight up, with prolonged hairs forming a crest; body weight 6–10 kg; light cheek beard - 7

6

 * (a) Fur on hands and feet white - 8
 * (b) Fur on hands and feet not white - 9

7

 * (a) Cheek beard pale yellow or orange, standing out on sides as if "brushed" outwards, with the hairs spreading out from the corners of the mouth like a fan; fur on chest rusty brown, not black - Nomascus gabriellae (male)
 * (b) Cheek beard white, rarely pale yellow, not "brushed" outwards; fur on chest black - Nomascus leucogenys (male)

8

 * (a) White face ring relatively broad and usually complete; no light corona around crown of head; hands and feet white up to wrist and ankle, respectively; genital tuft not white - Hylobates lar
 * (b) White face ring relatively thin, broad only above eyes; light corona around crown of head; hands and feet white only distally of wrist and ankle, respectively; white genital tuft - Hylobates pileatus (male)

9

 * (a) White brow band; body weight 6–9 kg; no light cheeks; distinct goatee beard; long genital tuft (ca 7.5 cm) - Hoolock hoolock (male)
 * (b) White or grey brow band (often reduced in older females); body weight 4.5–7 kg; males often with light cheeks (grey or whitish), females often without these; no distinct goatee beard; males with distinct, but relatively small genital tuft (ca 5 cm) - Hylobates agilis

10

 * (a) Back and limbs pale yellow, yellow, orange or beige brown, not grey or dark brown; black cap usually set off sharply from surrounding lighter fur, with crown fur (including cap) standing straight up body weight 6–10 kg - 11
 * (b) Back and limbs pale yellow, yellow, orange or beige brown, but also grey or dark brown; black cap, if present, with crown hairs lying flat; body weight 4–9 kg - 13

11

 * (a) Fur on chest (and sometimes belly) black or dark brown, contrasting with light back - Nomascus concolor (female), but excluding N. sp. cf. nasutus hainanus or N. sp. cf. nasutus ssp. nov.
 * (b) Fur on chest and belly light, as back - 12

12

 * (a) Face ring usually yellowish (rarely white), often not contrastingly lighter than neck or incomplete; cheek fur usually standing out on sides, as if "brushed" outwards, with the hairs spreading out from the corners of the mouth like a fan - Nomascus gabriellae (female)
 * (b) Face ring usually white and distinctly lighter than neck, often thin, but usually complete; cheek fur not standing out on sides - Nomascus leucogenys (female) or N. sp. cf. nasutus hainanus or N. sp. cf. nasutus ssp. nov. (female)

13

 * (a) Crown fur lying flat and being light beige, contrastingly lighter than dark brown cheeks; body weight 6–9 kg; a thin stripe of white hair starts from cheeks and crossing the face below the eyes goes upwards across the nasal ridge; distinct light goatee beard - Hoolock hoolock (female)
 * (b) Crown fur variably lying flat or standing up, not contrastingly lighter than cheeks; body weight 4–8 kg; no stripe of white hair below eyes and across nasal ridge; with or without distinct light goatee beard - 14

14

 * (a) Cap and ventral area black, both sharply set off from surrounding light grey fur; ventral shield three-cornered, with lower tip reaching genital area; back and limbs light grey or cream; no light face ring; thin white brow band may be present but is usually lacking; long, white temple fringes hanging over black cheek region and ears - Hylobates pileatus (female)
 * (b) Cap and ventral area may or may not be darker than surrounding fur; if these areas are black, they are rarely set off sharply and the ventral area has no distinctly three-cornered pattern; back and limbs light or dark, of variable colouration; with or without light face ring; light brow band usually present; no long, white temple fringes hanging over black cheek region and ears - 15

15

 * (a) White hands and feet; fur colouration variable: pale yellow, yellow, beige brown, hazel, dark brown (or black, see also 8a), but not grey; usually no dark cap and no dark ventral area (if present, these areas are brown or dark brown, not blackish); face ring white and usually complete - Hylobates lar
 * (b) Hands and feet not white; fur colouration variable as 15a, but may also be grey; with or without dark (dark brown or blackish) cap and dark ventral area; face ring only partly white, usually incomplete, often reduced to brow band - 16

16

 * (a) Silvery grey fur colouration; only cap and ventral area may be contrastingly darker (dark-gray or black) in some animals; brow band broad and whitish, usually distinct and sharply set off, pointed laterally of the eyes; distinct, forward projecting, whitish goatee beard - Hylobates moloch
 * (b) Fur colouration variable; grey individuals are mouse grey rather than silvery grey; cap, underparts and limbs may be contrastingly darker (dark grey, blackish brown or black) than back; hands and toes contrastingly black in some animals; brow band variable in both colouration (whitish, light grey, light buff) and extent (broad, thin, absent); no distinct, forward projecting, whitish goatee beard - 17

17

 * (a) Fur colouration variable: mouse grey, grey brown, hazel, or dark brown, but not yellow; feet may be distinctly paler than legs in some animals (but not white as in H. lar); light brow band very variable in its extent, more or less distinct, but rarely missing; usually no distinctly light cheek patches; genital tuft of male very small (ca 2.5 cm) and usually blackish - Hylobates muelleri
 * (b) Fur colouration variable: pale yellow, yellow, beige brown, grey brown, nut-brown, dark brown (or almost black, see also 9b); feet not distinctly paler than legs; brow band usually white, but may be completely absent in older blackish females; males often with grey, whitish or pale brownish cheek patches (often joined under the chin), females often without these; males with distinct, but relatively small genital tuft (ca 5 cm) which may be of contrastingly light colour in some brown animals (intermediate phase), but not contrasting with ventral fur in others - Hylobates agilis