Mueller's gibbon, Grey gibbon, Borneo gibbon
Hylobates muelleri

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Immature gibbons

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Adult gibbons

Adult gibbons

These gibbons are mostly grey, grey brown or blackish brown. A light brow band is usually present, but the rest of the face ring is usually absent or only slightly paler. Although perviously believed to be very variable, their fur colouration is now known to be quite uniform at any locality. At least three distinctly coloured subspecies are recognized, but I suspect that there may be more:

H. muelleri muelleri (SE Borneo, east of the Barito River) is usually contrastingly coloured with black cap, black hands and feet and with a dark ventral area. Hylobates albibarbis (SW Borneo between the Kapuas and Barito RIvers) may also be of similar colouration but is often more warmly brown than grey. In addition, males in H. muelleri usually have a shorter genital tuft than males of H. albibarbis (2.5 cm vs.5 cm), and the tuft is in most cases blackish, not contrastingly light as in H. albibarbis.

H. muelleri funereus (N. Borneo) is similar to H. muelleri muelleri, but hands and feet are not contrastingly blackish (but individual exceptions are known). In contrast, the northernmost populations have distinctly lighter feet (albeit not white as in H. lar).

H. muelleri abbotti (W. Boreno, north of the Kapuas River) is less contrastingly patterned. Animals are almost uniformly mouse grey, often with little more than traces of a dark cap and a dark ventral area. These grey animals may resemble H. moloch, but the latter is often more silvery grey and its the brow band and chin part of the face ring are usually sharply defined and very light (sometimes white).

Mueller's gibbon (Hylobates muelleri) Mueller's gibbon (Hylobates muelleri)
Adult male "Fridolin", Münster Zoo, Germany, 2 July 1987.
The nominate subsecies usually has dark hands and feet and or at least dark digits.
H. muelleri muelleri		 Adult male, Singapore Zoo, 5 Sept. 1993.
The dark cap, dark hands and dark digits on the feet are particularly contrasting in this individual.
H. muelleri muelleri
Mueller's gibbon (Hylobates muelleri) Mueller's gibbon (Hylobates muelleri)
Adult female, Taman Safari Zoo, Cisarua, Indonesia, 16 Sept. 1998.
H. muelleri muelleri		 Young adult or subadult female, Taman Safari Zoo, Cisarua, Indonesia, 16 Sept. 1998.
H. muelleri muelleri
Mueller's gibbon (Hylobates muelleri) Mueller's gibbon (Hylobates muelleri)  
Adult female "Greko", Maruyama Zoo, Sapporo, Japan, 25 August 2008.
H. muelleri funereus		 Adult male "Tarou", Maruyama Zoo, Sapporo, Japan, 25 August 2008.
Phenotype: H. muelleri funereus, but reportedly suspected of being a hybrid with H. agilis. This gibbon's mother is a pure H. muelleri, shown in the photograph to the left.		  
Mueller's gibbon (Hylobates muelleri) Mueller's gibbon (Hylobates muelleri)
Adult male "Jacque", Banham Zoo, United Kingdom, 14 Oct. 1988.
This male is known to originate from Sarawak. The absence of distinct darkening on hands and feet or digits is typical of this subspecies.
H. muelleri funereus		 Adult male, Singapore Zoo, 5 Sept. 1993.
Animals from the northern part of this subspecies's distribution range often exhibit light hands and feet.
H. muelleri funereus
Mueller's gibbon (Hylobates muelleri) Mueller's gibbon (Hylobates muelleri)
Adult male, Taman Safari Zoo, Cisarua, Indonesia, 16 Sept. 1998.
Light grey brown gibbon with almost no contrasting pattern (e.g. face ring, cap). In this individual, hands and feet are slightly darkened. The mouse-grey individuals of this subspecies sometimes resemble H. moloch. The latter usually exhibits a more silvery grey fur colouration and a more contrasting white goatee beard.
H. muelleri abbotti		 Adult male "Banju" (="Silver"), Rostock Zoo, Germany, 7 July 1988.
Another mouse-grey Mueller's gibbon. This one comes without dark hands and feet and resembles H. moloch even more.
H. muelleri abbotti