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White-cheeked Gibbons
Shelby Alkek
Professor Sanchez
Physical Anthropology
30 November 2016
Alkek 2
ABSTRCT: The initial study of white-cheeked gibbon behavior
INTRO: My interest in gibbons first began on October 25th when we discussed the correlation
between sexual dimorphism and monogamous pairs. I found it highly intriguing that monogamy
could occur amongst non-human primates, and that sexual dimorphism played a vital role in
primate social structure. The curiosity of my egalitarian side was heightened when I learned that
gibbons present nearly no sexual dimorphism, therefore allowing the adult female and the adult
male in the group to exist without any major competition for dominance. Consequently, when the
day came for my visit to the San Antonio Zoological Gardens and Aquarium I had an
anticipatory two- hour drive. I longed to discover if the information I had learned about gibbons
would retain its accuracy when I performed my captive studies.
Through the use of ad libitum sampling, focal animal instantaneous sampling, and scan
sampling I recorded my data on three white-cheeked gibbons inhabiting the San Antonio Zoo. I
arrived at the “Gibbon Forest” enclosure at 12:01 and observed a nuclear gibbon family group
which consisted of an adult female: Maya, an adult male: Mel, and their male offspring:
Harrison. The group allowed for me to easily record engaging data because all three apes were
relatively active during the hours of my visit (according to a zoo staff member).
Lucky for me, the information I had found interesting prior to my visit did present itself
during my observation of the gibbons in the zoo environment. On the other hand, I also observed
something I had not thought about at all before my visit to the zoo. Upon arrival, the three white-
cheeked gibbons were sitting in two square shaped alcoves approximately twelve feet above the
exhibit floor. Within five minutes, all three gibbons had left the two alcoves and had begun
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brachiating around the enclosure. While I was observing the group, I noticed patterns in their
movement, specifically a pattern than Mel seemed to abide by.
Over time I created a hypothesis about Mel’s movement patterns and focused my ad libitum
sampling mainly on Mel to see if the hypothesis had any credibility. Through data collection, I
formulated a thought-provoking hypothesis, learned new information about a fascinating
primate, and reinforced the information that had initially sparked my attention in gibbons.
BACKGROUND FACTS: The species I observed while at the San Antonio Zoo are
scientifically known as Nomascus leucogenys, but are more commonly named white-cheeked
gibbons. White-cheeked gibbons are members of the Hylobatidae family, which means they are
classified as lesser apes. Gibbons are considered lesser apes because they do not have a tail and
they are relatively diminutive in size. An adult female weighs approximately 12.8 pounds and an
adult male weighs approximately 12.3 pounds (Cawthon). Due to the minimal difference in
relative size, white-cheeked gibbons are some of the least sexually dimorphic primates. Their
lack of sexual dimorphism contributes to their group social structure by allowing the adult
female and the adult male to exhibit codominance (Cawthon). An average gibbon group consists
of three or four members. Two of the group members are the codominant parents and the other
one or two group member(s) are their dependent offspring (Harding).
Once the offspring reaches sexual maturity, which is usually around age seven, the same-
sexed parent will become aggressive with their offspring to force the young out of the natal
group. This ritualized aggression decreases the possibility of inbreeding and allows the gibbon
parents to remain monogamous (Cawthon). When a gibbon reaches sexual maturity their pelage
color changes. In males, the fur on the majority of the body remains black, but they grow white
fur on their cheeks. In females, their fur changes completely from the black juvenile coat to the
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whitish yellow shade of the adult female white-cheeked gibbon. Thus making gibbons sexual
dichromic.
White-cheeked gibbons inhabited Yunnan, China for several years, but today it is unlikely
that any of this species remains in China. They do inhabit two countries in Southeast Asia:
Vietnam and Laos (Cawthon). In a status report recorded in 2003, it was assumed that twenty-
seven sites throughout China, Vietnam, and Laos were home to the white-cheeked gibbon
species. Lamentably, the species was only surviving in four of the supposed twenty-seven sites
(Bleisch). Fragmentation and deforestation are the cause of a detrimental amount of deaths in
this arboreal primate species. Most likely, the first extinction of a gibbon species has already
occurred. Furthermore, white-cheeked gibbons are expected to be extinct in both Lai Chau and
