Keep Calm and Swing On!

          Have you ever watched the gymnastics events during the summer Olympics, saw how the athletes flipped through the air by hanging onto two measly bars, and thought, “I can totally do that,” on your next trip to the playground? Well, you might fall on your bottom trying to reach the monkey bars, but an ape could do it hands down. Rather, an ape could do it hands up. Meet the awkward-limbed acrobat of the forest: the white-cheeked gibbon.

          The scientific name for the white-cheeked gibbon is Nomascus leucogenys. Its common name might include “northern” to distinguish it from the closely related southern white-cheeked gibbon. Humans, gorillas, chimpanzees, bonobos, and orangutans form a group called the great apes. Gibbons and siamangs make up the lesser apes. Take a look at our new friend’s profile.

iucn-red-list-species-range
Fig. 1. The native range of N. leucogenys spans over southern China, the northern borders of Laos, and a bit of northwest and north-central Vietnam. Conservation status relative to location is shown on the right. Taken from IUCN Red List.

          White-cheeked gibbons grow to be 46 to 64 centimeters, or about 1.5 to 2 feet, tall. Males weigh about 5.6 kilograms and females weigh a bit more at 5.8 kg for an overall range of 12 to 13 pounds for both sexes. Their average lifespan in the wild is about 28 years (Cawthon 2010). Northern Laos, northwestern Vietnam, and the southern part of China is the native range of these apes, among other species of gibbons. The white-cheeked gibbon is almost exclusively arboreal, rarely spending its time on the ground. (In captivity at the Lincoln Park Zoo, the adult mating pair actually spends a fair amount of time on the ground, but the juveniles are usually active higher up in the exhibit.) They live in dense subtropical rain forests far from human settlements at elevations between 300 and 1000 meters above sea level depending on the region (Bleisch et al. 2008). During the rainy season from May to October, fruits are ripe and plentiful. The white-cheeked gibbon is mainly frugivorous but a small portion of its diet consists of leaves, buds, shoots, flowers, and insects. Reliance on these other food sources increases during the dry season between November and April (Cawthon 2010).

sexual-dichromism
Fig. 2. Burma with her son. Photo taken at the Lincoln Park Zoo in Chicago, IL.

          Time to bond with our gibbon over style. Which hair color looks better? Lucky for the gibbon, it has both light and dark hairstyles in its lifetime. All white-cheeked gibbons are born blonde. Their fur progressively turns black over the first two years. As males mature, they retain the black fur and develop the characteristic handsome white-furred chops. Females on the other hand will regain their blonde hair with a cap of darker fur on the crown of their head. This difference in fur coat color between the sexes is called sexual dichromism (Cawthon 2010).

          Now that we are more familiar with the gibbon, we can discuss its love life. N. leucogenys is a monogamous species. A mature female and male will mate, form a pair bond, and protect a territory together. They sing a morning duet daily. Boundary defense may also involve visual displays and physical confrontation. What about after the honeymoon phase? Sexually mature females have a gestation period of about 7 months and can produce one offspring every 2 to 3 years. White-cheeked gibbons are often found in groups of 3 to 4 individuals which is the basic family unit. Juveniles leave the group when they mature at about 6 years old. In fact, the same sex parent will become aggressive, further encouraging them to leave. Offspring need to migrate out to avoid interbreeding (Cawthon 2010).

          So what exactly makes our friend so special? Where do its fun acrobatic abilities come in? Gibbons have a peculiar mode of locomotion called brachiation. They use their extremely long arms to swing through the dense forest canopies, similar to how gymnasts fluidly swing between, over, and under uneven or parallel bars, though gibbons are not as elegant. There are two general forms to describe their movement. Continuous contact brachiation is characterized by a hand-over-hand motion. Think of when you swing along the monkey bars on a playground. As you hang from one bar, you swing forward using your weight and velocity to grab onto the next bar with your free hand.

continuous-contact-progression
Fig. 3. A progression of continuous contact brachiation. (A) Initial state, two points of contact. (B) Lets go of one hand. (C) Forward swing. (D) Hangs onto branch with free hand. (E-G) Moves forward by repeating the release-swing-hang cycle. Screen captures of video, taken at the Lincoln Park Zoo.
ricochetal-brachiation-progression
Fig. 4. A progression of ricochetal brachiation. (A-B) Forward swing with one handed contact. (C-E) Kicks off the wall to propel and direct himself, period of aerial flight, no points of contact. (G) Hangs on to rope. (H-I) Forward swing. Screen captures of video, taken at the Lincoln Park Zoo.

Ricochetal brachiation involves a brief period of aerial flight between points of contact. The gibbon basically flings itself, or ricochets, from one point to another using its momentum. Both modes of brachiation are energy efficient. They allow for quick movement through dense vegetation in order to cover a lot of ground in a short amount of time. When fruits are plentiful, gibbons like to stay local. When fruits are more scarce and spread out during the dry season, swinging under the canopy branches makes foraging easier, faster, and less energy expensive (Bertram 2004).

limb-proportions-2
Notice the length of the right arm hanging. The upper arm is about the length of the trunk of the body. The forearm to the fingertips is about the length of the leg. Photo taken at the Lincoln Park Zoo.

          The morphology of the white-cheeked gibbon is extremely weird compared to humans, but it makes them excellent brachiators. Its arms are much longer than its legs. This lowers the center of mass, keeping it upright and allowing for smooth locomotion when swinging (Bertram 2004). It also has really long fingers that loosely hook around branches instead of grasping them (Gibbon 2016). If you think about humans in contrast, we have longer legs than arms and short toes, which are great traits for walking bipedally.

          Studies have been done on sequencing the white-cheeked gibbon’s genome and teasing apart the DNA. One paper briefly discussed genes that may be associated with adaptations for brachiation. The TBX5 gene is important for developing the forelimb elements. COL1A1 has to do with type I collagen in bones, tendons for muscle attachment, and teeth. The CHRNA1 gene is involved in the production of an acetylcholine (ACh) receptor subunit. The neurotransmitter and its receptor are essential in muscle contraction (Carbone et al. 2014).

          Regrettably, there are not many of these awesome acrobats left in the wild. They are designated on the IUCN Red List of Threatened Species as critically endangered. Deforestation, agricultural encroachment, and road development contribute to habitat fragmentation. In other words, their habitat is being broken up, further reducing their small geographic range and territories. This has implications for fragmenting the populations. If young adult gibbons cannot migrate from their natal group, it will be difficult to find mates and establish their own territory. Hunting is also a serious problem. Gibbons are desired for local consumption, traditional medicinal uses, and are sold into the illegal pet trade (Bleisch et al. 2008). In the past 10 years, researchers attempted to take surveys of the populations in two Chinese nature reserves with no luck. The last time these gibbons were seen at the reserves was in the 1980s with a count of only 36 individuals over 9 groups. The researchers concluded that the species is now functionally extinct in China (Fan et al. 2014). However, a population of over 100 white-cheeked gibbons estimated from an auditory survey (using recordings of their songs to determine population size based on response) was found in Vietnam in 2010.

          Conservation of N. leucogenys, its endangered cousins within the genus, and preservation of their habitat is crucial. Laws are in place to ban harmful logging and illegal poaching, but enforcement is weak and ineffective. Efforts such as the Gibbon Species Survival Plan® coordinated between facilities in the Association of Zoos and Aquariums in the US and overseas helps to combat the population decline in the wild through carefully managed breeding.

          The white-cheeked gibbon is a fascinating organism. It has incredible adaptations that help it live an arboreal life. Brachiation allows for efficient foraging and dispersal in the wild, and makes for a great show for zoo visitors. Please show these tropical gymnasts some love by spreading awareness and appreciation for the white-cheeked gibbon! Keep calm and swing on!


Works Cited

Bleisch, B., Geissmann,T., Manh Ha, N., Rawson, B. & Timmins, R.J. 2008. Nomascus leucogenys. The IUCN Red List of Threatened Species 2008: e.T39895A10272040. http://dx.doi.org/10.2305/IUCN.UK.2008.RLTS.T39895A10272040.en. Downloaded on 30 November 2016.

Bertram, J. E.A. 2004. New perspectives on brachiation mechanics. Am. J. Phys. Anthropol., 125: 100–117. doi:10.1002/ajpa.20156

Carbone, L., Aken, B., Barrell, D., Anaclerio, F., Archidiacono, N., Capozzi, O., & … Campbell, M. S. 2014. Gibbon genome and the fast karyotype evolution of small apes. Nature, 513(7517), 195-201. doi:10.1038/nature13679

Cawthon Lang KA. 2010. Primate Factsheets: White-cheeked gibbon (Nomascus leucogenys) Taxonomy, Morphology, & Ecology . Retrieved from http://pin.primate.wisc.edu/factsheets/entry/white-cheeked_gibbon/taxon. Accessed 2016 November 30.

Endangered white-cheeked gibbon born at Lincoln Park Zoo. 2013, August 26. Chicago Tribune. Retrieved November 30, 2016, from http://www.chicagotribune.com/news/local/breaking/chi-endangered-whitecheeked-gibbon-born-at-lincoln-park-zoo-20130826-story.html

Fan, P., Fei, H., & Luo, A. 2014. Ecological extinction of the Critically Endangered northern white-cheeked gibbon Nomascus leucogenys in China. Oryx, 48(1), 52-55. doi:10.1017/S0030605312001305

Geissmann, T. 2014. Nomascus leucogenys. Gibbon Research Lab. Retrieved from http://www.gibbons.de/main/sound/11leuc.html

Gibbon. 2016. Funk & Wagnalls New World Encyclopedia, 1p. 1.

Largest Population of Critically Endangered Gibbon Discovered in Vietnam. 2011, July 18. Conservation International. Retrieved November 30, 2016, from http://www.conservation.org/NewsRoom/pressreleases/Pages/Critically-Endangered-Gibbon-Discovered-Vietnam.aspx

*All photos of gibbons in this post were taken personally.

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