When visitors see Kien Nahn swinging through the gibbon exhibit at the Lincoln Park Zoo, they often don’t notice his arm right away.

The 6-year-old white-cheeked gibbon moves quickly, gripping ropes and branches with his long fingers. He doesn’t look much different from his younger brother Sovann: long limbs, a blur of black fur leaping, swinging and tumbling through the air.

But when zoo patrons look closer, they gasp in surprise. While Sovann swings from branch to branch on two arms, Kien uses only his left. His right arm is a short stump that ends above his elbow.

Kien lost the lower part of his right arm in 2005, after a visitor tossed something towards the mesh of the outdoor gibbon enclosure.

“He reached out through the mesh to pick it up and then reached back in through another opening,” recalls Sue Margulis, the zoo’s primate curator. “He got caught, he got scared and he spun around and did a lot of vascular damage to his arm.”

A surgeon tried to repair the damage but Kien had torn his arteries, veins and ligaments too badly. The safest option was to amputate his arm.

Margulis knew of one other one-armed gibbon in North America, an adult female  in Pittsburgh whose entire arm had been amputated.

“I called Pittsburgh Zoo to find out how she manages, and was told she has no problems,” Margulis said. “That gave us some comfort in knowing that there was a very good chance that Kien would recover normal activity.”

Still, Margulis said, the zoo worried about how Kien would adjust to having only one arm. Gibbons move from tree to tree using only their arms, a kind of movement called brachiation.

“Often you will see other apes move slowly arm over arm through the trees but gibbons swing at great speed,” Margulis said. “This is their primary means of getting around.”

While one-armed swinging is not uncommon among wild gibbons, it is exhausting. Most wild gibbons do it only when holding food or recovering from a temporary injury. For Kien, it would now be his sole method of transportation.

But he adapted quickly, propelling himself with one arm and often using his feet to carry objects and eat. And within the past year, Margulis said, he’s begun to use his stump “to hold objects and to help balance when he’s on the vines and in the trees.”

The evidence for Kien’s success is not only anecdotal. A study co-authored last year by Margulis and two University of Chicago students found that surprisingly, Kien was on average more active than his family members. He also was usually the first to approach new objects.

Not all gibbons may respond to the loss of an arm as well as Kien did. Zoo environments are less harsh than the wild, and no doubt Kien’s controlled habitat enabled him to adapt quickly.

But zookeepers can only control so much. A large part of why Kien succeeded, Margulis said, had to do with the behavior of his parents and his brother upon his return.

Kien’s six-week separation from his family during his recovery was an unnatural situation for a gibbon, she said.

“There was certainly a chance that a 4-year-old gibbon might not be allowed back in the family group by his father,” Margulis said, because his father, Caruso, might see Kien as a competitor for his mother, Burma.

“But that turned out not to be a problem at all,” she said. “In fact, his mother was very excited when he first came back and he was even more excited, having been alone for about six weeks. And his younger brother also appeared to be very happy to have his playmate back.”

Like any other happy family, however, Kien’s must evolve. He is nearing the mating age, and zookeepers are on the hunt for a suitable female.

His handicap “shouldn’t matter at all” to a potential mate, Margulis added. “Obviously it’s not a genetic problem so there’s no risk of it being passed on and he clearly has no problems at all getting around.”
 
His future should be as normal as his present, she said. “Just like any primate child, he will leave his parents and set out on his own.”