With one misstep and the snap of a trap, Tinka was broken. The 50-year-old chimpanzee’s hands were mangled and left severely deformed and almost useless. Most of the muscles of his left wrist were paralyzed, and he was left with a limited range of movement. His left hand just sat there in a hooked position, all four fingers permanently flexed and incapable of independent movement. His right hand was even worse, with the wrist completely paralyzed.
One out of every three adult chimpanzees from the Sonso community in Uganda’s Budongo Forest has suffered some sort of injury or disability because of the snare traps employed by local bushmeat hunters to catch duiker (a type of antelope) and bush pigs. Commonly fashioned from wire or nylon, they trap the limbs of passing chimpanzees and are pulled tight across the wrist or ankle as the chimps struggle to remove them. The nylon snares can sometimes be chewed off before they cause permanent damage, but the wire snares can stay on a limb for weeks or months and cut off circulation and cause nerve and tissue damage. Tinka’s injuries are by far the worst among the Sonso chimps, and one might assume that he would be an easy target for predators. That he would starve. That nature, red in tooth and claw, would have no place for the bloodied, broken animal. That life would not find a way and Tinka, faced with an obstacle many other animals – including humans – would struggle to cope with, would be gone.
Instead, Tinka hung on, survived and made a name for himself among primatologists.
This was, in part, because he was itchy. As if Tinka didn’t have it hard enough, he also suffered from chronic skin mite problems that caused dry, flaky skin, rashes and hair loss. The limited movement he had in his wrists, hands and fingers made it difficult for him to groom or scratch himself the way most other chimps do, so Tinka came up with his own scratching technique: He grasped a liana vine with his toes, pulled it down or to the side, held it taut and rubbed his head, back and lower body against it.
The actual technique isn’t very elegant or impressive, but it suited Tinka just fine. It’s not the technique that’s got Tinka our attention, anyway, it’s the fact that the technique was Tinka’s own and that the other chimps in the community quickly began aping it.
In recent years, researchers have accumulated many examples of chimp habits that appear to be learned and passed down through generations, suggesting that culture is something we share with other animals, not a "human spark" that separates us from the lower beasts.
Cultural traditions require transmissions, though, so chimpanzees need some means of social learning, a way to pass these habits along. A major hurdle to detecting these transmissions in the wild has been identifying the original model for a habit that has been copied and learned by others.
Tinka and his imitators helped ease this problem in two ways. First, the liana scratching technique was highly idiosyncratic and hadn’t been reported in any other chimps at Sonso or in other chimpanzee communities. For Catherine Hobaiter and Richard W. Byrne, who reported their observations of Tinka in PLoS ONE last year, there was little doubt that the technique was invented by Tinka and that he was the original model. Second, the technique was clearly different from the natural behaviors of able-bodied chimps and served no useful function for them, since they had full use of their hands for grooming and scratching. In Tinka, an animal that seemingly had little to offer, Hobaiter and Byrne found a unique opportunity to observe and identify imitation among chimps in a natural setting.
Hobaiter and Byrne, both from the Centre for Social Learning and Cognitive Evolution at the University of St Andrews in the UK, noticed several chimps using the liana scratch technique while they were collecting and recording data for a different study on chimpanzee gestures, and decided to look into it further. Analyzing the video recordings, they found 21 instances of liana scratching among seven able-bodied chimps between October 2007 and August 2009.
In 13 of the recorded liana scratch instances, involving six different individuals, the technique closely mirrored Tinka's. The able-bodied chimps gripped a liana, applied tension by pulling and rubbed a body part against it side-to-side. Unlike Tinka, the able-bodied chimps often used a hand rather than a foot to pull on the liana. In five other cases, chimps pushed against the liana with the back of the hand or wrist, rather than gripping and pulling it, to get tension. None of these liana scratch instances occurred within the same party of chimps on the same day. There was only one occasion where Tinka was present when an able-bodied chimp used his scratching technique. This was when a juvenile named Zed tried the technique out after watching Tinka employ it, and was caught on video.
Long-term records kept for the other project they had been working on allowed Hobaiter and Byrne to at the opportunities that able-bodied chimps had to learn from Tinka's behavior. They found that for able-bodied chimpanzees 4-13 years of age, using the liana scratching technique was significantly associated with sharing range area with Tinka. Able-bodied chimpanzees that used the liana scratching technique were also recorded in a group with Tinka during more than twice as many hours as chimps who didn’t attempt the technique.
That liana scratching was used only by able-bodied chimps that had the opportunity to observe Tinka's technique, and was then used even when he wasn’t around, was, for the researchers, a clear demonstration of chimpanzees’ ability to pick up a new habit by social learning. Tinka was not actively teaching the other chimps his technique, and Hobaiter and Byrne think that the other chimps learned liana scratching by watching and imitating him. While Tinka coped with his disability for years and his scratching technique wasn’t recently developed, Hobaiter and Byrne noticed that all the able-bodied chimps who copied the technique were young. No older chimps adopted the technique, or they might have and then abandoned it as they got older. Behavioral "fads" have been observed in young chimps before (like mimicking the hunched-over walk of an older chimp) and a natural tendency to copy behaviors, useful or not, is probably important for the cultural transmission of valuable survival skills.
Hobaiter and Byrne thought the chimps were using one of three types of procedural imitation in learning the liana scratch technique.
Impersonation, where the imitator tries to behave as much like the model as possible, resulting in a very close match in specific details of behavior, seemed an unlikely answer. The able-bodied chimps didn’t copy easily observable details Tinka’s technique; he always gripped the liana with his toes, for example, but the able-bodied chimps sometimes gripped or pushed the liana with their hands.
Emulation, where the imitator learns by copying the end results rather than actions, didn’t fit the scenario, either. The "result" of liana-scratching was body maintenance, and there was no evidence Tinka’s technique was more efficient for an able-bodied chimp than simply scratching with a hand or getting another chimp to groom them.
In program-level imitation, the imitator copies the overall organization of motor actions, without necessarily duplicating the precise actions. The "gist" of a behavioral routine is learned by piecing together actions already familiar to the imitator into an arrangement that looks like the behavior being copied. This is seen in humans when children try pronouncing a word they haven’t heard before. The gist is there, but the sounds are different from the model, often with higher-pitched vowels and simplified consonant clusters.
Hobaiter and Byrne thought that this sort of imitation could explain the liana-scratch copying and, together with the chimpanzee's natural tendency to copy actions that it repeatedly sees demonstrated by others, fuel the social learning of behaviors. Watching Tinka, everyone has something to learn. The other chimps pick up a neat scratching trick and we see that the divide between man and beast is even narrower than we thought.
Sadly, Tinka passed away at the end of January, while this story was being written. He died of natural causes unrelated to his snare injuries. The human primates who lived and worked with him at the Budongo Conservation Field Station gave him a fitting eulogy on their blog: "We will all miss Tinka, despite (or perhaps because of) his ornery nature, he was a memorable and much loved character in the Sonso community. His indomitable spirit was an unfailing source of inspiration about the possibilities that are always there if you try hard enough – or are simply too stubborn to recognize the obstacles in the way! RIP Tinka – I’m sure wherever you are there’ll be a good scratching post near-by."
Reference: Hobaiter C, & Byrne RW. (2010) Able-bodied wild chimpanzees imitate a motor procedure used by a disabled individual to overcome handicap. PLoS ONE, 5(8). PMID: 20700527
Images: Chimpanzee by Graham Racher via Animal Photos. Used under a Creative Commons license.
About the Author: Matt Soniak writes about science, history and Bruce Springsteen. He frequently writes explainers for mental_floss, and his work has also appeared in Men’s Health, Philadelphia Weekly, Chemical Heritage and on Yahoo! News and CNN.com. He tweets as @flossymatt and blogs about animal behavior and communication at mattsoniak.com. He lives in Philadelphia with his girlfriend and two cats.
The views expressed are those of the author and are not necessarily those of Scientific American.