The chimpanzee grapevine

Victoria Horner and colleagues (2006) set up two "diffusion chains" of chimpanzees, to see if a learned task could be transmitted faithfully from one chimp to another for several iterations.

Using a powerful three-group, two-action methodology, we found that alternative methods used to obtain food from a foraging device ("lift door" versus "slide door") were accurately transmitted along two chains of six and five chimpanzees, respectively, such that the last chimpanzee in the chain used the same method as the original trained model. The fidelity of transmission within each chain is remarkable given that several individuals in the no-model control group were able to discover either method by individual exploration. A comparative study with human children revealed similar results. This study is the first to experimentally demonstrate the linear transmission of alternative foraging techniques by non-human primates. Our results show that chimpanzees have a capacity to sustain local traditions across multiple simulated generations (Horner et al. 2006:13878)

Essentially, they trained one individual in each of two chimpanzee groups to open a box with a reward inside -- but there were two ways to open the box, and each of these models was taught a different method. Another chimpanzee was given a period of time, with several trials, to observe one of these models opening the box. Then when the learner acquired the method, another chimp became the learner observing the second. And so on.

They found that the two methods were transmitted essentially intact across as many chimpanzees as they tried, with a couple of limits -- some chimpanzees couldn't be paired as model-learner pairs because they were aggressive toward each other, and some just didn't watch the model and learned the task independently. As the paper notes, these cases are interesting because they present limits on the ability of groups to maintain such traditions:

Side branches occurred in both chains because either the model was unsuccessful/unmotivated (RN in FS1, AM in FS2) or aggression occurred bet ween the model and observer (KT to BO in FS1, CY to VV in FS2). The latter highlights the importance of tolerance and reinforces the hypothesis that opportunities for social learning in the wild may be restricted by the level of tolerance between individuals (50) and that not all individuals within a population may be good models for social learning (51) (Horner et al. 2006:13881).

For me, this study helps to clarify some of the constraints on social learning:

It is not the function of diffusion studies to dissect in depth the underlying mechanisms of transmission, although these must be sophisticated enough to ensure the replication of behavior across the generations. However, some limited inferences are suggested by the contrasts deriving from the three-group design. The no-model control condition indicates that for about half the chimpanzees (and children), opening an object like the Doorian fruit can be said to be within their untutored competence. Therefore, it seems reasonable to assume that half the participants from the diffusion chains would also have been able to open the Doorian fruit in a control condition. Their exclusive use of only one of the two available techniques may represent a form of "canalization" (46), whereby a chimpanzee's potentially limitless exploration of a problem is focused around only a subset of behaviors that they see performed by others. Similarly, it is likely that half of the participants in the chains would have failed the control condition, and hence their behavior suggests a more complex social learning mechanism, such as emulation or imitation (28), but further experiments will be required to establish this (Horner et al. 2006:13881).

Of course this is an artificial situation -- with two possible solutions -- but it suggests several of the balancing factors in the learning and transmission of natural behaviors. Simple things ought to be transmitted very readily. But then, if an adaptive behavior is really that simple, then maybe it should be genetically assimilated. One wonders also whether the chimpanzees who failed to pick up the behavior independently would have ever been able to figure it out, or whether they would just remain oblivious to an adaptive resource in the wild. There is also, after all, a possible reason to be bad at trying new things -- that being, that sometimes new things will kill you.


Horner V, Whiten A, Flynn E, de Waal FBM. 2006. Faithful replication of foraging techniques along cultural transmission chains by chimpanzees and children. Proc Nat Acad Sci USA 103:13878-13883. PNAS online