The authors have declared that no competing interests exist.
Conceived and designed the experiments: SY TH MT. Performed the experiments: SY. Analyzed the data: SY. Wrote the paper: SY TH MT.
The evidence for culture in non-human animals has been growing incrementally over the past two decades. However, the ability for cumulative cultural evolution, with successive generations building on earlier achievements, in non-human animals remains debated. Faithful social learning of incremental improvements in technique is considered to be a defining feature of human culture, differentiating human from non-human cultures. This study presents the first experimental evidence for chimpanzees' social transmission of a more efficient tool-use technique invented by a conspecific group member.
The chimpanzees were provided with a straw-tube, and spontaneously demonstrated two different techniques in obtaining juice through a small hole: “dipping” and “straw-sucking”. Both the “dipping” and “straw-sucking” techniques depended on the use of the same tool (straw-tube) for the same target (juice) accessible from exactly the same location (small hole 1 cm in diameter). Therefore the difference between “dipping” and “straw-sucking” was only in “technique”. Although the two techniques differed significantly in their efficiency, their cognitive and perceptuo-motor complexity were comparable. All five chimpanzees who initially performed the “dipping” technique switched to using the more efficient “straw-sucking” technique upon observing a conspecific or human demonstrate the more proficient alternate “straw-sucking” technique.
The social learning mechanism involved here was clearly not local or stimulus enhancement, but imitation or emulation of a tool-use technique. When there is no biologically relevant difference in cognitive or perceptuo-motor complexity between two techniques, and when chimpanzees are dissatisfied with their own technique, chimpanzees may socially learn an improved technique upon close observation of a proficient demonstrator. This study provides important insights into the cognitive basis for cumulative culture in chimpanzees, and also suggests possible conditions in which cumulative cultural evolution could arise even in non-human animals.
Culture in non-human animals is one of the hottest and most debated questions within the social and biological sciences. Putative cultural variants are by definition independent of environmental or genetic differences and are maintained via social learning mechanisms
Chimpanzees, one of our closest living relatives, display in the wild not only an array of different tool-use types but also tool-use techniques that vary among communities
It remains to be examined how this variation in tool-use techniques emerges and how it is maintained within communities
It is also unclear whether or not chimpanzees are able to switch their technique to a more efficient one via social learning. Chimpanzees appear to be conservative when it comes to incorporating novel and more efficient techniques into their behavioral repertoire. Captive studies suggest that when chimpanzees become proficient at employing a particular technique, they stick to this technique even if given the opportunity to observe others demonstrating an alternate more efficient technique
We tested nine captive chimpanzees at the Primate Research Institute, Kyoto University. Each participant was provided with an 18 cm-long silicon straw-tube. This tube could be used as a tool to obtain juice contained in a bottle externally fixed to the panel wall of the experimental booth, and accessible via a small hole (1 cm in diameter). In a pre-test examination, four of the nine chimpanzees performed the “straw-sucking” technique, while the other five adopted the “dipping” technique (
Actually in the scene depicted in this photo, the chimpanzee thereafter retrieved the tube and licked its tip (“dipping”).
When we paired each of the five “dipping” participants with a “straw-sucking” conspecific non-kin demonstrator in the same booth, four of the five participants subsequently adopted the “straw-sucking” technique (
Individual | Paired 1 | Individual | Paired 2 | |||||||||||||||||
Pal | D | D | D | D | D | DS | S | S | S | S | S | S | S | S | S | |||||
Ayumu | D | D | D | D | D | D | DS | S | S | S | S | S | S | S | S | |||||
Puchi | D | - | - | - | - | D | - | - | S | S | S | S | S | S | S | |||||
Pan | D | D | - | - | - | D | - | - | - | - | D | - | - | - | - | - | S | S | S | S |
Mari | - | - | - | - | - | - | - | D | - | D | - | - | D | - | - | - | D | D | D | D |
Note: Individual: condition where each participant was tested individually; Paired: condition where each participant was tested with the conspecific “straw-sucking” demonstrator (1: together in a booth with two juice bottles; 2: separated in two adjacent booths each equipped with a juice bottle); D: “dipping” technique; S: “straw-sucking” technique; “DS”: firstly “dipping” technique, and then “straw-sucking” technique after observing the demonstrator's straw-sucking; -: no try; trials highlighted in grey indicate that a participant observed the demonstrator's “straw-sucking” within a distance of 50 cm.
The chimpanzees appeared to socially learn the tool-use “technique” they observed their partner perform. Both the “dipping” and “straw-sucking” techniques involved the same tool (straw-tube), the same target (juice), and exactly the same location (a hole of 1 cm in diameter drilled into the panel wall). When dipping, the chimpanzees sometimes used their mouth to manipulate and insert the tube into the bottle to dip for juice (
The chimpanzee participants of this study switched their technique to a more efficient one through social learning, although previous studies
Chimpanzees can therefore rely on simpler cognitive mechanisms for cumulative culture than previously assumed. A study of wild chimpanzees indicated that a young chimpanzee invented and modified a novel tool-use behavior based on the existing behavioral repertoire customary of his community
Our results also indicate that clear evidence of cumulative cultural evolution among our closest evolutionary neighbors may be constrained by other factors than their cognitive capacity. It is possible that we are currently unable to appreciate the extent of cumulative cultural evolution in chimpanzees because of the relatively short timescale of studies conducted in the wild. In addition, chimpanzees may infrequently experience ecological and/or social selective factors that would give rise to innovations reflecting improved increments in technology in combination with conditions favorable to the social transmission of the novel behavior
Participants were socially housed chimpanzees at the Primate Research Institute, Kyoto University (KUPRI). The participants spend their daily life with other group members in enriched facilities
All participants, nine chimpanzees in total, had previously taken part in a variety of perceptual and cognitive studies, including experiments which examined their honey-dipping behavior and social learning of tool-use
The chimpanzee participants were tested in an experimental booth (291 cm×192 cm, 200 cm high). In the “individual” and “paired 1” conditions, two juice bottle containers (2 m apart) and two portable translucent silicon tubes (18 cm long, 8 mm in external diameter and 3 mm in internal diameter) were available to the subjects. In the “paired 2” condition, when the subjects were tested separately in two adjacent experimental booths divided by a transparent wall (136 cm×142 cm and 155 cm×142 cm, 200 cm high), each booth was equipped with a tube and a juice container affixed to the panel wall. We first examined individually each participants' spontaneous tube-use behavior. We then selected all five “dipping” participants and one “straw-sucking” demonstrator. The participants were thereafter tested in at most 4 blocks of 5 trials in the individual and the paired conditions alternately (
Supplementary data, with video clips of the two tool-use techniques and observational learning, are available as supporting materials.
The “dipping” technique performed by a chimpanzee Ayumu. Note that he uses his mouth to insert the tube into the bottle. In form, his technique is identical to the “straw-sucking” technique. However, instead of leaving the tube in and retrieving the juice via sucking, he removes the tube and licks the tip.
(MPG)
Close observation and subsequent switch in technique used. Pal (out of sight in the first view) closely observes the demonstrator, then fetches a tube from the floor (out of sight), and then proceeds to suck the remainder of the juice in the bottle container. Pal had just performed the “dipping” technique prior to observing the alternate technique being demonstrated during the same trial.
(MPG)
We thank T. Matsuzawa, M. Tomonaga, C. Martin, and other staff members of the Language and Intelligence Section of the Primate Research Institute, Kyoto University for their help and invaluable comments. Thanks are also due to the Centre for Human Evolution Modeling Research at the Primate Research Institute for daily care of the chimpanzees.