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A normal day for the chimpanzee colony FS1 at the Yerkes Regional Primate Research Center Field Station consists of mostly eating, sleeping, and playing. Housed as a captive social group, they lead a much less active life than they would otherwise in the wild. Yet lack of land and foliage has not disrupted their basic behavioral activities; juveniles and adults must still learn how to interact with their environment. The FS1 chimpanzees, nineteen individuals ranging in age from 3 years to 37 years, live and learn much as they would in Africa. Young Claus follows around his dominant older brother Socko, observing behaviors around females and challengers to his alpha position. Although only a few years older than three year old Azalea, Missy takes the youngster from her mother, Peony, and “practices” carrying an infant around. These are social mechanisms of learning, similar to those occurring throughout the animal kingdom.
The processes of social learning have fascinated scientists and behaviorists for hundreds of years. Over time, the definitions encompassing transmission of knowledge and behavior have frequently been altered, amended, and disputed. Throughout the debate, however, one thing has remained clear: the presence and behavior of conspecifics can highly influence the behavior and learning of an individual. The unknown variable is “how?”. One proposed mechanism of information transfer is the method of stimulus enhancement. Stimulus enhancement occurs when an individual is attracted to a type of object after observing its manipulation by another individual, but does not necessarily attend to the actions of the individual (Roberts, 1941). Put simply, I may become interested in an object because you are interested in it. This enhanced interest can facilitate the acquisition of a novel behavior in observing conspecifics and play an integral role in the transmission of knowledge between individuals. Not surprisingly, even human infants less than one year old show increased interest in objects that they have seen others manipulate.
The term stimulus enhancement arose from the imitation debate in animal studies. Originally, learning by imitation was thought to be simple, involving little to no rational thought. By the late 19th century, that view was beginning to change, and investigators such as Romanes began to view imitation as a reflection of intelligence and intentionality, demonstrating the continuity between the mental abilities of humans and other animals. C. L. Morgan, strongly believing animals should not be considered intelligent until it can be scientifically demonstrated, introduced the concept of instinctive imitation, challenging the intelligent imitation theory. Thorndike took the budding debate even further and attempted to experimentally demonstrate imitative learning in many different species. With his failure, Thorndike adopted the philosophy that processes of imitation were to be referred to as an explanation for behavior only after alternative explanations had been excluded. By the 20th century, the term imitation had already become ambiguous and problematic. As a result, research during the past nine decades has narrowed the definition of imitation and introduced an array of related mechanisms for social learning. Imitation is now generally considered to be “learning that is mediated by cognitive process” (Zentall, p.221), while terms such as local enhancement, stimulus enhancement, social facilitation, and social contagion are considered largely non-cognitive.
All social learning mechanisms, including stimulus enhancement, are dependent on the types of interactions among members of a social group and the structure of the social group. For example, behaviors could spread among members of the same sex, age, or kin group, or from individuals of high social status to those of low social status. A famous example of social effects on behavior propagation is the sweet-potato washing of Japanese monkeys. The washing behavior originated with Imo, a 1-1/2 year old female, and within 5 years spread to over 75 percent of the monkeys aged two to seven. Yet only eighteen percent of the adults acquired this new behavior. Of those adults, none were male, and one of the females was Imo’s mother. Thus, those individuals that associated most closely with one another learned the behavior.
Japanese monkeys are not the only animals in which sex and age are important factors for transmission of a new behavior. Zuberbuhler, for example, allowed a dominant male long-tailed macaque (a monkey native to Asia) to rake food into his enclosure with a stick, while other group members looked on. The other group members who subsequently changed their own behavior were either males or members of the dominant male's family. Similarly, Petit and Thierry studied stone use in baboons and found that age and sex class were the most important factors for spread of the behavior. Transmission of the behavior among kin was not as apparent, and this is likely a consequence of the social structure, which is not centered around kin so much as it is in macaque monkeys. For each of these primate groups sociality among individuals of the same age group is important to social transmission, yet many other animals don’t necessarily show this age preference. Adult Mongolian gerbils, for example, form monogamous pairs, living with juvenile offspring for long periods of time. As a result, individuals learn food preferences based on familiarity or kinship, but not sex or age. For hens, social status may be important in the transmission of new behaviors. More hens on average watch dominant demonstrators and exhibit more apparent social learning effects.
Investigators at the Yerkes Regional Primate Research Field Station have recently designed a study intended to examine how the social relationships within a captive group of chimpanzees impact stimulus enhancement. The chimpanzees were given two identical objects and allowed to freely explore them. It was expected that the chimps would show more interest in the activities of their kin than of non-kin, and that adults would garner more attention than juveniles. However, this was not the case. Different individuals varied widely in who they attended to the most, and there was no evidence that one type of individual (e.g., kin) was a more attractive model than another (e.g., non-kin). Although kinship and age did not appear to influence who received the most attention, these variables did appear to influence who was able to gain access to the objects of interest. Examining the transfers between individuals, we found that juveniles and individuals related to the possessor obtained one of the objects more often than would be expected if the objects were randomly transferred among individuals. For learning to occur, which animals have access to both the information and the supplies is extremely important. While it appears that the chimps have no bias beyond individual preferences for observing, the possessor appears to demonstrate a bias for who may obtain the manipulated object.
With the toss of the experimenters arms, the two test objects fly into the chimpanzee arena and are abruptly nabbed by Georgia and Natasha. Georgia is quickly joined by her daughter Liza and her niece Tara. Maintaining close proximity, Liza rarely leaves her mother’s side, keeping an eye on the highly desired object. The moment Georgia releases the plastic jug, Liza runs off with her prize. Meanwhile, Tara, still interested in the milk jug, races after Liza. In pursuit, Tara spots the other milk jug, abandoned by Natasha in the corner. Without a moment’s hesitation she sprints toward the second object and quickly becomes possessor of what she had seen to be extremely desirable and interesting. This is how stimulus enhancement works and why it is so important to social learning. An unconscious interest in the objects that others manipulate provides a perfect medium for trial and error learning about what is most important in the environment. Although Tara now holds a different object than the one she had been watching, the stimulus remains the same. Similarly, in the wild, stimulus enhancement may provide the mechanism for transmission of stone tool use and termite fishing. The extent of learning in such complex social animals has only begun to be understood.
Referenced Articles:
- Roberts, D. 1941. Imitation and Suggestion in Animals. Bulletin of Animal Behavior, 1(3), 11-19
- Zentall, T. R. 1996. An Analysis of Imitative Learning in Animals. In: C. M. Heyes & B. G. Galef, Jr. (Eds.), Social Learning in Animals: the Roots of Culture. (pp. 221-243) San Diego: Academic Press, Inc
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