Peter Richerson
Robert Boyd
Not By Genes Alone
Considerable evidence suggests that the ability to acquire novel behaviors by observation is essential for cumulative cultural change. Students of animal social learning distinguish observational learning or true imitation (hereafter, plain imitation) from other kinds of social transmission. Imitation occurs when animals learn a novel behavior by observing the behavior of more-experienced animals. Simpler kinds of social transmission are much more common. For example, local enhancement occurs when the activity of older animals in a particular location increases the chance that younger animals will visit that spot and then learn the older animal's behavior on their own. Thus, young chimpanzees that frequently accompany their mothers to termite mounds are more likely to acquire termiting skills than individuals whose mothers never termite. A similar mechanism, stimulus enhancement, occurs when a social cue makes a given stimulus salient to the animal. For example, smelling food particles on nest mates makes Norway rats more likely to sample these foods when foraging. Young individuals do not acquire the information necessary to perform the behavior by observing older individuals in either of these cases. Instead, the activity of others causes them to be more likely to acquire this information through their own interaction with the environment.
Local and stimulus enhancement and imitation both can lead to persistent behavioral differences among populations, but only imitation gives rise to the cumulative cultural evolution of complex behaviors and artifacts. To see why, consider the cultural transmission of stone tool use. Suppose that an early hominid learned, on its own, to strike rocks to make useful flake tools. Her companions, who spent time near her, would be exposed to the same kinds of conditions, and some of them might learn to make flakes, too, entirely on their own. This behavior could be preserved by local enhancement, because groups in which tools were used would spend more time in proximity to the appropriate stones. However, that would be as far as toolmaking would go. Even if an especially talented individual found a way to improve the flakes, say by blunting the back to protect the hand, this innovation would not spread to other members of the group because each individual has to learn the behavior independently, and individual learning is time consuming and chancy. Local and stimulus enhancement are limited by the learning capabilities of individuals, and by the fact that each new learner must start from scratch with only the barest clues from other animals to go by. Imitation allows each new innovation to be added to an individual's behavioral repertoire, because the information about how to perform the behavior is acquired by observing the behavior of others. To the extent that observers can rapidly and accurately use the behavior of models as a starting point, imitation leads to the cumulative evolution of behaviors that no single individual could invent on its own.
Several lines of evidence suggest that imitation is usually,not responsible for protocultural traditions in other animals. First, as we have already said, many socially learned behaviors, like potato washing in Japanese macaques, are relatively simple and could be learned independently by individuals in each generation. Second, new behaviors like potato washing often take a long time to spread through the group, a pace more consistent with the idea that each individual had to learn the behavior on its own, aided only by weak clues of stimulus or local enhancement. Finally, sophisticated laboratory experiments capable of distinguishing imitation from other forms of social transmission like local enhancement have usually failed to demonstrate observational learning, except for the specialized song-learning system of some birds.
Adaptation by cumulative cultural evolution is not a byproduct of intelligence and social life. We say 'monkey see, monkey do,' and use ape as a verb, but in fact monkeys and even apes do not seem to be especially clever imitators compared to humans. The best evidence comes from experiments in which the imitative capacities of children and apes have been compared. Primatologists Andrew Whiten and Deborah Custance designed an artificial 'fruit,' a rugged, transparent plastic box that held treats inside. Experimental participants could open the box by manipulating a latch consisting of either bolts or a pin-and-handle arrangement. The participants were eight chimpanzees three to eight years of age and three groups of children with mean ages of 2.5, 3.5, and 4.5 years. They watched a familiar human demonstrate a specific technique for opening the fruit, and then were allowed to attempt,open it themselves. The experimenters recorded whether the participants used the same technique that they had been shown. By most measures, chimpanzee imitative performances exceeded chance. However, 2.5-year-old children did even better, and older children were dramatically more proficient imitators than the chimpanzees.
Psychologist Michael Tomasello and his coworkers conducted similar experiments in which chimpanzees and children were shown how to use rakelike toots to obtain food that was out of reach. The chimps who watched expert demonstrators were more successful than untrained chimps in using the tool to obtain the food reward, but they did hot imitate the precise method that their demonstrators had used. Children, on the other hand, followed the method they had been shown: Tomasello describes the ape technique as emulation rather than imitation; apes learn that a tool can be used to cause some desired effect by watching a demonstrator, but they don't pay close attention to the details of how the tool is used. Children imitate so faithfully that they persist in using an inefficient technique, one that the chimpanzees usually abandon in favor of the more-efficient alternative. Children aren't smarter than chimpanzees in general, just much more imitative. Taken together, these experiments suggest that social learning in apes and humans is not the same. Children imitate very faithfully, while apes emulate or at least imitate less faithfully.
Although the evidence on hand suggests that most cultural traditions in other animals are not the product of imitation, some caution is in order. Negative results are always difficult to interpret; experiments can fail for many reasons. A recent clear demonstration of imitation by marmosets suggests that better experiments might detect imitation in a wider range of species. Experimental data from bottle-nosed dolphins suggests that they are excellent vocal and motor imitators, consistent with the field evidence. Thus, we don't claim that imitation is unique to humans. However, the current evidence suggests that (1) cumulative cultural evolution is rare, and perhaps absent, in other species; and (2) even our closest relatives, the chimpanzees, rely on different modes of social learning than humans.
So far, we know of no convincing evidence that any other species has a cultural item as complex as a stone-tipped spear. Rudimentary forms of observational learning are certainly present in chimpanzees, orangutans, whales, 'crows, various songbirds, and parrots, but as Darwin put it, a 'great gap' exists between humans and other animals. No other species seems to depend on culture to anywhere near the degree that humans do, and none seem adept at piling innovation atop innovation to create culturally evolved adaptations of extreme perfection. In fact, there is no evidence that humans made tools as complex as a stone-tipped spear until about four hundred thousand years ago.
