Biological anthropologists use the comparative perspective in their efforts to reconstruct human evolutionary history. As our closest living relatives, primates are often used to frame comparisons and to test hypotheses about various human features. A feature (behavioral, genetic, or anatomical) that appears in all primate species is at least initially assumed to also characterize the last common ancestor of those species; features present in only one form presumably evolved at some point after it diverged, and hypotheses explaining the features are developed in the context of unique aspects of the organism’s ecology and anatomy. The large-bodied apes (orangutans, gorillas, chimpanzees, and bonobos) are most closely related to humans, so those species, particularly chimpanzees, are the preferred ones to compare to fossil and living humans. However, comparisons between humans and other more distantly related species are also informative and serve to demonstrate instances of evolutionary convergences (similar selective pressures lead to similar outcomes in distantly related forms) or features that evolved before the ape/human split as our primate or mammalian heritage.
Students of Darwin assume continuity between species, and they use a comparative approach to understand biology, behavior, and cognition of primates. These evolutionary anthropologists predict that few traits, including cognitive ones, will arise de novo—evolutionary precursors are the norm. In contrast, cultural anthropologists have sometimes focused on the uniqueness of the human mind, particularly with respect to language and culture, and assume gaps in the phylogenetic scale.
The dictionary defines cognition as “the act or process of knowing, including both awareness and judgment; also a product of this act.” These constructs are impossible to observe in humans and nonhumans; thus researchers are left with studying behaviors and defining those behaviors as indicators of a particular cognitive function. The methods of observation and the definitions of behaviors should be used consistently across species to increase the validity of comparisons.
Primatologists are scientists who study the behavior, biology, and evolution of nonhuman primates. The field of primatology draws from individuals trained as psychologists, biologists, or anthropologists, and one’s training has a profound impact on research questions asked. Traditionally, primatologists trained as anthropologists studied wild nonhuman primates and used resulting data to model hominids and to better understand modern humans. Primatologists trained as psychologists focused more heavily on cognitive processes, intelligence, and language and usually explored these topics using captive nonhuman primates where experimental conditions are more easily controlled.
Here we trace the quest to understand, however imperfectly, the ape mind. The study of ape cognitive abilities includes research conducted with captive individuals, where more precise control over experimental and rearing conditions are possible, and individuals living in the wild, where relationships between particular cognitive abilities and aspects of the organism’s environment can more readily be explored.
History of Primate Cognition
Studies of nonhuman primate cognition began in the Western intellectual tradition in the early 20th century and were conducted by psychologists. Nadie Kohts studied the perceptual and conceptual skills of a young chimpanzee, Joni, from 1913 to 1916, and compared them to those of her son, Roody. She published her observations in 1935 in Russian in the book Infant Chimpanzee and Human Child, which has recently been translated into English (2002). She used a comparative developmental approach and established a tradition of rearing the research subject in a home setting, which was to be revisited later in the century by other scientists. Wolfgang Kohler, a German psychologist, presented a variety of problems to captive chimpanzees. The chimpanzees had access to materials that, when assembled, could be used to obtain a reward, such as bananas. Kohler described his findings in his 1925 book The Mentality of Apes. The roots of American primatology can be traced to Kohler’s contemporary, Robert M. Yerkes, a psychologist fascinated with the evolution of intelligence. Yerkes explored this subject in captive apes and established what eventually became known as Yerkes Regional Primate Center in Atlanta, Georgia. After Yerkes’s pioneering research, primate cognition continued to be studied in laboratory settings by scientists such as William Mason, Emil Menzel Jr., Duane Rumbaugh, David Premack, and Allen and Beatrix Gardner, among others. The realization that cognition could also be examined in wild populations came slowly, in part as a consequence of the long-term ape research of Dian Fossey, Birute Galdikas, and Jane Goodall. While their projects were not intended to focus on primate cognition, their work demonstrated apes’ complex mental abilities, including long memory, tool manufacture and use, and the use of social stratagems. Sophisticated social and cognitive skills were also emerging from data collected from wild baboons by Barbara Smuts, Jeanne Altmann, Shirley Strum, and Joan Silk and from wild vervets by Dorothy Cheney and Robert Seyfarth. The shift to the study of cognition in wild populations offers exciting opportunities to explore relationships between particular cognitive abilities and aspects of the organism’s environment—that is, to understand the evolutionary significance of a particular mental capacity.
Ethologists use the procedure called “cross-fostering” to study the interaction between environmental and genetic factors by having parents of one species rear the young of a different species. Primate cross-fostering projects date to the 1930s, when Winthrop and Luella Kellog raised the infant chimpanzee Gua for a period of 9 months with their son. In the 1950s, Keith and Cathy Hayes cross-fostered the chimpanzee Viki while attempting to teach her to talk. After four years she was able to say four words, mama, papa, cup, and up. This research demonstrated that chimpanzees cannot speak, leading to the search for other means of testing language and other cognitive abilities of apes.
Allen and Beatrix Gardner cross-fostered the infant chimpanzee Washoe and immersed her in American Sign Language (ASL). In teaching ASL to Washoe, caregivers imitated human parents teaching human children in human homes. For example, they called attention to objects, expanded on fragmentary utterances, and molded Washoe’s hands into the shape of new signs. In a second project, the Gardners cross-fostered four more chimpanzees, Moja, Pili, Tatu, and Dar. All of these cross-fosterlings acquired and used signs in ways that paralleled human children. The size of their vocabularies, appropriate use of sentence constituents, number of utterances, proportion of phrases, and inflection all grew robustly throughout the 5-year cross-fostering process.
In 1979, at the University of Oklahoma under the care of Roger and Deborah Fouts, Washoe adopted a 10-month-old chimpanzee son, Loulis. Human signing was restricted in Loulis’s presence to test whether he could learn ASL from other chimpanzees rather than from humans. Loulis began to sign in 1 week, and at 73 months of age had a vocabulary of 51 signs. Washoe, Loulis, Tatu, and Dar now live together at the Chimpanzee and Human Communication Institute (CHCI) at Central Washington University in Ellensburg, Washington. Current research shows that they sign to each other and to themselves. The chimpanzees initiate conversations and maintain topics. When human interlocutors feign a misunderstanding, the chimpanzees adjust their responses appropriately. The chimpanzees’ patterns of conversation with human caregivers resemble patterns of conversation found in similar studies of human children.
In 1979, Herb Terrace claimed to have replicated Allen and Beatrix Gardners’s cross-fostering project with a chimpanzee named Nim. The young chimpanzee spent 6 hours each day in a classroom while a string of teachers drilled him with questions and demands for signing. If he wanted something, the teachers withheld it until he signed for it. Terrace found that Nim made few spontaneous utterances and often interrupted his teachers. This procedure differed greatly from the Gardners’s cross-fostering project, in which the young chimpanzees were treated like children. Terrace’s failure to create a comparable environment for language acquisition led to Nim’s failures. Later studies showed that Nim made more spontaneous utterances and interrupted less in a con-versational context.
In 1972, Francine Patterson began teaching signs to an infant gorilla, Koko, and later Michael. The gorillas acquired many signs and used them in all of their interactions with their caregivers. At the University of Tennessee at Chattanooga, Lyn Miles taught signs to a young orangutan, Chantek, in 1978. He produced signs spontaneously and combined them into phrases. In 1986, Chantek was returned to Yerkes Regional Primate Center where he was born, and the project continued on a modified basis.
Also in the 1970s, David Premack used plastic tokens and Duane Rumbaugh used Yerkish, both artificial systems to examine grammatical skills in chimpanzees. Later Sue Savage-Rumbaugh attempted to train a bonobo, Matata, to use lexigrams, the symbols of Yerkish. While Matata failed to use the lexigrams, her infant son, Kanzi, who was present during training, did use them. Devoid of face-to-face interaction, these studies reveal little about conversational behavior, but they do demonstrate apes’ capacities to understand language and to respond appropriately to requests.
Knowledge of the Physical World
From studies of how primates interact with their physical world, scientists make inferences about primates’ understandings of (1) object permanence, (2) cause and effect relationships, (3) spatial knowledge, and (4) quantitative abilities, among many other topics. These skills likely evolved as part of a suite of foraging behaviors that, among other functions, facilitate tracking, finding, and extracting food.
Object permanence refers to how an individual responds to an object that an experimenter hides. In the 1950s, Jean Piaget devised a series of tests to show the development of object permanence in stages. In one test, the experimenter places an object under a cover while the child watches. If the child lifts the cover, he or she has achieved Stage 4 of object permanence. Later stages are tested by multiple and invisible displacements. Piaget claimed that Stage 6 required mental representation and is achieved by 18-24 months in humans. Chimpanzees and gorillas have been tested and demonstrated Stage 6 object permanence.
Cause and Effect
Ape tool manufacture and use gives us insight into apes’ understandings of cause-and-effect relations because the tool is constructed or modified to achieve a particular goal. Gorillas, chimpanzees, bonobos, and orangutans use tools in captivity, but only the latter three have been observed using tools in the wild (presumably the gorilla’s leafy diet and great strength renders tools useless in its natural environment). For example, orangutans use sticks to extract honey from bee nests and leaves as gloves to protect their hands from thorny branches. Chimpanzees put leaves on the ground as seats or crumple leaves to sponge water. They use large sticks as clubs and pestles. Furthermore, they use specific tools for specific tasks. For example, in the Tai Forest (Ivory Coast), they use stones to crack hard Panda nuts and wood to crack soft Coula nuts. Some stone anvils have depressions from years of repeated use. Chimpanzees use tool sets to obtain termites from a mound; the different tools are used in a particular sequence to first perforate the mound and then extract the termites. Apes that have had opportunities to observe humans use a tool will use the same tool in the same way. For example, an ex-pet orangutan living at Camp Leakey (Tanjung Puting, Borneo) built a fire with a flint and gasoline, following the tools and techniques used by humans at the camp.
Tests of spatial knowledge include analyses of how individuals navigate through, or “cognitively map,” the environment. In the 1970s, Emil Menzel Jr. hid food in various locations in a 1-acre enclosure while a chimpanzee watched. When released into the enclosure, the chimpanzee foraged for the food in the most efficient pattern using a least distance strategy while moving from one item to the next. Researchers studying wild apes, including gibbons, have found that individuals move efficiently through the forest rather than in random patterns.
Apes’ quantitative skills have been indexed in a variety of ways. They are able to distinguish groupings of more or fewer objects in a set. Some chimpanzees have learned to use Arabic numerals to represent numbers of objects in sets (“3” for three jelly beans) and to count the number of objects in a set. Some chimpanzees have learned to add the numbers in two sets to reach a sum (three jelly beans plus four jelly beans totals seven).
Captive apes produce drawings and paintings using drawing implements, paints, and brushes. The signing chimpanzees at CHCI provide names for their artistic pieces, and their drawings or representations of a particular object are consistent in appearance from one illustration to the next. For example, pictures labeled as “bird” always have a V shape in them. When asked to draw a berry, chimpanzee Moja usually drew a small round object.
Intense sociality is one of the most distinctive characteristics of primates. Species living in large, complex groups might be expected to exhibit behaviors that enhance individuals’ abilities to operate effectively in a variety of social arenas with considerable insight into the likely actions of group mates. A long period of dependence on parents’ care and long lives contribute to this sociality, as individuals have lifelong relationships that may last 50 years or more. Social intelligence is the study of behaviors that occur in the realm of social interactions.
While controversial, it is apparent that primates learn by watching and imitating the behaviors of others in the social group. Young chimpanzees living in the wild spend years observing other chimpanzees cracking nuts with stones or dipping sticks into termite mounds. These foraging skills take years to master, and a chimpanzee may not be fully proficient at them until she or he is 8 years old or older. Field researchers have observed mothers actively teaching their offspring nutcracking skills by careful and slow demonstration. The cross-fostered chimpanzee Washoe modeled signs for her son Loulis, signed on his body, and demonstrated some signs to him, although he learned most of his signs by watching and imitating Washoe and others signing chimpanzees.
Primates form coalitions in which two individuals cooperate and support each other to compete with a third individual. Typically, support goes to a more dominant individual. In these cases, the dominant individual’s high status is reinforced, while the lower-ranking supporter may gain status or improved access to resources. Primates also engage in more risky, tactical coalitions in which both coalition partners will benefit from their relationship if their coalition is strong enough to overturn (immediately or eventually) the dominant individual. If not, they both stand to lose. Among wild chimpanzees, grooming behaviors are sometimes directed strategically at an individual to obtain or maintain his support in future dominance contests. A dominant chimpanzee, Ntologi, consistently separated two less dominant males when they were together or grooming, apparently because their coalition threatened his status. In his study of captive chimpanzees, Frans de Waal described a dominant male’s repeated attempts to break up coalitions between two males ranked below him. Eventually, one of these lower-ranked males displaced the dominant through the assistance of his coalition partner, whose rank was also elevated by the displacement.
In chimpanzee communities, high dominance is not always predicted by mere physical power, but instead is associated with social prowess and the ability to form and maintain coalitions. One chimpanzee at Gombe (Tanzania) famously obtained top rank through his intimidation displays using noisy, empty gas cans pilfered from Jane Goodall’s camp. These examples indicate that strategic skills are important in determining one’s status.
Chimpanzees of the Tai Forest (Ivory Coast) engage in hunting where individuals take cooperative and complementary roles. For example, some individuals block the prey’s escape route while others chase the prey. Again, such data reflect tactical and strategic skills of chimpanzees because they require an understanding of others’ actions and the likely outcome of those actions.
Tactical deception is defined as “acts from the normal repertoire of the agent, deployed such that another individual is likely to misinterpret what the acts signify, to the advantage of the agent.” Byrne and Whiten surveyed primatologists working with captive and wild populations. They obtained 132 descriptions of primate deception that they then categorized into three types. Examples of active concealment include a gorilla covering a play face and a chimpanzee covering an erection. Active misleading occurs when an individual provides misinformation. For example, as part of usual experimental procedures designed to test spatial knowledge, chimpanzee Belle alone saw the location of food hidden in the enclosure. Rock, a more dominant chimpanzee, often took the food from her when they later were released together in the foraging trial. On several occasions, Belle led her group mates to the wrong location and ran back to enjoy the true food source on her own. On other trials, the experimenters hid a single piece of food away from the main cache. Belle then led Rock to the single piece of food; then she alone rushed back to the main cache. In counterdeception, the deceiver is deceived by another. In an example of this, after Belle’s misleading, Rock sometimes walked away from her and then suddenly spun around to watch her.
Theory of Mind
Theory of mind is defined as one individual having the ability to take the perspective of another. Tests of theory of mind in human children entail creating situations that show what children know about others’ beliefs, knowledge, and attention. For example, in one experiment a child sat in a room with two adults. One adult placed an object in one of three containers and then left the room. The second adult and the child moved the object to another container. The adult asked the child, “When the absent adult returns will he look for the object in its original location, or the new location?” Autistic and very young children answered that he will look in the new location; older children responded that he will look in the original location.
Apes in captivity and in the wild demonstrate skills that fall under the heading of theory of mind. Apes, for example, gesture to others if the receiver is able to see the sender, but if the receiver is oriented so that his or her view is blocked, the sender instead uses auditory or tactile signals. In a test similar to the one given to children described above, an orangutan indicated an object’s new location to an experimenter who did not know that the object had been moved during his absence. In another test, captive chimpanzees watched a human put food into a container. In the test phase, the chimpanzees could ask one of two humans to help: the human who hid the food or another human who did not know the food’s location. Most chimpanzees chose the knowledgeable human.
Many of these highly inferential studies of social intelligence have found that apes reared in cages perform poorly on tests when compared to home-reared human children or human-reared apes. When the apes demonstrate success, it is often attributed to “enculturation,” or the human-rearing conditions, as if this endows the ape with new abilities rather than enabling the normal expression of capacities that are already present. Cage-reared chimpanzees have delays in motor development when compared to their wild counterparts. Likewise, human children reared in orphanages often show delays in motor development, especially when there is a high infant-to-caregiver ratio, as compared to home-reared children. Cognitive research comparing cage-reared apes to human children raised in stimulating environments probably tells us more about the effect of environment on cognitive development than about the cognitive capacities of the two species. We urge caution when interpreting results that claim that a cognitive ability is exclusively human when the comparison subjects include apes reared in restricted, unenriched environments.
Mirror studies were developed in the 1970s by Gordon G. Gallup Jr. as an experimental means of assessing an organism’s sense of self. When presented with a mirror, many organisms either have no reaction to it or respond to what they see reflected in a social manner. A dog, for example, might ignore or bark at its mirror image. Large-bodied apes use their mirror images for self-inspection, as do human adults and older children. Experimenters use the dye test to examine responses to mirror images. In this test, the experimenter secretly marks the subject. When the subject sees his or her mirror image, the experimental question is whether the subject touches or inspects the mark. If the subject touches it, he or she has demonstrated the use of the mirror for self-inspection. Researchers argue that apes and human children that inspect the mark have a sense of self.
Human children engage in imaginary play in a variety of ways. They use objects as though they are something else, treat inanimate objects as if alive, play roles, and create pretend scenarios. Apes also engage in pretend play. The cross-fostered chimpanzee Viki had an imaginary pull toy, and cross-fostered CHCI chimpanzee Dar played tickle games with a stuffed animal. Wild chimpanzees also engage in pretend play, such as carrying logs as if they were young chimpanzees. The gorilla Koko and the bonobo Kanzi have gone through the motions of eating imaginary objects. Similarly, wild chimpanzee Wanda dipped an imaginary stick into an imaginary termite mound.
Culture is a central concept of anthropology, and many scholars have devoted their careers to attempting to capture the tangible and intangible aspects of it. Culture is sometimes viewed by anthropologists as being exclusively human—”something humans do.” Biologists, however, have developed their own definitions of culture that expand its possible existence to nonhuman and nonprimate forms. Fish biologists Laland and Hoppitt define cultures as “those group-typical behavior patterns shared by members of a community that rely on socially learned and transmitted information.” More important than definitions, which exclude or include particular species, are descriptions of the behaviors.
Chimpanzees, orangutans, and humans use a variety of tools to extract foods, and the types of tools they use vary from one region to another. These tool-using traditions are socially transmitted. Monkeys, apes, and humans communicate visually and use various gestures, postures, and facial expression in their interactions with each other. Chimpanzees, bonobos, and orangutans in different regions use different gestures. For example, chimpanzees at Gombe Stream Reserve in Tanzania grasp an overhead branch during grooming, while just south in the Mahale Mountains the chimpanzees grasp each other’s hands during grooming. Researchers working at nine different long-term chimpanzee field sites collaborated and developed a list of 65 behavior patterns. The behaviors were classified in terms of their local frequency of occurrence. There were 39 behaviors that the researchers determined were cultural variants, since they were absent in some communities and customary or habitual in others. The behaviors involved tool use and grooming and courtship behaviors. This same analysis has been applied to orangutans and bonobos, and cultural variants have been identified in both of these species. For example, orangutans in one location place a kiss squeak on a leaf, while in another location they place a kiss squeak on the back of the hand.
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