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...this system, such as those underlying recursion. As we argue below, many acrimonious debates in this field have been launched by a failure to distinguish between these problems. According to one view =-=(1)-=-, questions concerning abstract computational mechanisms are distinct from those concerning communication, the latter targeted at problems at the interface between abstract computation and both sensor...

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...ded by the comparative method, which uses empirical data from living species to draw detailed inferences about extinct ancestors (3, 10–12). The comparative method was the primary tool used by Darwin =-=(13, 14)-=- to analyze evolutionary phenomena and continues to play a central role throughout modern evolutionary biology. Although scholars interested in language evolution have often ignored comparative data a...

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...that have been subjected to selection and perfected in recent human evolutionary history. This appears to represent the null hypothesis for many scholars who take the complexity of language seriously =-=(27, 28)-=-. The argument starts with the assumption that FLB, as a whole, is highly complex, serves the function of communication with admirable effectiveness, and has an ineliminable genetic component. Because...

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...nature of the constraints on FLN. More than 50 years of research using classical training studies demonstrates that animals can represent number, with careful controls for various important confounds =-=(80)-=-. In the typical experiment, a rat or pigeon is trained to press a lever x number of times to obtain a food reward. Results show that animals can hit the target number to within a closely matched mean...

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...le effectiveness, and has an ineliminable genetic component. Because natural selection is the only known biological mechanism capable of generating such functional complexes [the argument from design =-=(29)-=-], proponents of this view conclude that natural selection has played a powerful role in shaping many aspects of FLB, including FLN, and, further, that many of these are without parallel in nonhuman a...

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...mechanisms are involved; as pointed out above, there is no evidence for vocal imitation in nonhuman primates, and although human children may use domain-general mechanisms to acquire and recall words =-=(98, 99)-=-, the rate at which children build the lexicon is so massively different from nonhuman primates that one must entertain the possibility of an independently evolved mechanism. Furthermore, unlike the b...

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...ities of FLN constitute an adaptation. The viewpoint stated in hypothesis 2, especially the notion that FLN in particular is a highly evolved adaptation, has generated much enthusiasm recently [e.g., =-=(36)-=-], especially among evolutionary psychologists (37, 38). At present, however, we see little reason to believe either that FLN can be anatomized into many independent but interacting traits, each with ...

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...d by natural selection to the extent that they can be reasonably seen as constituting novel traits, perhaps exapted from other contexts [e.g., social intelligence, tool-making (7, 30–32)]. S C I E N C E ’ S C O M P A S S 22 NOVEMBER 2002 VOL 298 SCIENCE www.sciencemag.org1572 Hypothesis 3: Only FLN is uniquely human. On the basis of data reviewed below, we hypothesize that most, if not all, of FLB is based on mechanisms shared with nonhuman animals (as held by hypothesis 1). In contrast, we suggest that FLN—the computational mechanism of recursion—is recently evolved and unique to our species (33, 34). According to this hypothesis, much of the complexity manifested in language derives from complexity in the peripheral components of FLB, especially those underlying the sensory-motor (speech or sign) and conceptual-intentional interfaces, combined with sociocultural and communicative contingencies. FLB as a whole thus has an ancient evolutionary history, long predating the emergence of language, and a comparative analysis is necessary to understand this complex system. By contrast, according to recent linguistic theory, the computations underlying FLN may be quite limited. In fact, we propos...

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...cative efficacy (the surface or phenotypic function upon which selection presumably acted). We consider the possibility that certain specific aspects of the faculty of language are “spandrels”—by-products of preexisting constraints rather than end products of a history of natural selection (39). This possibility, which opens the door to other empirical lines of inquiry, is perfectly compatible with our firm support of the adaptationist program. Indeed, it follows directly from the foundational notion that adaptation is an “onerous concept” to be invoked only when alternative explanations fail (40). The question is not whether FLN in toto is adaptive. By allowing us to communicate an endless variety of thoughts, recursion is clearly an adaptive computation. The question is whether particular components of the functioning of FLN are adaptations for language, specifically acted upon by natural selection—or, even more broadly, whether FLN evolved for reasons other than communication. An analogy may make this distinction clear. The trunk and branches of trees are near-optimal solutions for providing an individual tree’s leaves with access to sunlight. For shrubs and small trees, a wide vari...

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...delineations. For example, Liberman and his associates (8) have argued that the sensory-motor systems were specifically adapted for language, and hence should be considered part of FLN. There is also a long tradition holding that the conceptualintentional systems are an intrinsic part of language in a narrow sense. In this article, we leave these questions open, restricting attention to FLN as just defined but leaving the possibility of a more inclusive definition open to further empirical research. The internal architecture of FLN, so conceived, is a topic of much current research and debate (4). Without prejudging the issues, we will, for concreteness, adopt a particular conception of this architecture. We assume, putting aside the precise mechanisms, that a key component of FLN is a computational system (narrow syntax) that generates internal representations and maps them into the sensory-motor interface by the phonological system, and into the conceptual-intentional interface by the (formal) semantic system; adopting alternatives that have been proposed would not materially modify the ensuing discussion. All approaches agree that a core property of FLN is recursion, attributed to ...

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...itself central to the language capacity. Thus, the capacity to imitate was a crucial prerequisite of FLB as a communicative system. Vocal imitation and learning are not uniquely human. Rich multimodal imitative capacities are seen in other mammals (dolphins) and some birds (parrots), with most songbirds exhibiting a well-developed vocal imitative S C I E N C E ’ S C O M P A S S 22 NOVEMBER 2002 VOL 298 SCIENCE www.sciencemag.org1574 capacity (65). What is surprising is that monkeys show almost no evidence of visually mediated imitation, with chimpanzees showing only slightly better capacities (66). Even more striking is the virtual absence of evidence for vocal imitation in either monkeys or apes (3). For example, intensively trained chimpanzees are incapable of acquiring anything but a few poorly articulated spoken words, whereas parrots can readily acquire a large vocal repertoire. With respect to their own vocalizations, there are few convincing studies of vocal dialects in primates, thereby suggesting that they lack a vocal imitative capacity (3, 65). Evidence for spontaneous visuomanual imitation in chimpanzees is not much stronger, although with persistent training they can learn...

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...esis 1: FLB is strictly homologous to animal communication. This hypothesis holds that homologs of FLB, including FLN, exist (perhaps in less developed or otherwise modified form) in nonhuman animals =-=(3, 10, 26)-=-. This has historically been a popular hypothesis outside of linguistics and closely allied fields, and has been defended by some in the speech sciences. According to this hypothesis, human FLB is com...

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... studies demonstrates that animals can represent number, with careful controls for various important confounds (80). In the typical experiment, a rat or pigeon is trained to press a lever x number of times to obtain a food reward. Results show that animals can hit the target number to within a closely matched mean, with a standard deviation that increases with magnitude: As the target number increases, so does variation around the mean. These results have led to the idea that animals, including human infants and adults, can represent number approximately as a magnitude with scalar variability (101, 102). Number discrimination is limited in this system by Weber’s law, with greater discriminability among small numbers than among large numbers (keeping distances between pairs constant) and between numbers that are farther apart (e.g., 7 versus 8 is harder than 7 versus 12). The approximate number sense is accompanied by a second precise mechanism that is limited to values less than 4 but accurately distinguishes 1 from 2, 2 from 3, and 3 from 4; this second system appears to be recruited in the context of object tracking and is limited by working memory constraints (103). Of direct relevance to...

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...t satisfies the interface conditions to FLB. Many of the details of language that are the traditional focus of linguistic study [e.g., subjacency, Wh- movement, the existence of garden-path sentences =-=(4, 44)-=-] may represent by-products of this solution, generated automatically by neural/computational constraints and the structure of FLB— components that lie outside of FLN. Even novel capacities such as re...

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...esis 1: FLB is strictly homologous to animal communication. This hypothesis holds that homologs of FLB, including FLN, exist (perhaps in less developed or otherwise modified form) in nonhuman animals =-=(3, 10, 26)-=-. This has historically been a popular hypothesis outside of linguistics and closely allied fields, and has been defended by some in the speech sciences. According to this hypothesis, human FLB is com...

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...th and formant dispersion in birds and primates (54–61) Biomechanics of sound production Studies of primate vocal production, including the role of mandibular oscillations (121, 122) Modalities of language production and perception Cross-modal perception and sign language in humans versus unimodal communication in animals (3, 25, 123) FLB—conceptual-intentional system Theory of mind, attribution of mental states Studies of the seeing/knowing distinction in chimpanzees (84, 86–89) Capacity to acquire nonlinguistic conceptual representations Studies of rhesus monkeys and the object/kind concept (10, 76, 77, 124) Referential vocal signals Studies of primate vocalizations used to designate predators, food, and social relationships (3, 78, 90, 91, 93, 94, 97) Imitation as a rational, intentional system Comparative studies of chimpanzees and human infants suggesting that only the latter read intentionality into action, and thus extract unobserved rational intent (125–127) Voluntary control over signal production as evidence of intentional communication Comparative studies that explore the relationship between signal production and the composition of a social audience (3, 10, 92, 128) FLN—recursion Sponta...

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...C E ’ S C O M P A S S www.sciencemag.org SCIENCE VOL 298 22 NOVEMBER 2002 1571 tion of analysis of fossil skull shape and cranial endocasts has led to little consensus about the evolution of language =-=(7, 9)-=-. A more tractable and, we think, powerful approach to problems of language evolution is provided by the comparative method, which uses empirical data from living species to draw detailed inferences a...

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...ing it are some subset of those underlying FLB. Others have agreed on the need for a restricted sense of “language” but have suggested different delineations. For example, Liberman and his associates =-=(8)-=- have argued that the sensory-motor systems were specifically adapted for language, and hence should be considered part of FLN. There is also a long tradition holding that the conceptualintentional sy...

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...exemplars of different phonemes (52). Further, in the absence of training, nonhuman primates can discriminate sentences from two different languages on the basis of rhythmic differences between them (53). On the side of production, birds and nonhuman primates naturally produce and perceive formants in their own species-typical vocalizations (54–59). The results also shed light on discussions of the uniquely human structure of the vocal tract and the unusual descended larynx of our species (7, 48, 60), because new evidence shows that several other mammalian species also have a descended larynx (61). Because these nonhuman species lack speech, a descended larynx clearly has nonphonetic functions; one possibility is exaggerating apparent size. Although this particular anatomical modification undoubtedly plays an important role in speech production in modern humans, it need not have first evolved for this function. The descended larynx may thus be an example of classic Darwinian preadaptation. Many phenomena in human speech perception have not yet been investigated in animals [e.g., the McGurk effect, an illusion in which the syllable perceived from a talking head represents the interactio...

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...us eyes reveal the stringent constraints placed by the laws of optics and the contingencies of development on an organ capable of focusing a sharp image onto a sheet of receptors. Detailed analogies between the parts of the vertebrate and cephalopod eye also provide independent evidence that each component is an adaptation for image formation, shaped by natural selection. Furthermore, the discovery that remarkably conservative genetic cascades underlie the development of such analogous structures provides important insights into the ways in which developmental mechanisms can channel evolution (21). Thus, although potentially misleading for taxonomists, analogies provide critical data about adaptation under physical and developmental constraints. Casting the comparative net more broadly, therefore, will most likely reveal larger regularities in evolution, helping to address the role of such constraints in the evolution of language. An analogy recognized as particularly relevant to language is the acquisition of song by birds (12). In contrast to nonhuman primates, where the production of species-typical vocalizations is largely innate (22), most songbirds learn their species-specific so...

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...mants in their own species-typical vocalizations (54–59). The results also shed light on discussions of the uniquely human structure of the vocal tract and the unusual descended larynx of our species =-=(7, 48, 60)-=-, because new evidence shows that several other mammalian species also have a descended larynx (61). Because these nonhuman species lack speech, a descended larynx clearly has nonphonetic functions; o...

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...nal factors shared with other vertebrates. Hypothesis 3 raises the possibility that structural details of FLN may result from such preexisting constraints, rather than from direct shaping by natural selection targeted specifically at communication. Insofar as this proves to be true, such structural details are not, strictly speaking, adaptations at all. This hypothesis and the alternative selectionist account are both viable and can eventually be tested with comparative data. Comparative Evidence for the Faculty of Language Study of the evolution of language has accelerated in the past decade (45, 46). Here, we offer a highly selective review of some of these studies, emphasizing animal work that seems particularly relevant to the hypotheses advanced above; many omissions were necessary for reasons of space, and we firmly believe that a broad diversity of methods and perspectives will ultimately provide the richest answers to the problem of language evolution. For this reason, we present a broader sampler of the field’s offerings in Table 1. How “special” is speech? Comparative study of the sensory-motor system. Starting with early work on speech perception, there has been a tradition of c...

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...nist account are both viable and can eventually be tested with comparative data. Comparative Evidence for the Faculty of Language Study of the evolution of language has accelerated in the past decade =-=(45, 46)-=-. Here, we offer a highly selective review of some of these studies, emphasizing animal work that seems particularly relevant to the hypotheses advanced above; many omissions were necessary for reason...

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...f mental states Studies of the seeing/knowing distinction in chimpanzees (84, 86–89) Capacity to acquire nonlinguistic conceptual representations Studies of rhesus monkeys and the object/kind concept =-=(10, 76, 77, 124)-=- Referential vocal signals Studies of primate vocalizations used to designate predators, food, and social relationships (3, 78, 90, 91, 93, 94, 97) Imitation as a rational, intentional system Comparat...

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...L 298 SCIENCE www.sciencemag.org1574 capacity (65). What is surprising is that monkeys show almost no evidence of visually mediated imitation, with chimpanzees showing only slightly better capacities =-=(66)-=-. Even more striking is the virtual absence of evidence for vocal imitation in either monkeys or apes (3). For example, intensively trained chimpanzees are incapable of acquiring anything but a few po...

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...ions to this foundational system alone seem inadequate to generate the fundamental difference—discrete infinity—between language and all known forms of animal communication. This claim is one of several reasons why we suspect that hypothesis 3 may be a productive way to characterize the problem of language evolution. A primary issue separating hypotheses 2 and 3 is whether the uniquely human capacities of FLN constitute an adaptation. The viewpoint stated in hypothesis 2, especially the notion that FLN in particular is a highly evolved adaptation, has generated much enthusiasm recently [e.g., (36)], especially among evolutionary psychologists (37, 38). At present, however, we see little reason to believe either that FLN can be anatomized into many independent but interacting traits, each with its own independent evolutionary history, or that each of these traits could have been strongly shaped by natural selection, given their tenuous connection to communicative efficacy (the surface or phenotypic function upon which selection presumably acted). We consider the possibility that certain specific aspects of the faculty of language are “spandrels”—by-products of preexisting constraints ra...

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...the faculty of language as a whole relies on some uniquely human capacities that have evolved recently in the approximately 6 million years since our divergence from a chimpanzee-like common ancestor =-=(35)-=-. Hypothesis 3, in its strongest form, suggests that only FLN falls into this category (34). By this hypothesis, FLB contains a wide variety of cognitive and perceptual mechanisms shared with other sp...

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...h our firm support of the adaptationist program. Indeed, it follows directly from the foundational notion that adaptation is an “onerous concept” to be invoked only when alternative explanations fail =-=(40)-=-. The question is not whether FLN in toto is adaptive. By allowing us to communicate an endless variety of thoughts, recursion is clearly an adaptive computation. The question is whether particular co...

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... is based on mechanisms shared with nonhuman animals (as held by hypothesis 1). In contrast, we suggest that FLN—the computational mechanism of recursion—is recently evolved and unique to our species =-=(33, 34)-=-. According to this hypothesis, much of the complexity manifested in language derives from complexity in the peripheral components of FLB, especially those underlying the sensory-motor (speech or sign...

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...) suggests the possibility that at least the narrow-syntactic component satisfies conditions of highly efficient computation to an extent previously unsuspected. Thus, FLN may approximate a kind of “optimal solution” to the problem of linking the sensory-motor and conceptual-intentional systems. In other words, the generative processes of the language system may provide a near-optimal solution that satisfies the interface conditions to FLB. Many of the details of language that are the traditional focus of linguistic study [e.g., subjacency, Wh- movement, the existence of garden-path sentences (4, 44)] may represent by-products of this solution, generated automatically by neural/computational constraints and the structure of FLB— components that lie outside of FLN. Even novel capacities such as recursion are implemented in the same type of neural tissue as the rest of the brain and are thus constrained by biophysical, developmental, and computational factors shared with other vertebrates. Hypothesis 3 raises the possibility that structural details of FLN may result from such preexisting constraints, rather than from direct shaping by natural selection targeted specifically at communication...

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...y that remarkably conservative genetic cascades underlie the development of such analogous structures provides important insights into the ways in which developmental mechanisms can channel evolution =-=(21)-=-. Thus, although potentially misleading for taxonomists, analogies provide critical data about adaptation under physical and developmental constraints. Casting the comparative net more broadly, theref...

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...ases, so does variation around the mean. These results have led to the idea that animals, including human infants and adults, can represent number approximately as a magnitude with scalar variability =-=(101, 102)-=-. Number discrimination is limited in this system by Weber’s law, with greater discriminability among small numbers than among large numbers (keeping distances between pairs constant) and between numb...

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...ecific song by listening to conspecifics, and they develop highly aberrant song if deprived of such experience. Current investigation of birdsong reveals detailed and intriguing parallels with speech =-=(11, 23, 24)-=-. For instance, many songbirds pass through a critical period in development beyond which they produce defective songs that no amount of acoustic input can remedy, reminiscent of the difficulty adult ...

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...t stated in hypothesis 2, especially the notion that FLN in particular is a highly evolved adaptation, has generated much enthusiasm recently [e.g., (36)], especially among evolutionary psychologists =-=(37, 38)-=-. At present, however, we see little reason to believe either that FLN can be anatomized into many independent but interacting traits, each with its own independent evolutionary history, or that each ...

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...ng with early work on speech perception, there has been a tradition of considering speech “special,” and thus based on uniquely human mechanisms adapted for speech perception and/or production [e.g., =-=(7, 8, 47, 48)-=-]. This perspective has stimulated a vigorous research program studying animal speech perception and, more recently, speech production. Surprisingly, this research has turned up little evidence for un...

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...s of no truly novel traits in the speech domain appears to stand. There is, however, a striking ability tied to speech that has received insufficient attention: the human capacity for vocal imitation =-=(63, 64)-=-. Imitation is obviously a necessary component of the human capacity to acquire a shared and arbitrary lexicon, which is itself central to the language capacity. Thus, the capacity to imitate was a cr...

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...ntations Studies of rhesus monkeys and the object/kind concept (10, 76, 77, 124) Referential vocal signals Studies of primate vocalizations used to designate predators, food, and social relationships =-=(3, 78, 90, 91, 93, 94, 97)-=- Imitation as a rational, intentional system Comparative studies of chimpanzees and human infants suggesting that only the latter read intentionality into action, and thus extract unobserved rational ...

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... Voluntary control over signal production as evidence of intentional communication Comparative studies that explore the relationship between signal production and the composition of a social audience =-=(3, 10, 92, 128)-=- FLN—recursion Spontaneous and training methods designed to uncover constraints on rule learning Studies of serial order learning and finite-state grammars in tamarins and macaques (114, 116, 117, 129...

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...t defined but leaving the possibility of a more inclusive definition open to further empirical research. The internal architecture of FLN, so conceived, is a topic of much current research and debate =-=(4)-=-. Without prejudging the issues, we will, for concreteness, adopt a particular conception of this architecture. We assume, putting aside the precise mechanisms, that a key component of FLN is a comput...

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...ogy recognized as particularly relevant to language is the acquisition of song by birds (12). In contrast to nonhuman primates, where the production of species-typical vocalizations is largely innate =-=(22)-=-, most songbirds learn their species-specific song by listening to conspecifics, and they develop highly aberrant song if deprived of such experience. Current investigation of birdsong reveals detaile...

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...esis 1: FLB is strictly homologous to animal communication. This hypothesis holds that homologs of FLB, including FLN, exist (perhaps in less developed or otherwise modified form) in nonhuman animals =-=(3, 10, 26)-=-. This has historically been a popular hypothesis outside of linguistics and closely allied fields, and has been defended by some in the speech sciences. According to this hypothesis, human FLB is com...

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... is based on mechanisms shared with nonhuman animals (as held by hypothesis 1). In contrast, we suggest that FLN—the computational mechanism of recursion—is recently evolved and unique to our species =-=(33, 34)-=-. According to this hypothesis, much of the complexity manifested in language derives from complexity in the peripheral components of FLB, especially those underlying the sensory-motor (speech or sign...

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...t stated in hypothesis 2, especially the notion that FLN in particular is a highly evolved adaptation, has generated much enthusiasm recently [e.g., (36)], especially among evolutionary psychologists =-=(37, 38)-=-. At present, however, we see little reason to believe either that FLN can be anatomized into many independent but interacting traits, each with its own independent evolutionary history, or that each ...

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...nist account are both viable and can eventually be tested with comparative data. Comparative Evidence for the Faculty of Language Study of the evolution of language has accelerated in the past decade =-=(45, 46)-=-. Here, we offer a highly selective review of some of these studies, emphasizing animal work that seems particularly relevant to the hypotheses advanced above; many omissions were necessary for reason...

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...f mental states Studies of the seeing/knowing distinction in chimpanzees (84, 86–89) Capacity to acquire nonlinguistic conceptual representations Studies of rhesus monkeys and the object/kind concept =-=(10, 76, 77, 124)-=- Referential vocal signals Studies of primate vocalizations used to designate predators, food, and social relationships (3, 78, 90, 91, 93, 94, 97) Imitation as a rational, intentional system Comparat...

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... have rich conceptual representations (76, 77). Surprisingly, however, there is a mismatch between the conceptual capacities of animals and the communicative content of their vocal and visual signals =-=(78, 79)-=-. For example, although a wide variety of nonhuman primates have access to rich knowledge of who is related to whom, as well as who is dominant and who is subordinate, their vocalizations only coarsel...

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... Voluntary control over signal production as evidence of intentional communication Comparative studies that explore the relationship between signal production and the composition of a social audience =-=(3, 10, 92, 128)-=- FLN—recursion Spontaneous and training methods designed to uncover constraints on rule learning Studies of serial order learning and finite-state grammars in tamarins and macaques (114, 116, 117, 129...

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...d by natural selection to the extent that they can be reasonably seen as constituting novel traits, perhaps exapted from other contexts [e.g., social intelligence, tool-making (7, 30–32)]. S C I E N C E ’ S C O M P A S S 22 NOVEMBER 2002 VOL 298 SCIENCE www.sciencemag.org1572 Hypothesis 3: Only FLN is uniquely human. On the basis of data reviewed below, we hypothesize that most, if not all, of FLB is based on mechanisms shared with nonhuman animals (as held by hypothesis 1). In contrast, we suggest that FLN—the computational mechanism of recursion—is recently evolved and unique to our species (33, 34). According to this hypothesis, much of the complexity manifested in language derives from complexity in the peripheral components of FLB, especially those underlying the sensory-motor (speech or sign) and conceptual-intentional interfaces, combined with sociocultural and communicative contingencies. FLB as a whole thus has an ancient evolutionary history, long predating the emergence of language, and a comparative analysis is necessary to understand this complex system. By contrast, according to recent linguistic theory, the computations underlying FLN may be quite limited. In fact, we propos...

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...At present, however, we see little reason to believe either that FLN can be anatomized into many independent but interacting traits, each with its own independent evolutionary history, or that each of these traits could have been strongly shaped by natural selection, given their tenuous connection to communicative efficacy (the surface or phenotypic function upon which selection presumably acted). We consider the possibility that certain specific aspects of the faculty of language are “spandrels”—by-products of preexisting constraints rather than end products of a history of natural selection (39). This possibility, which opens the door to other empirical lines of inquiry, is perfectly compatible with our firm support of the adaptationist program. Indeed, it follows directly from the foundational notion that adaptation is an “onerous concept” to be invoked only when alternative explanations fail (40). The question is not whether FLN in toto is adaptive. By allowing us to communicate an endless variety of thoughts, recursion is clearly an adaptive computation. The question is whether particular components of the functioning of FLN are adaptations for language, specifically acted upon by...

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...e evidence not only of categorical perception, but also of the ability to discriminate among prototypical exemplars of different phonemes (52). Further, in the absence of training, nonhuman primates can discriminate sentences from two different languages on the basis of rhythmic differences between them (53). On the side of production, birds and nonhuman primates naturally produce and perceive formants in their own species-typical vocalizations (54–59). The results also shed light on discussions of the uniquely human structure of the vocal tract and the unusual descended larynx of our species (7, 48, 60), because new evidence shows that several other mammalian species also have a descended larynx (61). Because these nonhuman species lack speech, a descended larynx clearly has nonphonetic functions; one possibility is exaggerating apparent size. Although this particular anatomical modification undoubtedly plays an important role in speech production in modern humans, it need not have first evolved for this function. The descended larynx may thus be an example of classic Darwinian preadaptation. Many phenomena in human speech perception have not yet been investigated in animals [e.g., the McGur...

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... reveal that animals acquire and use a wide range of abstract concepts, including tool, color, geometric relationships, food, and number (66, 76–82). More controversially, but of considerable relevance to intentional aspects of language and conditions of felicitous use, some studies claim that animals have a theory of mind (83–85), including a sense of self and the ability to represent the beliefs and desires of other group members. On the side of positive support, recent studies of chimpanzees suggest that they recognize the perceptual act of seeing as a proxy for the mental state of knowing (84, 86, 87). These Fig. 4. The distribution of imitation in the animal kingdom is patchy. Some animals such as songbirds, dolphins, and humans have evolved exceptional abilities to imitate; other animals, such as apes and monkeys, either lack such abilities or have them in a relatively impoverished form. [Illustration: John Yanson] S C I E N C E ’ S C O M P A S S www.sciencemag.org SCIENCE VOL 298 22 NOVEMBER 2002 1575 studies suggest that at least chimpanzees, but perhaps no other nonhuman animals, have a rudimentary theory of mind. On the side of negative support, other studies suggest that even chimpa...

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...object/kind concept (10, 76, 77, 124) Referential vocal signals Studies of primate vocalizations used to designate predators, food, and social relationships (3, 78, 90, 91, 93, 94, 97) Imitation as a rational, intentional system Comparative studies of chimpanzees and human infants suggesting that only the latter read intentionality into action, and thus extract unobserved rational intent (125–127) Voluntary control over signal production as evidence of intentional communication Comparative studies that explore the relationship between signal production and the composition of a social audience (3, 10, 92, 128) FLN—recursion Spontaneous and training methods designed to uncover constraints on rule learning Studies of serial order learning and finite-state grammars in tamarins and macaques (114, 116, 117, 129) Sign or artificial language in trained apes and dolphins Studies exploring symbol sequencing and open-ended combinatorial manipulation (130, 131) Models of the faculty of language that attempt to uncover the necessary and sufficient mechanisms Game theory models of language acquisition, reference, and universal grammar (72–74) Experiments with animals that explore the nature and content of numbe...