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AbstractYoung human children at around 2 years of age fail to predict the correct location of an object when it is dropped from the top of an S‐shape opaque tube. They search in the location just below the releasing point (Hood, 1995). This type of error, called a 'gravity bias', has recently been reported in dogs and monkeys. In the present study, we investigated whether young and adult chimpanzees also show such a gravity bias in a modified version of the original opaque‐tube task. The original task by Hood and colleagues required the subject to search in a location after the object had fallen, while in the task reported here, subjects were required to predict the location before the object was dropped. Thus the present procedure does not involve explicit invisible displacement operations, one of the important components of the original procedure. InExperiment 1both young (1.5–2.5‐year‐old) and adult chimpanzees predicted the location of falling food items below the releasing point even when crossed tubes were used. These gravity errors remained after the extensive experience of using the tubes themselves.Experiment 2further tested adult and 4‐year‐old chimpanzees under the set‐up in which the straight and crossed tubes were simultaneously presented. The results were the same as those in the previous test, suggesting that developmental changes and learning effect do not affect the gravity bias in chimpanzees.

Abstract The use of gaze shifts as social cues has various evolutionary advantages. To investigate the developmental processes of this ability, we conducted an object‐choice task by using longitudinal methods with infant chimpanzees tested from 8 months old until 3 years old. The experimenter used one of six gestures towards a cup concealing food; tapping, touching, whole‐hand pointing, gazing plus close‐pointing, distant‐pointing, close‐gazing, and distant‐gazing. Unlike any other previous study, we analyzed the behavioral changes that occurred before and after choosing the cup. We assumed that pre‐choice behavior indicates the development of an attentional and spatial connection between a pointing cue and an object (e.g. Woodward, 2005); and post‐choice behavior indicates the emergence of object permanence (e.g. Piaget, 1954). Our study demonstrated that infant chimpanzees begin to use experimenter‐given cues with age (after 11 months of age). Moreover, the results from the behavioral analysis showed that the infants gradually developed the spatial link between the pointing as an object‐directed action and the object. Moreover, when they were 11 months old, the infants began to inspect the inside of the cup, suggesting the onset of object permanence. Overall, our results imply that the ability to use the cues is developing and mutually related with other cognitive developments. The present study also suggests what the standard object‐choice task actually measures by breaking the task down into the developmental trajectories of its component parts, and describes for the first time the social‐physical cognitive development during the task with a longitudinal method.

Abstract The development of visual interaction between mother and infant has received much attention in developmental psychology, not only in humans, but also in non‐human primates. Recently, comparative developmental approaches have investigated whether the mechanisms that underlie these behaviors are common in primates. In the present study, we focused on the question of whether chimpanzee mother and infant replace physical contact with visual contact. To test this hypothesis, we measured non‐synchronous looking ('looking') between mother and infant. A unique setting, in which the mother chimpanzee stayed in one location and the infant chimpanzee moved freely, allowed us to analyze the relation between the visual interaction and the distance of a mother–infant pair during the first year of life. Our results showed that 'looking' increased when body contact decreased or when the distance between mother and infant increased. We also show a behavioral sequence of typical 'secure base' behavior, a behavior characterized by the infant regularly returning to its mother when exploring the environment. These findings imply that attachment between mother and infant chimpanzee appears to develop in a similar fashion as in humans.

AbstractThis paper provides evidence for imitative abilities in neonatal chimpanzees (Pan troglodytes), our closest relatives. Two chimpanzees were reared from birth by their biological mothers. At less than 7 days of age the chimpanzees could discriminate between, and imitate, human facial gestures (tongue protrusion and mouth opening). By the time they were 2 months old, however, the chimpanzees no longer imitated the gestures. They began to perform mouth opening frequently in response to any of the three facial gestures presented to them. These findings suggest that neonatal facial imitation is most likely an innate ability, developed through natural selection in humans and in chimpanzees. The relationship between the disappearance of neonatal imitation and the development of social communicative behavior is discussed from an evolutionary perspective.

Abstract We directly compared chimpanzee infants and human infants for categorical representations of three global‐like categories (mammals, furniture and vehicles), using the familiarization–novelty preference technique. Neither species received any training during the experiments. We used the time that participants spent looking at the stimulus object while touching it as a measure. During the familiarization phase, participants were presented with four familiarization objects from one of three categories (e.g. mammals). Then, they were tested with a pair of novel objects, one was a familiar‐category object and another was a novel‐category object (e.g. vehicle) in the test phase. The chimpanzee infants did not show significant habituation, whereas human infants did. However, most important, both species showed significant novelty‐preference in the test phase. This indicates that not only human infants, but also chimpanzee infants formed categorical representations of a global‐like level. Implications for the shared origins and species‐specificity of categorization abilities, and the cognitive operations underlying categorization, are discussed.