Relative numerousness judgment and summation in young and old Western lowland gorillas

The relationship between age, relative numerousness judgment, and summation was investigated in 11 Western lowland gorillas (Gorilla gorilla gorilla). Experiments 1 and 2 evaluated the gorillas' ability to select the larger of 2 food quantities before and with training. The majority of gorillas did not reliably select the larger quantity in Experiment 1 until receiving training to do so in Experiment 2. Experiment 3 evaluated their ability to select the larger of 2 pairs of quantities. All gorillas selected the larger pair more often than chance, and the old were less accurate and slower than were the young. For most gorillas, performance in relative numerousness judgment with training and summation was comparable with previous reports in chimpanzees and orangutans.     

Hand preference for food processing in wild western lowland gorillas (Gorilla gorilla gorilla)

Western lowland gorillas (Gorilla gorilla gorilla) have not previously been represented in studies of laterality in wild great apes. The discovery of swampy clearings frequented by gorillas in northern Congo has provided the first opportunity to redress this imbalance. Hand preference data are presented from 33 gorillas in seated and standing postures, covering the procurement and processing of 2 to 4 plant species. Levels of hand preference exhibited were low. When data from all postures and plant species were pooled, 33% of gorillas showed hand preferences in excess of chance. In the standing posture, more gorillas exhibited significant left-hand preferences than right, but an overall population-level bias was not evident.     

Quantity estimation and comparison in western lowland gorillas (Gorilla gorilla gorilla)

We investigated the quantity judgment abilities of two adult male western lowland gorillas (Gorilla gorilla gorilla) by presenting discrimination tasks on a touch-screen computer. Both gorillas chose the larger quantity of two arrays of dot stimuli. On some trials, the relative number of dots was congruent with the relative total area of the two arrays. On other trials, number of dots was incongruent with area. The gorillas were first tested with static dots, then with dots that moved within the arrays, and finally on a task where they were required to discriminate numerically larger subsets within arrays of moving dots. Both gorillas achieved above-chance performance on both congruent and incongruent trials with all tasks, indicating that they were able to use number as a cue even though ratio of number and area significantly controlled responding, suggesting that number was not the only relevant dimension that the gorillas used. The pattern of performance was similar to that found previously with monkeys and chimpanzees but had not previously been demonstrated in gorillas within a computerized test format, and with these kinds of visual stimuli.     

Response of captive lowland gorillas (Gorilla gorilla gorilla) to different housing conditions: testing the aggression-density and coping models

Several studies have suggested that primates react differently to spatial reduction. In this article, the authors tested some general hypotheses on primate response to spatial reduction by studying the Apenheul lowland gorillas (Gorilla gorilla gorilla; Apeldoorn, the Netherlands). The frequency of conflicts did not greatly change between the 2 housing settings, thus not supporting the density- aggression model. Indoor, gorillas performed touching behavior more often and increased their level of reconciliation. These findings support the coping model. Indoor, the gorillas also maintained broader interindividual distances by increasing the levels of sitting alone, avoidance, and dismissing behaviors. In conclusion, the Apenheul gorillas modified selectively the distribution of some patterns typical of their behavioral repertoire in response to a high-density condition. Both avoidance and coping tactics were used, thus revealing high levels of behavioral flexibility in this species.     

Mother-infant interactions in western Lowland Gorillas (Gorilla gorilla gorilla): spatial relationships,communication, and opportunities for social learning

This study investigated mother-infant interactions in lowland gorillas (Gorilla gorilla gorilla) with particular focus on the relative role of mothers and infants in creating situations that are potentially conducive to infant social learning. Eleven gorilla mother-infant dyads were focally observed in weekly 1-hr sessions for 12 months. Spatial relationships were affected by age as well as by ambient temperature. Although the youngest infant was encouraged by its mother to walk and climb, mothers showed little or no encouragement in other contexts. In contrast, infants were quite interested in their mothers' activities, on some occasions repeated their mother's behavior, and actively encouraged their mothers to share food, play, or follow them. These findings suggest that gorilla infants are more active than their mothers in creating situations that are potentially conducive to the acquisition of knowledge or skills.     

Local traditions in gorilla manual skill: evidence for observational learning of behavioral organization

Elaborate manual skills of food processing are known in several species of great ape; but their manner of acquisition is controversial. Local, “cultural” traditions show the influence of social learning, but it is uncertain whether this includes the ability to imitate the organization of behavior. Dispute has centered on whether program-level imitation contributes to the acquisition of feeding techniques in gorillas. Here, we show that captive western gorillas at Port Lympne, Kent, have developed a group-wide habit of feeding on nettles, using two techniques. We compare their nettle processing behavior with that of wild mountain gorillas in Rwanda. Both populations are similar in their repertoires of action elements, and in developing multi-step techniques for food processing, with coordinated asymmetric actions of the hands and iteration of parts of a process as “subroutines”. Crucially, however, the two populations deal in different ways with the special challenges presented by nettle stings, with consistently different organizations of action elements. We conclude that, while an elaborate repertoire of manual actions and the ability to develop complex manual skills are natural characteristics of gorillas, the inter-site differences in nettle-eating technique are best explained as a consequence of social transmission. According to this explanation, gorillas can copy aspects of program organization from the behavior of others and they use this ability when learning how to eat nettles, resulting in consistent styles of processing by most individuals at each different site; like other great apes, gorillas have the precursor abilities for developing culture.     

Natural concepts in a juvenile gorilla (gorilla gorilla gorilla) at three levels of abstraction

The extent to which nonhumans are able to form conceptual versus perceptual discriminations remains a matter of debate. Among the great apes, only chimpanzees have been tested for conceptual understanding, defined as the ability to form discriminations not based solely on simple perceptual features of stimuli, and to transfer this learning to novel stimuli. In the present investigation, a young captive female gorilla was trained at three levels of abstraction (concrete, intermediate, and abstract) involving sets of photographs representing natural categories (e.g., orangutans vs. humans, primates vs. nonprimate animals, animals vs. foods). Within each level of abstraction, when the gorilla had learned to discriminate positive from negative exemplars in one set of photographs, a novel set was introduced. Transfer was defined in terms of high accuracy during the first two sessions with the new stimuli. The gorilla acquired discriminations at all three levels of abstraction but showed unambiguous transfer only with the concrete and abstract stimulus sets. Detailed analyses of response patterns revealed little evidence of control by simple stimulus features. Acquisition and transfer involving abstract stimulus sets suggest a conceptual basis for gorilla categorization. The gorilla's relatively poor performance with intermediate-level discriminations parallels findings with pigeons, and suggests a need to reconsider the role of perceptual information in discriminations thought to indicate conceptual behavior in nonhumans.     

Spontaneously occurring mother-infant swapping and the relationships of infants with their biological and foster mothers in a captive group of lowland gorillas (Gorilla gorilla gorilla)

The present study describes spontaneously occurring infant-mother swapping and the relationships of infants with their biological and foster mothers after swapping in a captive social group of lowland gorillas (Gorilla gorilla gorilla). After infant-swapping took place between a primiparous mother with a neonate and a multiparous mother with a 10-month-old infant, the mothers provided appropriate maternal care, including nursing, to their adopted infants, but the older infant still sought refuge with its biological mother more frequently than with its foster mother. Almost 2 years later, the younger infant who was weaned by its foster mother began nursing from its biological mother; the latter subsequently weaned her adopted infant. Thereafter, the older infant who was weaned exhibited an abnormal hair-plucking behavior. These results indicate that it was psychologically stressful for the gorilla infant to quickly transfer its attachment figure from the mother to another adult female when the mother continued to remain in the group.     

Target animacy influences gorilla handedness

We investigated the unimanual actions of a biological family group of twelve western lowland gorillas (Gorilla gorilla gorilla) using a methodological approach designed to assess behavior within social context from a bottom-up perspective. Measures of both the lateralization of unimanual actions (left, right) and the target of the action (animate, inanimate) were assessed during dual, synchronized video observations of naturalistic behavior. This paper demonstrates a corelationship between handedness and the animate quality of the target object. Analyses demonstrated a significant interaction between lateralized unimanual actions and target animacy and a right-hand bias for actions directed toward inanimate targets. We suggest that lateralized motor preference reflects the different processing capabilities of the left and right hemispheres, as influenced by the emotive (animate) and/or functional (inanimate) characteristics of the target, respectively.     

Single-trial learning of "what" and "who" information in a gorilla (Gorilla gorilla gorilla): implications for episodic memory

Single-trial learning and long-term memory of "what" and "who" information were examined in an adult gorilla (Gorilla gorilla gorilla). We presented the gorilla with a to-be-remembered food item at the time of study. In Experiment 1, following a retention interval of either approximately 7 min or 24 h, the gorilla responded with one of five cards, each corresponding to a particular food. The gorilla was accurate on 70% of the short retention-interval trials and on 82% of the long retention-interval trials. In Experiment 2, the food stimulus was provided by one of two experimenters, each of whom was represented by a card. The gorilla identified the food (55% of the time) and the experimenter (82% of the time) on the short retention-interval trials. On the long retention-interval trials, the gorilla was accurate for the food (73%) and for the person (87%). The results are interpreted in light of theories of episodic memory. Electronic Publication     

Comparative assessment of handedness for a coordinated bimanual task in chimpanzees (Pan troglodytes), gorillas (Gorilla gorilla) and orangutans (Pongo pygmaeus)

Hand preferences for a coordinated bimanual task were assessed in a sample of 31 captive gorillas (Gorilla gorilla) and 19 captive orangutans (Pongo pygmaeus) and were compared with chimpanzee (Pan troglodytes) hand preferences in subjects that were matched on the basis of age, sex, and rearing history. The task required that the apes remove food from the inside edges of a symmetrical polyvinyl chloride pipe presented to them in their home cages. The results indicate significant species differences with chimpanzees showing population-level right-handedness and orangutans showing population-level left-handedness. The gorillas showed a nonsignificant trend toward right-handedness. The results are discussed in terms of possible ecological or biomechanical factors that may influence hand preferences in different ape species.     

Gestural communication of the gorilla (Gorilla gorilla): repertoire, intentionality and possible origins

Social groups of gorillas were observed in three captive facilities and one African field site. Cases of potential gesture use, totalling 9,540, were filtered by strict criteria for intentionality, giving a corpus of 5,250 instances of intentional gesture use. This indicated a repertoire of 102 gesture types. Most repertoire differences between individuals and sites were explicable as a consequence of environmental affordances and sampling effects: overall gesture frequency was a good predictor of universality of occurrence. Only one gesture was idiosyncratic to a single individual, and was given only to humans. Indications of cultural learning were few, though not absent. Six gestures appeared to be traditions within single social groups, but overall concordance in repertoires was almost as high between as within social groups. No support was found for the ontogenetic ritualization hypothesis as the chief means of acquisition of gestures. Many gestures whose form ruled out such an origin, i.e. gestures derived from species-typical displays, were used as intentionally and almost as flexibly as gestures whose form was consistent with learning by ritualization. When using both classes of gesture, gorillas paid specific attention to the attentional state of their audience. Thus, it would be unwarranted to divide ape gestural repertoires into ‘innate, species-typical, inflexible reactions’ and ‘individually learned, intentional, flexible communication’. We conclude that gorilla gestural communication is based on a species-typical repertoire, like those of most other mammalian species but very much larger. Gorilla gestures are not, however, inflexible signals but are employed for intentional communication to specific individuals.     

Discrete quantity judgments in the great apes (Pan paniscus, Pan troglodytes, Gorilla gorilla, Pongo pygmaeus): the effect of presenting whole sets versus item-by-item

The authors examined quantity-based judgments for up to 10 items for simultaneous and sequential whole sets as well as for sequentially dropped items in chimpanzees (Pan troglodytes), gorillas (Gorilla gorilla), bonobos (Pan paniscus), and orangutans (Pongo pygmaeus). In Experiment 1, subjects had to choose the larger of 2 quantities presented in 2 separate dishes either simultaneously or 1 dish after the other. Representatives of all species were capable of selecting the larger of 2 quantities in both conditions, even when the quantities were large and the numerical distance between them was small. In Experiment 2, subjects had to select between the same food quantities sequentially dropped into 2 opaque cups so that none of the quantities were ever viewed as a whole. The authors found some evidence (albeit weaker) that subjects were able to select the larger quantity of items. Furthermore, the authors found no performance breakdown with the inclusion of certain quantities. Instead, the ratio between quantities was the best performance predictor. The authors conclude that quantity-based judgments rely on an analogical system, not a discrete object file model or perceptual estimation mechanism, such as subitizing.     

Event memory and misinformation effects in a gorilla (<Emphasis Type="Italic">Gorilla gorilla gorilla</Emphasis>)

Event memory and misinformation effects were examined in an adult male gorilla ( Gorilla gorilla gorilla). The gorilla witnessed a series of unique events, involving a familiar person engaging in a novel behavior (experiment 1), a novel person engaging in a novel behavior (experiment 2), or the presentation of a novel object (experiment 3). Following a 5- to 10-min retention interval, a tester gave the gorilla three photographs mounted on wooden cards: a photograph depicting the correct person or object and two distractor photographs drawn from the same class. The gorilla responded by returning a photograph. If correct, he was reinforced with food. Across three experiments, the gorilla performed significantly above chance at recognizing the target photograph. In experiment 4, the gorilla showed at-chance performance when the event was followed by misinformation (a class-consistent, but incorrect photograph), but significantly above-chance performance when no misinformation occurred (either correct photograph or no photograph). Although the familiarity can account for these data, they are also consistent with an episodic-memory interpretation.     

Gorillas (Gorilla gorilla) and orangutans (Pongo pygmaeus) encode relevant problem features in a tool-using task

Two important elements in problem solving are the abilities to encode relevant task features and to combine multiple actions to achieve the goal. The authors investigated these 2 elements in a task in which gorillas (Gorilla gorilla) and orangutans (Pongo pygmaeus) had to use a tool to retrieve an out-of-reach reward. Subjects were able to select tools of an appropriate length to reach the reward even when the position of the reward and tools were not simultaneously visible. When presented with tools that were too short to retrieve the reward, subjects were more likely to refuse to use them than when tools were the appropriate length. Subjects were proficient at using tools in sequence to retrieve the reward.     

Differences in gorilla nettle-feeding between captivity and the wild: local traditions,species typical behaviors or merely the result of nutritional deficiencies?

Behavioral and cognitive studies on captive apes often pay little attention to the specific environmental conditions of their study subjects. A recent report form Byrne et al. (Anim Cogn doi:, 2011), comparing nettle-feeding techniques between captive and wild gorillas, claimed to document “the strongest evidence yet to come from any great ape that observational learning of a skilled conspecific” can allow social learning and culture in gorillas. An earlier study with similar findings placed emphasis instead on the many similarities and claims for species typical behavior, thus a genetic hypothesis instead of a cultural hypothesis. This commentary aims at formulating a third environmental hypothesis based on path-dependent behavioral differences owing to different diet and availability of nutritional resources of wild and captive gorillas. Captive diet provides gorillas with a much lower concentration of fibers. Gorillas are hindgut fermenters, and this deficit of natural fermentation of fibers may impact their health and their behavior in zoos. Results of Byrne et al.’s study will be discussed comparing feeding choice and availability of nutritional resources of wild and captive gorillas, showing that in captivity gorilla, motivation to consume certain food or certain plant parts may differ drastically from that of wild gorillas. This view does not intend to deny that social learning and culture may exist in gorillas, but to guide and encourage future works investigating social learning in great apes to take more accurately into account the living conditions and, when comparing populations, the possible environmental differences.     

Social learning by orangutans (Pongo abelii and Pongo pygmaeus) in a simulated food-processing task

Increasing evidence for behavioral differences between populations of primates has created a resurgence of interest in examining mechanisms of information transfer between individuals. The authors examined the social transmission of information in 15 captive orangutans (Pongo abelii and Pongo pygmaeus) using a simulated food-processing task. Experimental subjects were shown 1 of 2 methods for removing a suite of defenses on an "artificial fruit." Control subjects were given no prior exposure before interacting with the fruit. Observing a model provided a functional advantage in the task, as significantly more experimental than control subjects opened the fruit. Within the experimental groups, the authors found a trend toward differences in the actual behaviors used to remove 1 of the defenses. Results support observations from the wild implying horizontal transfer of information in orangutans and show that a number of social learning processes are likely to be involved in the transfer of knowledge in this species.     

Orangutans (Pongo abelii) and a gorilla (Gorilla gorilla gorilla) match features in familiar and unfamiliar individuals

Great apes can perceive images as representative of corresponding real-life objects. Coupled with the potential advantages of identifying specific members of one’s species and mounting evidence for individual recognition in other non-humans, it seems likely that great apes would have the ability to identify conspecifics in photographs. The ability of four orangutans and a gorilla to match images of individuals of their own and a closely related but unfamiliar species was examined here for the first time. First, the subjects matched photographs of familiar conspecifics taken at various time points in a delayed matching-to-sample procedure (Experiment 1). Second, they matched different photographs of unfamiliar individuals of a different species (Experiment 2) at above chance levels. These results suggest that the subjects matched photographs by matching physical features, not necessarily by recognizing the identity of the individuals depicted. However, they also quickly learned to select photographs of familiar individuals when these photographs were paired with photographs of unfamiliar individuals of their own species (Experiment 3), and three subjects showed transfer to novel images of familiar and unfamiliar individuals. Thus, the findings support the idea that subjects attended to physical features to identify individuals that they could categorize on the basis of familiarity.     

Inferences about the location of food in the great apes (Pan paniscus, Pan troglodytes, Gorilla gorilla, and Pongo pygmaeus)

Bonobos (Pan paniscus; n = 4), chimpanzees (Pan troglodytes; n = 12), gorillas (Gorilla gorilla; n = 8), and orangutans (Pongo pygmaeus; n = 6) were presented with 2 cups (1 baited) and given visual or auditory information about their contents. Visual information consisted of letting subjects look inside the cups. Auditory information consisted of shaking the cup so that the baited cup produced a rattling sound. Subjects correctly selected the baited cup both when they saw or heard the food. Nine individuals were above chance in both visual and auditory conditions. More important, subjects as a group selected the baited cup when only the empty cup was either shown or shaken, which means that subjects chose correctly without having seen or heard the food (i.e., inference by exclusion). Control tests showed that subjects were not more attracted to noisy cups, avoided shaken noiseless cups, or learned to use auditory information as a cue during the study. It is concluded that subjects understood that the food caused the noise, not simply that the noise was associated with the food.     

Tracking the displacement of objects: a series of tasks with great apes (Pan troglodytes, Pan paniscus, Gorilla gorilla, and Pongo pygmaeus) and young children (Homo sapiens)

The authors administered a series of object displacement tasks to 24 great apes and 24 30-month-old children (Homo sapiens). Objects were placed under 1 or 2 of 3 cups by visible or invisible displacements. The series included 6 tasks: delayed response, inhibition test, A not B, rotations, transpositions, and object permanence. Apes and children solved most tasks performing at comparable levels except in the transposition task, in which apes performed better than children. Ape species performed at comparable levels in all tasks except in single transpositions, in which chimpanzees (Pan troglodytes) and bonobos (Pan paniscus) performed better than gorillas (Gorilla gorilla) and orangutans (Pongo pygmeaus). All species found nonadjacent trials and rotations especially difficult. The number of elements that changed locations, the type of displacement, and having to inhibit predominant reaching responses were factors that negatively affected the subjects' performance.