Chimpanzees utilize video as reference in a spatiotemporally distant search for hidden food.

Referring to things that are displaced in space and time is one of the defining features of human language. In order to better understand the evolution of human language, it is therefore important to explore how widely the ability for displaced reference is shared in animal kingdom. In this study, we explored whether chimpanzees are capable of uzsing video as a displaced reference in a spatiotemporally distant task. We used video to inform chimpanzees about an otherwise unobservable food-hiding. We examined the extent to which chimpanzees would make use of video as a source of information to guide their retrieval of hidden food from a target container. We found that when the event of observing food-hiding and the event of retrieving hidden food were close in space and time within the same room, all chimpanzees solved the task. Some chimpanzees continued to solve the task even when the two events were distant and separated spatiotemporally, in which they had to move to the next room between the events. Our findings suggested that chimpanzees can uzse video as a displaced reference to retrieve hidden food later when solving real-life problems.

Are chimpanzees futurists? Effects of motion lines and motion blur on the judgments of global motion direction in chimpanzees (Pan troglodytes).

Based on the invention and development of photography and movie in the 19th century, schools of contemporary art, such as Futurism, have emerged that express the dynamism of motion in painting. Painting techniques such as multiple stroboscopic images, motion blur, and motion lines are culturally based, but the biological basis of their perception has also been intensively investigated recently. Then what are the evolutionary origins of such pictorial representations of motion? Do nonhuman animals also have sensitivity to such representations? To address this question, we examined the effects of motion blur and motion lines on the judgments of global motion directions in chimpanzees. The results showed that the motion lines biased the chimpanzees' judgments toward the direction of motion implied by them, whereas the effect of the motion blur was either absent or weak (Experiment 1). In Experiment 2, we manipulated the length and number of motion lines to examine the effect of "speed" and "distance" in addition to the motion direction implied by the motion lines. The results showed that the effect of motion lines became stronger as the length and the number of lines increased within a specific range. These results indicate that the motion lines also imply the direction of motion in chimpanzees and provide a clue to the evolutionary basis for the pictorial representations of motion. (PsycInfo Database Record (c) 2024 APA, all rights reserved).

Assessing the motivational value of infant video clips on chimpanzees through discrimination learning task.

The motivational value of visual infant stimuli in humans is considered to encourage parental behavior. To explore the evolutionary roots of this preference for infants, we examined the reward value of conspecific infant videos compared to adult ones in nine chimpanzees. We employed a novel approach, a simultaneous discrimination task with differential sensory reinforcement. In Experiments 1 and 2, we tested if watching conspecific infant videos is more rewarding than watching adult ones. Participants were required to discriminate between two visual stimuli by a touch panel task. In video reward trials, a video clip featuring a chimpanzee infant followed a correct choice, while one featuring an adult followed an incorrect choice. However, the percentage of correct choices did not significantly differ from chance except in one chimpanzee, indicating that chimpanzees did not exhibit a preference for watching infant videos over those of adult. In Experiment 3, we tested if chimpanzees prefer conspecific videos over a blank screen; however, we did not find evidence either at a group level. These results suggest that the incentive salience of infant stimuli may not be universally compelling across species. Additionally, we discuss the limitations of the task using sensory reinforcement.

May the force be with you: exploring force discrimination in chimpanzees using the force-feedback device.

While force-feedback devices have been developed in areas such as virtual reality, there have been very few comparative cognitive studies in nonhuman animals using these devices. In addition, although cross-modal perception between vision and touch has been actively studied in nonhuman primates for several decades, there have been no studies of their active haptic perception. In this study, we attempted to train force discrimination in chimpanzees using a force-feedback device modified from a trackball. Chimpanzees were given different levels of force feedback (8.0 vs. 0.5 N) when moving the on-screen cursor to the target area by manipulating the trackball and were required to select one of two choice stimuli based on the force cue. The experiment was conducted using a trial-block procedure in which the same force stimulus was presented for a fixed number of trials, and the force stimulus was changed between blocks. The block size was progressively reduced from ten trials. Four chimpanzees were trained, but none reached the learning criterion (80% or more correct responses under the condition that the force stimuli were presented randomly). However, a detailed analysis of the chimpanzees' performance before and after the trial-block switching revealed that their choice behavior could not be explained by a simple win-stay/lose-shift strategy, suggesting that the switching of the force stimuli affected the chimpanzees' choice behavior. It was also found that the chimpanzees performed better when switching from small to large force stimuli than when switching from large to small force stimuli. Although none of the chimpanzees in this study acquired force discrimination, future studies using such force-feedback devices will provide new insights for understanding haptic cognition in nonhuman primates from a comparative cognitive perspective.

Endodontic Treatment of a Maxillary Incisor Tooth in a Chimpanzee (Pan troglodytes).

Chimpanzees (Pan troglodytes) with teeth severely damaged by dental caries and/or periodontal disease are often managed with medication and/or tooth extraction. A common endodontic treatment for severely decayed teeth in a 26-year-old female chimpanzee is reported. The left maxillary central incisor tooth had lost its crown, probably due to trauma that was not recent, and it had a fistula most likely due to chronic apical periodontitis. The diagnosis was confirmed radiographically before treatment. To treat the infected root canal, endodontic treatment used in humans was adapted for a chimpanzee. After the treatment, the tooth was sealed using an adhesive resin composite. At 11-years post-treatment, there were no signs of recurrence of the lesion or of failure of the tooth seal. The results of this case report suggest that common endodontic treatments used in humans are also effective in chimpanzees.

Slowly walking down to the more food: relative quantity discrimination in African spurred tortoises (Centrochelys sulcata).

Quantity discrimination, is thought to be highly adaptive as it allows an organism to select greater amounts of food or larger social groups. In contrast to mammals, the processes underlying this ability are not as well understood in reptiles. This study examined the effects of ratio and number size on relative quantity discrimination in African spurred tortoises (Centrochelys sulcata). To assess these effects, tortoises were presented with trays containing favored food pieces in all possible number combinations between 1 and 7. The tortoises had to approach the tray they perceived as having the larger quantity. If correct, they received one piece of food as reinforcement. The results revealed that relative quantity discrimination was influenced by the ratio between the numbers of pieces, with performance improving as the ratio between the numbers increased. This finding suggests that the approximate number system or analogue magnitude estimation may control their behavior. However, as the number size increased, their performance declined, also suggesting that the approximate number system alone could not explain the present results.

Left or right, that is the question: use of egocentric frame of reference and the right-eye advantage for understanding gestural signs in bottlenose dolphins (Tursiops truncatus).

How do bottlenose dolphins visually perceive the space around them? In particular, what cues do they use as a frame of reference for left-right perception? To address this question, we examined the dolphin's responses to various manipulations of the spatial relationship between the dolphin and the trainer by using gestural signs for actions given by the trainer, which have different meanings in the left and right hands. When the dolphins were tested with their backs to the trainer (Experiment 1) or in an inverted position underwater (Experiments 2 and 3), correct responses from the trainer's perspective were maintained for signs related to movement direction instructions. In contrast, reversed responses were frequently observed for signs that required different sounds for the left and right hands. When the movement direction instructions were presented with symmetrical graphic signs such as " × " and "●", accuracy decreased in the inverted posture (Experiment 3). Furthermore, when the signs for sounds were presented from either the left or right side of the dolphin's body, performance was better when the side of the sign movement coincided with the body side on which it was presented than when it was mismatched (Experiment 4). In the final experiment, when one eye was covered with an eyecup, the results showed that, as in the case of body-side presentation, performance was better when the open eye coincided with the side on which the sign movement was presented. These results indicate that dolphins used the egocentric frame for visuospatial cognition. In addition, they showed better performances when the gestural signs were presented to the right eye, suggesting the possibility of a left-hemispheric advantage in the dolphin's visuospatial cognition.

Don't look back on failure: spontaneous uncertainty monitoring in chimpanzees.

During computer-controlled cognitive tasks, chimpanzees often look up at the food dispenser, which activates at the same time as feedback for the correct choice but not for feedback for the incorrect choice. Do these "looking back" behaviors also indicate signs of spontaneous monitoring of their confidence in their choices? To address this question, we delayed the feedback for 1 s after their choice responses and observed their look-back behaviors during the delay period. Two chimpanzees looked up at the food dispenser significantly less frequently when their choice was incorrect (but the feedback was not given) than when it was correct. These look-back behaviors have not been explicitly trained under experimental contexts. Therefore, these results indicate that chimpanzees spontaneously change the frequency of their look-back behaviors in response to the correctness or incorrectness of their own choices, even without external feedback, suggesting that their look-back behaviors may reflect the level of "confidence" or "uncertainty" of their responses immediately before.

Attention to inattention: Effect of the other's attentional state on visual search performance in chimpanzees.

In the present study, we examined the effects of the other's triadic attention to objects on visual search performances in chimpanzees. We found the search-asymmetry-like effect of the other's attentional state; the chimpanzees searched a target object not attended by the other individual more efficiently than that attended (Experiment 1). Additional experiments explored the possibility that the other individual "holding an object but not looking at it" led to expectancy violation (Experiment 2) or the role of nonsocial cues such as the proximity relation between the head and the object (Experiment 3). Still, these accounts alone did not explain this effect. It was also shown that the other's attentional state affected the chimpanzees' performances more readily as the interference effect than the facilitation effect (Experiment 4). Furthermore, the same effect was observed in the visual search for the gaze (head direction) of others (Experiment 5). We obtained the same results using photographs of chimpanzees (Experiment 6). Contrary to the chimpanzees, humans detected the object to which attention was directed more efficiently than vice versa (Experiment 7). The present results may reflect species differences between chimpanzees and humans in processing triadic social attention.

Do chimpanzees see a face on Mars? A search for face pareidolia in chimpanzees.

We sometimes perceive meaningful patterns or images in random arrangements of colors and shapes. This phenomenon is called pareidolia and has recently been studied intensively, especially face pareidolia. In contrast, there are few comparative-cognitive studies on face pareidolia with nonhuman primates. This study explored behavioral evidence for face pareidolia in chimpanzees using visual search and matching tasks. Faces are processed in a configural manner, and their perception and recognition are hampered by inversion and misalignment of top and bottom parts. We investigated whether the same effect occurs in a visual search for face-like objects. The results showed an effect of misalignment. On the other hand, consistent results were not obtained with the photographs of fruits. When only the top or bottom half of the face-like object was presented, chimpanzees showed better performance for the top-half condition, suggesting the importance of the eye area in face pareidolia. In the positive-control experiments, chimpanzees received the same experiment using human faces and human participants with face-like objects and fruits. As a result, chimpanzees showed an inefficient search for inverted and misaligned faces and humans for manipulated face-like objects. Finally, to examine the role of face awareness, we tested matching a human face to a face-like object in chimpanzees but obtained no substantial evidence that they saw the face-like object as a "face." Based on these results, we discussed the extents and limits of face pareidolia in chimpanzees.

Going forward: perceptual bias for forward-facing motion in chimpanzees.

When a row of objects surrounded by a frame suddenly shifts a certain distance so that part of the row is occluded by the frame, humans perceive ambiguous apparent motion either to the left or the right. However, when the objects have "directionality," humans perceive them as moving forward in the direction in which they are pointing, which is termed forward-facing motion bias. In the present study, five experiments were conducted to address whether, and if so how, physical properties or prior knowledge about the objects affected the perception of their apparent motion in two juvenile chimpanzees (Pan troglodytes). In experiment 1, the chimpanzees did not show a clear forward-facing bias in judging the direction of motion when directed triangles were presented, whereas the human participants did. In contrast, when pictures of the lateral view of chimpanzees with quadrupedal postures were shown, there was a clear bias for going "forward" with regards to the side with the head (experiment 2). We presented pictures of dogs looking back to explore what features caused the forward-facing motion bias (experiment 3). Chimpanzees did not show any bias for these stimuli, suggesting that the direction of the head and body interactively affected the perceptual bias. Experiment 4 tested the role of the head and found that only the lateral view of the heads of chimpanzees or humans caused the bias (experiment 4). Additional tests also showed that the chimpanzees could not solve the task based only on the direction of the stimuli without motion (experiment 5). These results indicate that the perception of motion in the chimpanzees was affected by the biological features of the stimuli, suggesting their prior knowledge of the "body" from a biological (morphological and kinetic) perspective.

Chimpanzees (Pan troglodytes) detect strange body parts: an eye-tracking study.

This study investigated chimpanzee body representation by testing whether chimpanzees detect strangeness in body parts. We tested six chimpanzees with edited chimpanzee body pictures in eye-tracking tasks. The target body parts were arms or legs. For either target, there were four conditions: "normal" condition as control, where all bodies were normal; "misplaced" condition, where one arm or one leg was misplaced to an incorrect body location in each picture; "replaced by a chimpanzee part" condition, where one arm or one leg was replaced by a chimpanzee leg or arm, respectively, in its original place in each picture; and "replaced by a human part" condition, where one arm or one leg was replaced by a human arm or leg in each picture. Compared to the looking times toward the normal parts, chimpanzees had significantly longer looking times toward the human arms or legs. The looking times toward the misplaced parts were also longer than the normal parts, but the difference just failed to meet significance. These results indicate more interests toward strange body parts, compared to typical parts, suggesting that chimpanzees might have a body representation that is sufficiently sensitive to detect these aspects of strangeness.

Chimpanzees (Pan troglodytes) exhibit gaze bias for snakes upon hearing alarm calls.

Calls of several species of nonhuman animals are considered to be functionally referential. However, the cognitive mechanisms underlying audience behaviors remain unclear. This study used an audiovisual cross-modal preferential-looking paradigm to examine whether captive chimpanzees spontaneously associated a conspecific call with images of a corresponding item. Chimpanzees were presented with videos of snakes and fruit side-by-side while hearing playbacks of alarm calls, food-associated calls, or no sound (as a baseline condition). Chimpanzees looked at videos of snakes for longer when hearing alarm calls compared with food calls or baseline. However, chimpanzees did not look at videos of fruit for longer when hearing food calls compared with baseline. An additional experiment tested whether chimpanzees' gaze bias to the snake videos was driven by negative affective states in general via affect-driven attention biases. When chimpanzees were presented with the same snake and fruit videos while hearing playbacks of conspecific screams or no sound, they exhibited no gaze bias for snake videos. These results suggest that chimpanzees spontaneously associated alarm calls with images of a potential threat in a preferential-looking experiment and that this response was not simply driven by an affective state matching process. These findings should be interpreted in consideration of a procedural limitation related to pseudoreplication in the experimental stimuli. (PsycInfo Database Record (c) 2022 APA, all rights reserved).

The evolution of quantitative sensitivity.

The ability to represent approximate quantities appears to be phylogenetically widespread, but the selective pressures and proximate mechanisms favouring this ability remain unknown. We analysed quantity discrimination data from 672 subjects across 33 bird and mammal species, using a novel Bayesian model that combined phylogenetic regression with a model of number psychophysics and random effect components. This allowed us to combine data from 49 studies and calculate the Weber fraction (a measure of quantity representation precision) for each species. We then examined which cognitive, socioecological and biological factors were related to variance in Weber fraction. We found contributions of phylogeny to quantity discrimination performance across taxa. Of the neural, socioecological and general cognitive factors we tested, cortical neuron density and domain-general cognition were the strongest predictors of Weber fraction, controlling for phylogeny. Our study is a new demonstration of evolutionary constraints on cognition, as well as of a relation between species-specific neuron density and a particular cognitive ability. This article is part of the theme issue 'Systems neuroscience through the lens of evolutionary theory'.

No evidence of spatial representation of age, but "own-age bias" like face processing found in chimpanzees.

Previous studies have revealed that non-human primates can differentiate the age category of faces. However, the knowledge about age recognition in non-human primates is very limited and whether non-human primates can process facial age information in a similar way to humans is unknown. As humans have an association between time and space (e.g., a person in an earlier life stage to the left and a person in a later life stage to the right), we investigated whether chimpanzees spatially represent conspecifics' adult and infant faces. Chimpanzees were tested using an identical matching-to-sample task with conspecific adult and infant face stimuli. Two comparison images were presented vertically (Experiment 1) or horizontally (Experiment 2). We analyzed whether the response time was influenced by the position and age category of the target stimuli, but there was no evidence of correspondence between space and adult/infant faces. Thus, evidence of the spatial representation of the age category was not found. However, we did find that the response time was consistently faster when they discriminated between adult faces than when they discriminated between infant faces in both experiments. This result is in line with a series of human face studies that suggest the existence of an "own-age bias." As far as we know, this is the first report of asymmetric face processing efficiency between infant and adult faces in non-human primates.

Search asymmetries for threatening faces in chimpanzees (Pan troglodytes).

For primates, the ability to efficiently detect threatening faces is highly adaptive; however, it is not clear exactly how faces are detected. This study investigated whether chimpanzees show search asymmetries for conspecific threatening faces featuring scream and bared teeth expressions. Five adult female chimpanzees participated in a series of touchscreen matching-to-sample visual search tasks. In Experiment 1, search advantages for scream versus neutral targets and scream versus bared teeth targets were found. A serial search strategy indicated greater difficulty in disengaging attention from scream versus neutral distractors. In Experiments 2a and 2b, search advantages for scream versus neutral targets remained when the mouth was darkened, suggesting that the brightness contrast of the mouth was not critical for the efficient detection of scream targets. In Experiments 3a and 3b, search advantages for inverted scream versus neutral targets disappeared, indicating configural processing. Together, exclusion of the brightness contrast of the mouth as a low-level perceptual confound, and evidence of configural processing, suggested the scream faces may have been perceived as threatening. However, the search advantage for scream faces is most likely explained by the presence of teeth, independently of threat. The study provides further support that an attentional bias toward threatening faces is a homologous trait, which can be traced back to at least the last common ancestor of Old World monkeys and apes. (PsycInfo Database Record (c) 2022 APA, all rights reserved).

Impairment effect of infantile coloration on face discrimination in chimpanzees.

Impaired face recognition for certain face categories, such as faces of other species or other age class faces, is known in both humans and non-human primates. A previous study found that it is more difficult for chimpanzees to differentiate infant faces than adult faces. Infant faces of chimpanzees differ from adult faces in shape and colour, but the latter is especially a salient cue for chimpanzees. Therefore, impaired face differentiation of infant faces may be due to a specific colour. In the present study, we investigated which feature of infant faces has a greater effect on face identification difficulty. Adult chimpanzees were tested using a matching-to-sample task with four types of face stimuli whose shape and colour were manipulated as either infant or adult one independently. Chimpanzees' discrimination performance decreased as they matched faces with infant coloration, regardless of the shape. This study is the first to demonstrate the impairment effect of infantile coloration on face recognition in non-human primates, suggesting that the face recognition strategies of humans and chimpanzees overlap as both species show proficient face recognition for certain face colours.

Great apes' understanding of biomechanics: eye-tracking experiments using three-dimensional computer-generated animations.

Visual processing of the body movements of other animals is important for adaptive animal behaviors. It is widely known that animals can distinguish articulated animal movements even when they are just represented by points of light such that only information about biological motion is retained. However, the extent to which nonhuman great apes comprehend the underlying structural and physiological constraints affecting each moving body part, i.e., biomechanics, is still unclear. To address this, we examined the understanding of biomechanics in bonobos (Pan paniscus) and chimpanzees (Pan troglodytes), following a previous study on humans (Homo sapiens). Apes underwent eye tracking while viewing three-dimensional computer-generated (CG) animations of biomechanically possible or impossible elbow movements performed by a human, robot, or nonhuman ape. Overall, apes did not differentiate their gaze between possible and impossible movements of elbows. However, some apes looked at elbows for longer when viewing impossible vs. possible robot movements, which indicates that they may have had knowledge of biomechanics and that this knowledge could be extended to a novel agent. These mixed results make it difficult to draw a firm conclusion regarding the extent to which apes understand biomechanics. We discuss some methodological features that may be responsible for the results, as well as implications for future nonhuman animal studies involving the presentation of CG animations or measurement of gaze behaviors.

The contingency symmetry bias (affirming the consequent fallacy) as a prerequisite for word learning: A comparative study of pre-linguistic human infants and chimpanzees.

Humans are known to possess an "affirming the consequent fallacy," which assumes that a learned contingency holds true even when the order is reversed. In contrast, non-human animals do not fall for this fallacy, as they do not have the contingency symmetry bias. Importantly, language is founded on the symmetrical relationship between symbols and referents, and the contingency symmetry bias plays a key role in word learning. A critical problem for the ontogenesis of language is whether the contingency symmetry bias has been acquired through the experience of word learning or if it is present before infants begin word learning. Using a habituation switch paradigm, 8-month-old human infants and adult chimpanzees were familiarized with two object-then-movement sequences, whereby Object A (or B) was always paired with Movement A (or B). At test, the order of the contingency was reversed. The infants showed surprise when observing the violation of the object-movement pairings in the reversed sequence (Experiment 1). In contrast, despite the chimpanzees being able to detect the violation of the pairings in the original direction (Experiment 2a), they did not discriminate the learned and novel pairings when the order of the contingency was reversed (Experiment 2b). The results suggest that the contingency symmetry bias is a uniquely human cognitive bias, one which plays a critical role for language acquisition ontogenetically. This contingency symmetry bias likely gives humans a great advantage, by enabling them to rapidly expand their knowledge without direct training and making them strikingly different from other animal species. (250 words).