Consistency of hand preference across low-level and high-level tasks in Capuchin monkeys (Cebus apella)

Evaluation of the main disorders and microbiota of the oral cavity of capuchin monkeys (Sapajus apella) under human care

BACKGROUND

Although critical to the overall condition of animals under human care, there is still limited information about oral health in neotropical primates.

METHODS

We analyzed the main oral conditions and microbiota using mass spectrometry from 13 capuchin monkeys (Sapajus apella) under human care. The findings were registered on odontograms following the Triadan system.

RESULTS

The most prevalent conditions were dental fractures (n = 9), mainly enamel fractures, and periodontal disease (n = 8), mainly grade 1 calculi. When exanimating teeth, alterations were identified in 90 out of the 416 evaluated pieces, being periodontal disease the most common (n = 60), followed by enamel fracture (n = 15) and missing teeth (n = 10). In the oral microbiota analyses, Staphylococcus and Streptococcus species were the most prevalent, although no obvious association was observed between isolated organisms and oral conditions.

CONCLUSIONS

These findings hold the potential to prevent oral disorders, including fractures and periodontal diseases, contribute to molecular identification of oral microbiota, and to improve the well-being of primates under human care.

Measuring paw preferences in dogs, cats and rats: Design requirements and innovations in methodology

Studying behavioural lateralization in animals holds great potential for answering important questions in laterality research and clinical neuroscience. However, comparative research encounters challenges in reliability and validity, requiring new approaches and innovative designs to overcome. Although validated tests exist for some species, there is yet no standard test to compare lateralized manual behaviours between individuals, populations, and animal species. One of the main reasons is that different fine-motor abilities and postures must be considered for each species. Given that pawedness/handedness is a universal marker for behavioural lateralization across species, this article focuses on three commonly investigated species in laterality research: dogs, cats, and rats. We will present six apparatuses (two for dogs, three for cats, and one for rats) that enable an accurate assessment of paw preference. Design requirements and specifications such as zoometric fit for different body sizes and ages, reliability, robustness of the material, maintenance during and after testing, and animal welfare are extremely important when designing a new apparatus. Given that the study of behavioural lateralization yields crucial insights into animal welfare, laterality research, and clinical neuroscience, we aim to provide a solution to these challenges by presenting design requirements and innovations in methodology across species.

Hand preference in wild crab-eating capuchin monkeys (Sapajus libidinosus) in the coastal area of Northest Brazil

Handedness is a fundamental human trait, although recent research, especially on nonhuman primates, has shown that it is displayed by other animals as well (e.g., chimpanzees, gorillas). In this study, we explore hand preference in wild crab-eating tufted capuchin monkeys (Sapajus libidinosus) inhabiting a mangrove forest located on the coastal area of Northeast Brazil (Maranhão State). Tufted capuchin monkeys at our site use facultatively wooden tools to crack open crabs. We observed hand preference in 12 subjects who spontaneously participated in experiment sessions, in which we provided crabs and tools on wooden platforms. We recorded (using events and bouts) two unimanual tasks, (tool or crab) grabbing and (tool or crab) pounding, and one bimanual task, crab pulling, where one hand kept the crab in place while the other pulled off parts of the crab. Hand preference increased with greater strength needed to perform the task and its complexity. While only 17%-25% of capuchins showed hand preference during grabbing, 44%-64% showed hand preference during pounding, and most subjects 64%-80% displayed a right-hand preference when performing the bimanual task, for which all lateralized individuals were right-handed. Hand preference did not vary between adults and juvenile individuals and was not consistent across tasks. Group-level hand preference was found only for the bimanual task, for which all lateralized individuals were right-handed. Our findings are in concordance with those of other primate studies showing the emergence of hemispheric specialization for bimanual actions, highlight the importance of conducting such studies on diverse type of tasks, and show the feasibility to conduct experimental manipulation under natural conditions.

The evolution and biological correlates of hand preferences in anthropoid primates

The evolution of human right-handedness has been intensively debated for decades. Manual lateralization patterns in non-human primates have the potential to elucidate evolutionary determinants of human handedness, but restricted species samples and inconsistent methodologies have so far limited comparative phylogenetic studies. By combining original data with published literature reports, we assembled data on hand preferences for standardized object manipulation in 1786 individuals from 38 species of anthropoid primates, including monkeys, apes, and humans. Based on that, we employ quantitative phylogenetic methods to test prevalent hypotheses on the roles of ecology, brain size, and tool use in primate handedness evolution. We confirm that human right-handedness represents an unparalleled extreme among anthropoids and found taxa displaying population-level handedness to be rare. Species-level direction of manual lateralization was largely uniform among non-human primates and did not strongly correlate with any of the selected biological predictors, nor with phylogeny. In contrast, we recovered highly variable patterns of hand preference strength, which show signatures of both ecology and phylogeny. In particular, terrestrial primates tend to display weaker hand preferences than arboreal species. These results challenge popular ideas on primate handedness evolution, including the postural origins hypothesis. Furthermore, they point to a potential adaptive benefit of disparate lateralization strength in primates, a measure of hand preference that has often been overlooked in the past. Finally, our data show that human lateralization patterns do not align with trends found among other anthropoids, suggesting that unique selective pressures gave rise to the unusual hand preferences of our species.

Evaluating Self-Directed Behaviours and Their Association with Emotional Arousal across Two Cognitive Tasks in Bonobos (Pan paniscus)

Self-directed behaviours (SDBs) are widely used as markers of emotional arousal in primates, and are commonly linked to negative arousal, or are used as indicators of stress or poor welfare. However, recent studies suggest that not all SDBs have the same function. Moreover, lateralisation in the production of these behaviours has been suggested to be associated with emotional processing. Hence, a better understanding of the production and the asymmetry of these displacement behaviours is needed in a wider range of species in order to confirm their reliability as indicators of emotional arousal. In the current study, we experimentally evaluated the production and asymmetry of SDBs in zoo-housed bonobos during two cognitive touchscreen tasks. Overall, nose wipes were most commonly observed, followed by gentle self-scratches, and rough self-scratches. The rates of nose wipes and rough self-scratches increased with incorrect responses, suggesting that these behaviours indicate arousal and possibly frustration. Rough self-scratching was additionally more directed towards the left hemispace after incorrect responses. In contrast, gentle self-scratching increased after correct responses in one study, possibly linking it with positive arousal. We also tested if left-handed bonobos showed greater behavioural reactivity towards incorrect responses, but found no evidence to confirm this hypothesis. Our results shed light on potential different mechanisms behind separate SDBs. We therefore provide nuance to the use of SDBs as indicator of emotional arousal in bonobos.

Hand preferences in coordinated bimanual tasks in non-human primates: A systematic review and meta-analysis

The evolutionary significance of hand preferences among non-human primates and humans has been studied for decades with the aim of determining the origins of the population-level tendency. In this study, a meta-analysis was conducted to statistically integrate data on hand preferences in non-human primates performing the tube task and other bimanual tasks to determine the presence and direction of manual laterality. Significant individual-level lateralization was obtained for these bimanual tasks. In nonhuman primates, 82% of the animals analysed showed right or left-hand preference performing the tube task, this figure being 90% for other bimanual tasks. In contrast with humans, no asymmetry was found at the population level. Additionally, population-level preferences were not found in either of the tasks, although a strong manual preference was found when performing the tube task and other bimanual tasks. Species was studied as a variable moderator throughout the meta-analysis. These results highlight the importance of standardized testing methodologies across species and institutions to obtain comparable data and fill the gaps in the taxonomy.

Hand preference in unimanual and bimanual coordinated tasks in wild western lowland gorillas (Gorilla gorilla gorilla) feeding on African ginger (Zingiberaceae)

OBJECTIVES

Bimanual coordinated behaviors are critical for detecting robust individual hand preference in nonhuman primates but are particularly challenging to observe in the wild. This study focuses on spontaneous feeding behavior on African ginger (Aframomum sp. and Renealmia sp.), which involves a unimanual task (reaching and pulling out a ginger stem) and a bimanual coordinated task (extracting pith from a ginger stem) by wild western lowland gorillas.

MATERIALS AND METHODS

Study subjects were 21 gorillas in the Moukalaba-Doudou National Park, Gabon. We examined whether they exhibit significant hand preference at the individual and group levels for both tasks.

RESULTS

Sixteen gorillas showed significant hand preference in the unimanual task, whereas all 21 individuals showed significant hand preference in the bimanual coordinated task. Hand preference was significantly stronger in the bimanual coordinated task than in the unimanual task. It is noteworthy that gorillas showed a significant right-hand preference at the group level for the bimanual task (roughly 70% of the subjects).

DISCUSSION

This study confirmed that bimanual coordinated tasks are more sensitive in detecting hand preferences in nonhuman primates. In addition to the bimanual nature of the task, the precision grip for processing and the importance of African ginger as a food resource might influence the expression of hand preference. Evidence of a group-level right-hand preference may support the "postural origins theory." Because all wild African great apes feed on the pith of African ginger, comparing this task and its hand preferences can contribute toward a better understanding of the evolution of handedness in Hominidae.

Asymmetries in mother-infant behaviour in Barbary macaques (Macaca sylvanus)

Asymmetries in the maternal behaviour and anatomy might play an important role in the development of primate manual lateralization. In particular, early life asymmetries in mother’s and infant’s behaviour have been suggested to be associated with the development of the hand preference of the offspring. The aim of this study was to investigate the presence of behavioural asymmetries in different behavioural categories of mother-infant dyads of zoo-living Barbary macaques (Macaca sylvanus). The study subjects were 14 Barbary macaques involved in seven mother-infant dyads housed in Parco Natura Viva, Italy. For the mothers, bouts of hand preference for maternal cradling and infant retrieval were collected. For the infants, we focused on nipple preference and hand preference for clinging on mother ventrum. Moreover, we collected bouts of hand preference for food reaching in both groups. No significant group-level bias was found for any of the behavioural categories in either mothers or infants. However, at the individual level, six out of seven mothers showed a significant cradling bias, three toward the right hand and three toward the left hand. Moreover, all infants showed a significant nipple preference, six toward the mother’s right nipple, one toward the left nipple. Furthermore, a significant correlation was found between the infant nipple preference and their hand preference for food reaching, suggesting that maternal environment rather than behaviour might affect the development of hand preference in Old World monkeys. Our findings seem partially to add to previous literature on perceptual lateralization in different species of non-primate mammals, reporting a lateral bias in mother-infant interactions. Given the incongruences between our study and previous research in great apes and humans, our results seem to suggest possible phylogenetic differences in the lateralization of mothers and infants within the Primates order.

Hand preference during bimanual coordinated task in northern pig-tailed macaques Macaca leonina

In humans, handedness is one defining characteristic regardless of cultures and ethnicity. Population-level right handedness is considered to be related with the evolution of left hemisphere for manual control and language. In order to further understand evolutionary origins of human cerebral lateralization and its behavioral adaptation, standardized measures on hand preference are required to make reliable comparison in nonhuman primate species. In this study, we present the first evidence on hand preference during bimanual coordinated tasks in northern pig-tailed macaques Macaca leonina. The classical TUBE task was applied to examine hand preference among nine individuals from Tianjin Zoo of China. We recorded and made analysis on both frequency and bout data on manual laterality. The results consistently show that subjects displayed strong individual hand preferences, whereas no significant group-level handedness was found. There were no sex and age significant differences on both direction and strength of hand preference. The M. leonina preferred to use the index finger to extract the baited food inside the tube. Our findings fill the knowledge gap on primate handedness, and efficiently affirm the robustness of the TUBE task as one efficient measure of hand preference in primates.

Handedness for Unimanual Grasping in 564 Great Apes: The Effect on Grip Morphology and a Comparison with Hand Use for a Bimanual Coordinated Task

A number of factors have been proposed to influence within and between species variation in handedness in non-human primates. In the initial study, we assessed the influence of grip morphology on hand use for simple reaching in a sample of 564 great apes including 49 orangutans Pongo pygmaeus, 66 gorillas Gorilla gorilla, 354 chimpanzees Pan troglodytes and 95 bonobos Pan paniscus. Overall, we found a significant right hand bias for reaching. We also found a significant effect of the grip morphology of hand use. Grasping with the thumb and index finger was more prevalent in the right compared to left hand in all four species. There was no significant sex effect on the patterns of handedness. In a subsample of apes, we also compared consistency in hand use for simple reaching with previously published data on a task that measures handedness for bimanual actions. We found that the ratio of subjects with consistent right compared to left hand use was more prevalent in bonobos, chimpanzees and gorillas but not orangutans. However, for all species, the proportion of subjects with inconsistent hand preferences between the tasks was relatively high suggesting some measures may be more sensitive in assessing handedness than others.

Precise digit use increases the expression of handedness in Colombian spider monkeys (Ateles fusciceps rufiventris)

Decades of research on the hand use patterns of nonhuman primates can be aptly summarized by the following phrase: measurement matters. There is a general consensus that simple reaching is a poor indicator of handedness in most species, while tasks that constrain how the hands are used elicit individual, and in some cases, population-level biases. The TUBE task has become a popular measure of handedness, although there is variability in its administration across studies. The goal of this study was to investigate whether TUBE performance is affected by tube diameter, with the hypothesis that decreasing tube diameter would increase task complexity, and therefore the expression of handedness. We predicted that hand preference strength, but not direction, would be affected by tube diameter. We administered the TUBE task using a 1.3 cm tube to Colombian spider monkeys, and compared their performance to a previous study using a larger 2.5 cm diameter tube. Hand preference strength increased significantly on the smaller diameter tube. Hand preference direction was not affected. Notably, spider monkeys performed the TUBE task using a single digit, despite the longstanding view that this species has poor dexterity. We encourage investigators who use the TUBE task to carefully consider the diameter of the tube used in testing, and to report digit use consistently across studies. In addition, we recommend that researchers who cannot use the TUBE task try to incorporate the key features from this task into their own species appropriate measures: bimanual coordination and precise digit use.

Division of labor in hand usage is associated with higher hand performance in free-ranging bonnet macaques, Macaca radiate [corrected]

A practical approach to understanding lateral asymmetries in body, brain, and cognition would be to examine the performance advantages/disadvantages associated with the corresponding functions and behavior. In the present study, we examined whether the division of labor in hand usage, marked by the preferential usage of the two hands across manual operations requiring maneuvering in three-dimensional space (e.g., reaching for food, grooming, and hitting an opponent) and those requiring physical strength (e.g., climbing), is associated with higher hand performance in free-ranging bonnet macaques, Macaca radiate [corrected]. We determined the extent to which the macaques exhibit laterality in hand usage in an experimental unimanual and a bimanual food-reaching task, and the extent to which manual laterality is associated with hand performance in an experimental hand-performance-differentiation task. We observed negative relationships between (a) the latency in food extraction by the preferred hand in the hand-performance-differentiation task (wherein, lower latency implies higher performance), the preferred hand determined using the bimanual food-reaching task, and the normalized difference between the performance of the two hands, and (b) the normalized difference between the performance of the two hands and the absolute difference between the laterality in hand usage in the unimanual and the bimanual food-reaching tasks (wherein, lesser difference implies higher manual specialization). Collectively, these observations demonstrate that the division of labor between the two hands is associated with higher hand performance.

A comparative assessment of hand preference in captive red howler monkeys, Alouatta seniculus and yellow-breasted capuchin monkeys, Sapajus xanthosternos

There are two major theories that attempt to explain hand preference in non-human primates-the 'task complexity' theory and the 'postural origins' theory. In the present study, we proposed a third hypothesis to explain the evolutionary origin of hand preference in non-human primates, stating that it could have evolved owing to structural and functional adaptations to feeding, which we refer to as the 'niche structure' hypothesis. We attempted to explore this hypothesis by comparing hand preference across species that differ in the feeding ecology and niche structure: red howler monkeys, Alouatta seniculus and yellow-breasted capuchin monkeys, Sapajus xanthosternos. The red howler monkeys used the mouth to obtain food more frequently than the yellow-breasted capuchin monkeys. The red howler monkeys almost never reached for food presented on the opposite side of a wire mesh or inside a portable container, whereas the yellow-breasted capuchin monkeys reached for food presented in all four spatial arrangements (scattered, on the opposite side of a wire mesh, inside a suspended container, and inside a portable container). In contrast to the red howler monkeys that almost never acquired bipedal and clinging posture, the yellow-breasted capuchin monkeys acquired all five body postures (sitting, bipedal, tripedal, clinging, and hanging). Although there was no difference between the proportion of the red howler monkeys and the yellow-breasted capuchin monkeys that preferentially used one hand, the yellow-breasted capuchin monkeys exhibited an overall weaker hand preference than the red howler monkeys. Differences in hand preference diminished with the increasing complexity of the reaching-for-food tasks, i.e., the relatively more complex tasks were perceived as equally complex by both the red howler monkeys and the yellow-breasted capuchin monkeys. These findings suggest that species-specific differences in feeding ecology and niche structure can influence the perception of the complexity of the task and, consequently, hand preference.

Evaluating handedness measures in spider monkeys

Despite long-standing interest and a vast body of literature, there is still disagreement as to how handedness should be measured in nonhuman primates. The primary goal of this study was to evaluate two common measures of handedness in nonhuman primates using the spider monkey, a unique study species due to its lack of a thumb and limited dexterity. Contrary to our predictions and previous findings in Ateles, there was no evidence for group-level hand biases on either the coordinated bimanual TUBE task or a unimanual reaching task. At the individual level, monkeys exhibited preferences on both tasks. There was a leftward trend on the bimanual task and a rightward trend on the unimanual task. Monkeys that were strongly lateralized on the bimanual task showed a comparable hand preference on the unimanual task, whereas monkeys with a moderate preference on the bimanual task shifted to the opposite hand on the unimanual task. Comparing across measures, the two hand-use patterns reported (consistent and shift) might have obscured group-level findings, given the available sample size. Overall, these data reaffirm that task type influences hand use in primates, and multiple measures are needed to fully characterize the construct of handedness. Consideration should be given to the difficulty required between tasks as well as between species.

Comparing human and nonhuman primate handedness: challenges and a modest proposal for consensus

In the past 20-25 years, there have been a number of studies published on handedness in nonhuman primates. The goal of these studies has been to evaluate whether monkeys and apes show patterns of hand preference that resemble the right-handedness found in the human species. The extant findings on handedness in nonhuman primates have revealed inconsistent evidence for population-level handedness within and between species. In this article, I discuss some of the methodological and statistical challenges to comparative studies of handedness in human and nonhuman primates. I further offer a framework for developing some consensus on evaluating the validity of different handedness measures and the characterization of individual hand preferences.

Performance asymmetries in tool use are associated with corpus callosum integrity in chimpanzees (Pan troglodytes): a diffusion tensor imaging study

The authors examined the relationship of corpus callosum (CC) morphology and organization to hand preference and performance on a motor skill task in chimpanzees. Handedness was assessed using a complex tool use task that simulated termite fishing. Chimpanzees were initially allowed to perform the task wherein they could choose which hand to use (preference measure), then they were required to complete trials using each hand (performance measure). Two measures were used to assess the CC: midsagittal area obtained from in vivo magnetic resonance images and density of transcallosal connections as determined by fractional anisotropy values obtained from diffusion tensor imaging. The authors hypothesized that chimpanzees would perform better on their preferred hand compared to the nonpreferred hand, and that strength of behavioral lateralization (rather the direction) on this task would be negatively correlated to regions of the CC involved in motor processing. Results indicate that the preferred hand was the most adept hand. Performance asymmetries correlated with fractional anisotropy measures but not area measures of the CC.

Hand preference for tool-use in capuchin monkeys (Cebus apella) is associated with asymmetry of the primary motor cortex

Skilled motor actions are associated with handedness and neuroanatomical specializations in humans. Recent reports have documented similar neuroanatomical asymmetries and their relationship to hand preference in some nonhuman primate species, including chimpanzees and capuchin monkeys. We investigated whether capuchins displayed significant hand preferences for a tool-use task and whether such preferences were associated with motor-processing regions of the brain. Handedness data on a dipping tool-use task and high-resolution 3T MRI scans were collected from 15 monkeys. Capuchins displayed a significant group-level left-hand preference for this type of tool use, and handedness was associated with asymmetry of the primary motor cortex. Left-hand preferent individuals displayed a deeper central sulcus in the right hemisphere. Our results suggest that capuchins show an underlying right-hemisphere bias for skilled movement.

Forelimb preferences in quadrupedal marsupials and their implications for laterality evolution in mammals

BACKGROUND

Acquisition of upright posture in evolution has been argued to facilitate manual laterality in primates. Owing to the high variety of postural habits marsupials can serve as a suitable model to test whether the species-typical body posture shapes forelimb preferences in non-primates or this phenomenon emerged only in the course of primate evolution. In the present study we aimed to explore manual laterality in marsupial quadrupeds and compare them with the results in the previously studied bipedal species. Forelimb preferences were assessed in captive grey short-tailed opossum (Monodelphis domestica) and sugar glider (Petaurus breviceps) in four different types of unimanual behaviour per species, which was not artificially evoked. We examined the possible effects of sex, age and task, because these factors have been reported to affect motor laterality in placental mammals.

RESULTS

In both species the direction of forelimb preferences was strongly sex-related. Male grey short-tailed opossums showed right-forelimb preference in most of the observed unimanual behaviours, while male sugar gliders displayed only a slight, not significant rightward tendency. In contrast, females in both species exhibited consistent group-level preference of the left forelimb. We failed to reveal significant differences in manual preferences between tasks of potentially differing complexity: reaching a stable food item and catching live insects, as well as between the body support and food manipulation. No influence of subjects' age on limb preferences was found.

CONCLUSIONS

The direction of sex-related differences in the manual preferences found in quadrupedal marsupials seems to be not typical for placental mammals. We suggest that the alternative way of interhemispheric connection in absence of corpus callosum may result in a fundamentally distinct mechanism of sex effect on limb preferences in marsupials compared to placentals. Our data confirm the idea that non-primate mammals differ from primates in sensitivity to task complexity. Comparison of marsupial species studied to date indicate that the vertical body orientation and the bipedalism favor the expression of individual- and population-level forelimb preferences in marsupials much like it does in primates. Our findings give the first evidence for the effect of species-typical posture on the manual laterality in non-primate mammals.

Independence of data points in the measurement of hand preferences in primates: statistical problem or urban myth?

Lack of independence of data points or the pooling fallacy has been suggested as a potential problem in the study of handedness in nonhuman primates, particularly as it relates to whether hand use responses should be recorded as individual events or bouts of activity. Here, I argue that there is no evidence that the concept of statistical independence of data points or the pooling fallacy is a problem in the evaluation of population-level handedness in previous studies in nonhuman primates. I further argue these statistical concepts have been misapplied to the characterization of individual hand preferences. Finally, I argue that recording hand use responses as bouts rather than events has no significant effect on reports of hand use in nonhuman primates and, in fact, may unintentionally bias hand use toward the null hypothesis. Several suggestions for improvement in the measurement and statistical determination of individual handedness are offered in the article.

Unimanual to bimanual: tracking the development of handedness from 6 to 24 months

Manual skills change dramatically over the first two years of life, creating an interesting challenge for researchers studying the development of handedness. A vast body of work to date has focused on unimanual skills during the period from the onset of reaching to walking. The current study sought to connect such early unimanual hand use to later role-differentiated bimanual manipulation (RDBM), in which one hand stabilizes the object for the other hand's action. We examined hand use in 38 children over 16 monthly visits using a validated measure for assessing hand preference for acquiring objects when children were 6-14 months old. We also developed a new measure for assessing RDBM preference presented when children were 18-24 months old. The new measure reliably elicited RDBM actions in both toddlers and an adult control group (N = 15). Results revealed that some children show preferences for acquiring objects as infants; these preferences are stable and persist into their second year as new skills appear. Moreover, children with no hand preference during infancy shifted to left or right lateralized hand use as toddlers. Despite a higher incidence of left-handedness compared to adult norms, the majority of children were right-handed by 2 years of age.

Handedness in nature: first evidence on manual laterality on bimanual coordinated tube task in wild primates

Handedness is a defining feature of human manual skill and understanding the origin of manual specialization remains a central topic of inquiry in anthropology and other sciences. In this study, we examined hand preference in a sample of wild primates on a task that requires bimanual coordinated actions (tube task) that has been widely used in captive primates. The Sichuan snub-nosed monkey (Rhinopithecus roxellana) is an arboreal Old World monkey species that is endemic to China, and 24 adult individuals from the Qinling Mountains of China were included for the analysis of hand preference in the tube task. All subjects showed strong individual hand preferences and significant group-level left-handedness was found. There were no significant differences between males and females for either direction or strength of hand preference. Strength of hand preferences of adults was significantly greater than juveniles. Use of the index finger to extract the food was the dominant extractive-act. Our findings represent the first evidence of population-level left-handedness in wild Old World monkeys and broaden our knowledge on evaluating primate hand preference via experimental manipulation in natural conditions.

Complex tasks force hand laterality and technological behaviour in naturalistically housed chimpanzees: inferences in hominin evolution

Clear hand laterality patterns in humans are widely accepted. However, humans only elicit a significant hand laterality pattern when performing complementary role differentiation (CRD) tasks. Meanwhile, hand laterality in chimpanzees is weaker and controversial. Here we have reevaluated our results on hand laterality in chimpanzees housed in naturalistic environments at Fundació Mona (Spain) and Chimfunshi Wild Orphanage (Zambia). Our results show that the difference between hand laterality in humans and chimpanzees is not as great as once thought. Furthermore, we found a link between hand laterality and task complexity and also an even more interesting connection: CRD tasks elicited not only the hand laterality but also the use of tools. This paper aims to turn attention to the importance of this threefold connection in human evolution: the link between CRD tasks, hand laterality, and tool use, which has important evolutionary implications that may explain the development of complex behaviour in early hominins.

Kinematics of reaching and implications for handedness in rhesus monkey infants

Kinematic studies of reaching in human infants using two-dimensional (2-D) and three-dimensional (3-D) recordings have complemented behavioral studies of infant handedness by providing additional evidence of early right asymmetries. Right hand reaches have been reported to be straighter and smoother than left hand reaches during the first year. Although reaching has been a popular measure of handedness in primates, there has been no systematic comparison of left and right hand reach kinematics. We investigated reaching in infant rhesus monkeys using the 2-D motion analysis software MaxTRAQ Lite+ (Innovision Systems). Linear mixed-effects models revealed that left hand reaches were smoother, but not straighter, than right hand reaches. An early left bias matches previous findings of a left hand preference for reaching in adult rhesus monkeys. Additional work using this kind of kinematic approach will extend our understanding of primate handedness beyond traditional studies measuring only frequency or bouts of hand use.

Observations of hand preference in wild groups of white-faced sakis (Pithecia pithecia) in Suriname

Hand preference is well observed in humans and some primates. Unlike many other primates, however, humans show a consistent hand preference across a variety of tasks, and a distinct right-handed skew at the population level. Although there are a moderate number of published studies, primate hand preference literature is unbalanced by the large number of studies on only a few species. No previous studies have addressed hand preference in white-faced sakis (WFS; Pithecia pithecia). We followed three habituated groups of wild WFS in Suriname and recorded individual hand preference for six different manual behaviors. There was no consistent hand preference across a range of uni-manual behaviors for any individual. Likewise, there were significantly more ambidextrous individuals in the population than expected (χ(2) (df = 2) = 11.2, P = 0.004) and thus, no population level hand preference. Our findings contribute baseline data to the debate of primate hand lateralization, and support the notion that lateralization of hand function does not characterize all species.

Can Traditions Emerge from the Interaction of Stimulus Enhancement and Reinforcement Learning? An Experimental Model

The study of social learning in captivity and behavioral traditions in the wild are two burgeoning areas of research, but few empirical studies have tested how learning mechanisms produce emergent patterns of tradition. Studies have examined how social learning mechanisms that are cognitively complex and possessed by few species, such as imitation, result in traditional patterns, yet traditional patterns are also exhibited by species that may not possess such mechanisms. We propose an explicit model of how stimulus enhancement and reinforcement learning could interact to produce traditions. We tested the model experimentally with tufted capuchin monkeys (Cebus apella), which exhibit traditions in the wild but have rarely demonstrated imitative abilities in captive experiments. Monkeys showed both stimulus enhancement learning and a habitual bias to perform whichever behavior first obtained them a reward. These results support our model that simple social learning mechanisms combined with reinforcement can result in traditional patterns of behavior.

Brief communication: Captive gorillas are right-handed for bimanual feeding

Predominance of right-handedness has historically been considered as a hallmark of human evolution. Whether nonhuman primates exhibit population-level manual bias remains a controversial topic. Here, we investigated the hypothesis that bimanual coordinated activities may be a key-behavior in our ancestors for the emergence and evolution of human population-level right-handedness. To this end, we collected data on hand preferences in 35 captive gorillas (Gorilla gorilla) during simple unimanual reaching and for bimanual coordinated feeding. Unimanual reaching consisted of grasping food on the ground, while bimanual feeding consisted of using one hand for holding a food and processing the food item by the opposite hand. No population-level manual bias was found for unimanual actions but, in contrast, gorillas exhibited a significant population-level right-handedness for the bimanual actions. Moreover, the degree of right-handedness for bimanual feeding exceeds any other known reports of hand use in primates, suggesting that lateralization for bimanual feeding is robust in captive gorillas. The collective evidence is discussed in the context of potential continuity of handedness between human and nonhuman primates.