Valentina Truppa

Bimanual coordination is considered a crucial factor in the evolution of manual lateralization. Experimental tasks requiring the simultaneous and coordinated use of both hands induce hand preferences at the population level across various nonhuman primate species studied in captive environments. However, to date, research investigating laterality in bimanual coordination within natural settings remains scarce, especially with respect to platyrrhine taxa. We assessed hand preference in a bimanual coordinated extractive foraging task by wild bearded capuchin monkeys, Sapajus libidinosus , living in the Cerrado/Caatinga ecotone, Northeast Brazil. Nineteen individuals were filmed during the consumption of four different nut species for 4 months. A detailed video‐coding analysis was conducted on their hand movements to extract the edible kernel(s) from the nuts. Overall, most individuals were lateralized, and as a group, capuchins significantly preferred their right‐hand fingers for extracting the kernel, thus suggesting an advantage of the left hemisphere for fine finger movements. The comparison with captive capuchins previously tested with an experimental bimanual coordinated task (the tube task), revealed that the direction of the preference did not differ between wild and captive individuals, although the latter tended to use their preferred hand more consistently. Our findings are in line with a growing body of evidence suggesting that bimanual coordination might have played a major role in the evolution of limb preference for motor action in primates and indicate that the study of coordinated bimanual behaviors in natural settings provides a valuable contribution to the understanding of manual laterality.

Categorization skills enable animal species to sort entities based on their similarities, providing significant advantages in terms of learning efficiency. The extent of these skills and the mechanisms involved vary across taxa, making it critical to explore categorization in as many lineages as possible. From an evolutionary standpoint, fish are valuable models for studying categorization processes; however, until recently, this group of vertebrates has been neglected in visual categorization research. To address this gap, we conducted three experiments to investigate, for the first time, the visual categorical learning skills of the redtail splitfin (Xenotoca eiseni) and the zebrafish (Danio rerio), two teleost species studied in other contexts of animal cognition. Using a two-alternative forced-choice task, we first trained the fish to discriminate between several pairs of stimuli. Then, we tested their ability to generalize to pairs of new stimuli of the same categories. The stimulus categories varied across the experiments (Experiments 1 and 2: alphanumeric characters; Experiment 3: fish silhouettes) and were consistent with those used in past studies involving different taxa. Both fish species met the learning criterion in the training phase across all three experiments, but their ability to generalize to novel stimuli differed based on stimulus features. Moreover, both species presented interindividual differences in their ability to generalize what they learnt. Our results validate the use of X. eiseni and D. rerio as models for research on categorical learning. They also encourage further investigation into how experimental methods and stimuli could affect learning-based generalization abilities in these species.

Categorization skills enable animal species to sort entities based on their similarities, providing significant advantages in terms of learning efficiency. The extent of these skills and the mechanisms involved vary across taxa, making it critical to explore categorization in as many lineages as possible. From an evolutionary standpoint, fish are valuable models for studying categorization processes; however, until recently, this group of vertebrates has been neglected in visual categorization research. To address this gap, we conducted three experiments to investigate, for the first time, the visual categorical learning skills of the redtail splitfin (Xenotoca eiseni) and the zebrafish (Danio rerio), two teleost species studied in other contexts of animal cognition. Using a two-alternative forced-choice task, we first trained the fish to discriminate between several pairs of stimuli. Then, we tested their ability to generalize to pairs of new stimuli of the same categories. The stimulus categories varied across the experiments (Experiments 1 and 2: alphanumeric characters; Experiment 3: fish silhouettes) and were consistent with those used in past studies involving different taxa. Both fish species met the learning criterion in the training phase across all three experiments, but their ability to generalize to novel stimuli differed based on stimulus features. Moreover, both species presented interindividual differences in their ability to generalize what they learnt. Our results validate the use of X. eiseni and D. rerio as models for research on categorical learning. They also encourage further investigation into how experimental methods and stimuli could affect learning-based generalization abilities in these species.

We assessed whether wild geladas, highly specialized terrestrial grass eaters, are lateralized for bimanual grass‐plucking behavior. According to the literature, we expected that complex motor movements in grass feeding would favor the emergence of a population‐level hand bias in these primates. In addition, we described geladas' manual behavior based on systematic observations of several individuals. Our study group included 28 individuals belonging to a population of free‐ranging geladas frequenting the Kundi plateau, Ethiopia. We filmed monkeys while feeding on grass, and hand preference and performance were coded. Geladas performed more plucking movements per second with their left hand (LH) compared to the right one and preferred their LH both to start and finish collection bouts. Also, the rhythmic movements of each hand had a significant tendency toward isochrony. Finally, geladas used forceful pad‐to‐pad precision grips, in‐hand movements, and compound grips to pluck and collect grass blades, considered the most advanced manual skills in primate species. The LH's leading role suggests an advantage of the right hemisphere in regulating geladas' bimanual grass‐feeding behavior. The tactile input from the hands and/or rhythmic hand movements might contribute to explaining this pattern of laterality. Our findings highlighted the importance of adopting multiple laterality measures to investigate manual laterality. Moreover, the need to speed up the execution time of manual foraging might be a further important factor in studying the evolution of manual laterality and dexterity in primates.

Two experiments employing an identity matching-to-sample procedure were carried out to clarify the factors affecting global-local visual processing of capuchin monkeys (Sapajus spp.) in comparison with humans. In the first experiment, we assessed the relative ability of the two species to discriminate high, medium, or low spatial frequencies (HSFs, MSFs, or LSFs). Then, in a second experiment, we determined if the use of a procedure designed to induce a bias toward attending given spatial frequencies could produce a top-down or selection-history modulation of global-local visual processing in capuchins and humans. In the first experiment, monkeys discriminated better HSFs. By contrast, humans discriminated better MSFs and LSFs. The second experiment showed an effect of SF processing on global-local processing in both species. However, this effect was confined to local trials only and occurred under different conditions in the two species. In monkeys, it occurred following a bias toward attending HSFs, whereas in humans, it occurred following a bias toward attending LSFs. These results provide new information about the relative sensitivity of humans and capuchins to different spatial frequencies in vision. Moreover, they suggest that global-local visual processing can be modulated in both humans and monkeys by processes that are not confined to attending one or the other level of stimulus structure.

The way in which animals grasp objects to perform subsequent action execution allows studying their anticipatory abilities. We examined whether 11 capuchin monkeys (Sapajus spp.) were able to prospectively grasp a magnetic dowel to use it as a tool to retrieve a baited metallic container from a plexiglas box placed in front of them. We investigated whether and how initial dowel orientation (horizontal vs vertical) affected grasping and using the dowel to retrieve the container in two testing conditions: (1) 2-FE condition in which the dowel had two functional magnetic ends; (2) 1-FE condition in which the dowel had only one functional magnetic end. In the 2-FE condition, capuchins had to take into account the initial dowel orientation since both ends were functional, whereas in the 1-FE condition capuchins had also to take into account the initial functional end position when grasping the dowel. Capuchins were trained to grasp the dowel to put one functional end in contact with the metallic container. However, they did not learn to associate the functional end of the 1-FE dowel to successful retrieval. Capuchins showed better anticipatory planning (1) in 2-FE than in 1-FE condition and (2) when the dowel was initially positioned on the horizontal plane than on the vertical one. Moreover, hand preferences affected planning in the 1-FE condition. Results were discussed within the frameworks of primates’ abilities to use abstract cues and on their abilities to process functional features and spatial cues and to perform mental rotations.

Self‐directed object manipulation tasks illuminate development of motor planning. Grasping strategies that lead to good object control to perform the following action(s) reveal second‐order motor planning. Motor planning for efficient grips develops slowly in children. Age‐related differences in other primates have been poorly investigated. Here, we investigated anticipatory motor planning of infant, juvenile and adult wild capuchin monkeys grasping a horizontally positioned stick baited to the left or right side (a version of the elevated spoon task). We recorded the grasps capuchins used to bring the baited end of the stick to the mouth. The percentage of efficient radial grips positively correlated with age and adults used efficient grips significantly more frequently than infants. Adult wild capuchins' use of radial grips was higher than that reported for adult captive capuchins in similar tasks, suggesting that experience throughout life may influence motor anticipation. Self‐directed object manipulation tasks will be useful to compare this aspect of cognition across primates. A video abstract of this article can be viewed at https://youtu.be/a1Zbr_AQkb8.

Tool use in humans can be optional, that is, the same person can use different tools or no tool to achieve a given goal. Strategies to reach the same goal may differ across individuals and cultures and at the intra‐individual level. This is the first experimental study at the intra‐individual level on the optional use of a tool in wild nonhuman primates. We investigated optional tool use by wild bearded capuchins (Sapajus libidinosus) of Fazenda Boa Vista (FBV; Piauí, Brazil). These monkeys habitually succeed in cracking open the mesocarp of dry cashew nuts (Anacardiumspp.) by pounding them with stones and/or by biting. We assessed whether availability of a stone and resistance of the nut affected capuchins' choice to pound or to bite the nuts and their rates of success. Sixteen capuchins (1–16 years) received small and large dry cashew nuts by an anvil together with a stone (Stone condition) or without a stone (No‐Stone condition). In the Stone conditions, subjects used it to crack the nut in 89.1% (large nuts) and 90.1% (small nut) of the trials. Nut size significantly affected the number of strikes used to open it. Availability of the stone significantly increased the average percent of success. In the No‐Stone conditions, monkeys searched for and used other percussors to crack the nuts in 54% of trials. In all conditions, age affects percentage of success and number of strikes to reach success. We argue that exclusive use of stones in other sites may be due to the higher abundance of stones at these sites compared with FBV. Since capuchins opened cashews with a tool 1–2 years earlier than they succeed at cracking more resistant palm nuts, we suggest that success at opening cashew nuts with percussors may support the monkeys' persistent efforts to crack palm nuts.