Compound grips in tufted capuchin monkeys (Sapajus spp and Sapajus libidinosus)

Manual preference, performance, and dexterity for bimanual grass-feeding behavior in wild geladas (Theropithecus gelada)

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.

Moving

Examples of realized scientific careers can provide ideas and inspiration for others aiming to pursue such careers. Here I recount in brief the story of my long career in primatology (1973 to the present), focusing on one enduring theme in my research: the nature and genesis of goal-directed action (evident in movement). The story begins in graduate school, passes through developing my own laboratory, on to pursuing a spectrum of studies with mentees and collaborators, developing a theoretical explanatory framework for goal-directed action that I think holds promise for the field as a whole, and ends with an exciting field project that seems a suitable finale to my career. I mention the value to me, the field, and society of participation in scientific societies, including the American Society of Primatologists, throughout my career.

Manipulative repertoire of bonobos (Pan paniscus) in spontaneous feeding situation

Comparative behavioral studies of hand use amongst primate species, including humans, have been central in research on evolutionary mechanisms. In particular, the manipulative abilities of our closest relatives, the chimpanzee (Pan troglodytes), have been widely described in various contexts, showing a high level of dexterity both in zoo and in natural conditions. In contrast, the study of bonobos' manipulative abilities has almost exclusively been carried out in experimental contexts related to tool use. The objective of the present study is to describe the richness of the manipulative repertoire of zoo-housed bonobos, in a spontaneous feeding context including various physical substrates to gain a larger insight into our evolutionary past. Our study describes a great variety of grasping postures and grip associations in bonobos, close to the range of manipulative repertoire in chimpanzees, confirming that the two species are not markedly different in terms of cognitive and morphological constraints associated with food manipulation. We also observed differences in manipulative behaviors between juveniles and adults, indicating a greater diversity in grip associations and grasping postures used in isolation with age, and a sex-biased use of tools with females using tools more often than males. These results are consistent with the previous results in the Pan genus and reinforce the hypothesis that the evolutionary mechanisms underlying the flexibility of manipulative behaviors are shared by both species and that these ecological strategies would have already evolved in their common ancestor.