Cognitive performance of grey mouse lemurs (Microcebus murinus) during a discrimination learning task: Effect of the emotional valence of stimuli.

Emotions are omnipresent in many animals' lives. It is a complex concept that encompasses physiological, subjective, behavioural and cognitive aspects. While the complex relationship between emotion and cognition has been well studied in humans and in some nonhuman primates, it remains rather unexplored for other nonhuman primate species, such as lemurs. In our study, we evaluated the performance of N = 48 grey mouse lemurs (Microcebus murinus) in a discrimination learning task using visual emotional stimuli. We tested whether the type of visual stimulus (positive, negative or neutral) influenced the cognitive performance of mouse lemurs. Individuals had to learn to discriminate between two platforms according to the associated visual stimuli and to jump to the target platform (leading to a reward). Our main finding was that emotional stimuli, whether positive or negative in valence, impaired cognitive performance when used as a target. Specifically, the lowest success rate occurred when the target was associated with the emotional stimuli, and the highest success rate occurred when it was associated with neutral stimuli. Our results show a similar pattern to that found in other primate species and support the adaptive role of emotion. Our results also support that individual differences could be a factor impacting the relation between emotion and cognition. This study is the first to explore how emotions interfere with the cognitive abilities of a lemur species and highlights the importance of acknowledging emotion in mouse lemurs as well as studying the emotion-cognition interaction in a wider range of primate species.

The influence of body posture on the kinematics of prehension in humans and gorillas (Gorilla gorilla).

Much of our current understanding of human prehension in a comparative context is based on macaque models in a sitting, constrained body posture. In a previous study, we clearly showed differences in the amplitude of the forelimb joints between five primate species (lemur, capuchin, chimpanzee, gorilla and human) during unconstrained grasping where the animals were free to choose their body posture. One of our interrogations was to know if these differences could be due to the body posture. To address this question, this study compares humans with new data for gorillas during an unconstrained food prehension task in two body postures, a sitting and a quadrupedal one. The objective is to determine the behavioral and kinematic strategies (amplitudes and patterns of evolution of the articular angles) as well as differences and invariants of trunk and forelimb motions between species. The subjects were recorded by five cameras, and landmarks were digitized frame by frame to reconstruct 3D movement. Our results show that (1) despite significant influences of body postures on ranges of motion in gorillas and humans, species preserve their specific forelimb joint and trunk contribution; (2) body posture has a limited effect on the basic pattern of wrist velocity. Our study indicates that different primate species have specific kinematic features of limb coordination during prehension, which dose not alter with changes in posture. Therefore, across varying species, it is possible to compare limb kinematics irrespective of postural constraints and unconstrained condition need to be explored in other primates to understand the evolution of primate prehension.

Getting a grip on the evolution of grasping in musteloid carnivorans: a three-dimensional analysis of forelimb shape.

The ability to grasp and manipulate is often considered a hallmark of hominins and associated with the evolution of their bipedal locomotion and tool use. Yet, many other mammals use their forelimbs to grasp and manipulate objects. Previous investigations have suggested that grasping may be derived from digging behaviour, arboreal locomotion or hunting behaviour. Here, we test the arboreal origin of grasping and investigate whether an arboreal lifestyle could confer a greater grasping ability in musteloid carnivorans. Moreover, we investigate the morphological adaptations related to grasping and the differences between arboreal species with different grasping abilities. We predict that if grasping is derived from an arboreal lifestyle, then the anatomical specializations of the forelimb for arboreality must be similar to those involved in grasping. We further predict that arboreal species with a well-developed manipulation ability will have articulations that facilitate radio-ulnar rotation. We use ancestral character state reconstructions of lifestyle and grasping ability to understand the evolution of both traits. Finally, we use a surface sliding semi-landmark approach capable of quantifying the articulations in their full complexity. Our results largely confirm our predictions, demonstrating that musteloids with greater grasping skills differ markedly from others in the shape of their forelimb bones. These analyses further suggest that the evolution of an arboreal lifestyle likely preceded the development of enhanced grasping ability.

Food prehension and manipulation in Microcebus murinus (Prosimii, Cheirogaleidae).

Among primates, apes and monkeys are known to use their hands and to exhibit independent control of their fingers. In comparison, Prosimii are thought to have less digital individualization and to use their mouth more commonly for prehension. Unfortunately, prehension and manipulation studies in Prosimii have been conducted in conditions constraining the subject to grasp with the hand. Moreover, the effect of food size remains unexplored, even though it could affect the use of the hands versus the mouth. Thus, whether prosimians use the hand or the mouth to grasp and manipulate food items of different sizes in unconstrained conditions remains unclear. To address this question, we characterized the eating and manipulation patterns of Microcebus murinus in unconstrained conditions, using three food sizes. The results showed that M. murinus showed (i) an eating pattern similar to that of rodents, with smaller food items being grasped with the mouth, (ii) a greater tendency to use the hands for prehension of larger foods, and (iii) plasticity during food manipulation similar to that which has been observed in rodents. These results are discussed in the framework of grasping in mammals and are used to discuss the origins of prehension in primates.

Influence of the task on hand preference: individual differences among gorillas (Gorilla gorilla gorilla).

The degree of task complexity and bimanual complementarity have been proposed as factors affecting lateralization strength in humans. However, a large number of studies have demonstrated group-level lateral hand bias for different manual activities in numerous non-human primate species. However, no study has tested the effects that a variety of tasks may have in inducing differences in hand preference. Here, we aim to test if 3 adult gorillas exhibited a greater hand preference bias performing 4 tasks of varying complexity: grasping small versus large foods, proto-tool use task and tool use task involving greater visuospatial requirements. We found that (1) the complexity of the task does not necessarily induce a right-handed bias and (2) a subject can be right-handed for a complex task and left-handed for another one. These results, complemented by many publications on hand preference in non-human primates, reveal a great variability in hand preference, which makes it very difficult to deduce any details of hominin handedness with artefacts.

A clarification of Pouydebat et al., 2008, evolution of grasping among anthropoids.

Several statements by Pouydebat et al. (2008) do not adequately represent views of authors cited, in part because they reflect confusion in the literature about terminology regarding precision gripping. We address these problems, by tracing definitions of precision grips through the literature on manipulative behaviour and identifying the grip that is central to the Pouydebat et al. (2008) study. This allows us to offer a clarification of the statements by Pouydebat et al. (2008) regarding the sequence of appearance of human grip capabilities and possible morphological correlates to these capabilities in extant species.

Evolution of grasping among anthropoids.

The prevailing hypothesis about grasping in primates stipulates an evolution from power towards precision grips in hominids. The evolution of grasping is far more complex, as shown by analysis of new morphometric and behavioural data. The latter concern the modes of food grasping in 11 species (one platyrrhine, nine catarrhines and humans). We show that precision grip and thumb-lateral behaviours are linked to carpus and thumb length, whereas power grasping is linked to second and third digit length. No phylogenetic signal was found in the behavioural characters when using squared-change parsimony and phylogenetic eigenvector regression, but such a signal was found in morphometric characters. Our findings shed new light on previously proposed models of the evolution of grasping. Inference models suggest that Australopithecus, Oreopithecus and Proconsul used a precision grip.

Substrate optimization in nut cracking by capuchin monkeys (Cebus apella).

We conducted an experiment to examine the effect of substrate on the nut-cracking behavior of a group of semicaptive capuchin monkeys. We wanted to determine whether tufted capuchin monkeys were selective in choosing the substrate on which they pound nuts, and whether the choice of substrate affected the outcome. Eight adult females and eight juveniles were provided with nuts in the outdoor facility. We found that 1) all adult females and three young capuchins succeeded in cracking nuts; 2) they preferred the hardest substrates (concrete and stone); 3) there is a link between the substrate and the amount of time needed to crack a nut; 4) most young capuchins used various substrates, some of which were inadequate, in a haphazard manner; and 5) there are different forms of nut cracking. We conclude that adult capuchins choose the hardest substrates, and that these substrates support efficient cracking.