Substrate Diameter and Orientation in the Context of Food Type in the Gray Mouse Lemur, Microcebus murinus: Implications for the Origins of Grasping in Primates

Influence of food physical properties and environmental context on manipulative behaviors highlighted by new methodological approaches in zoo-housed bonobos (Pan paniscus)

Research on manipulative abilities in nonhuman primates, in the context of hominid evolution, has mostly focused on manual/pedal postures considered as static behaviors. While these behavioral repertoires highlighted the range of manipulative abilities in many species, manipulation is a dynamic process that mostly involves successive types of grips before reaching its goal. The present study aims to investigate the use of manual/pedal postures in zoo-housed bonobos in diverse dynamic food processing by using an innovative approach: the optimal matching analysis that compares sequences (i.e., succession of grasping postures) with each other. To characterize the manipulative techniques spontaneously employed by bonobos, we performed this sequential analysis of manual/pedal postures during 766 complete feeding sequences of 17 individuals. We analyzed the effectiveness with a score defined by a partial proxy of food intake (i.e., the number of mouthfuls) linked to a handling score measuring both the diversity and changes of manual postures during each sequence. We identified four techniques, used differently depending on the physical substrate on which the individual performed food manipulation and the food physical properties. Our results showed that manipulative techniques were more complex (i.e., higher handling score) for large foods and on substrates with lower stability. But the effectiveness score was not significantly lower for these items since manipulative complexity seemed to be compensated by a greater number of mouthfuls. It appeared that the techniques employed involved a trade-off between manipulative complexity and the amount of food ingested. This study allowed us to test and validate innovative analysis methods that are applicable to diverse ethological studies involving sequential events. Our results bring new data for a better understanding of the evolution of manual abilities in primates in association with different ecological contexts and both terrestrial and arboreal substrates and suggest that social and individual influences need to be explored further.

Effect of binocular visual cue availability on fruit and insect grasping performance in two cheirogaleids: Implications for primate origins hypotheses

Forward-facing eyes with parallel optic axes, which provide a wide field of binocular vision and precise depth perception, are among the diagnostic features of crown primates; however, the adaptive significance of this feature remains contentious. Two of the most prominent primate-origins hypotheses propose that either foraging for fruit or nocturnal predation on insects created selective pressures that led to the evolution of diagnostic primate traits, including a wide binocular field. To determine whether either of these hypotheses provides a viable explanation for the evolution of primates' derived eye orientation, the importance of binocular depth cues for the two tasks invoked by these hypotheses was evaluated experimentally in Microcebus murinus and Cheirogaleus medius, cheirogaleids' considered reasonable living analogs of the earliest euprimates. Performance in grasping insects and fruit was evaluated when the animals made use of their full binocular visual field and when their binocular visual field was restricted using a helmet-mounted blinder. Restriction of the binocular field had no effect on fruit grasping performance; however, restriction of the binocular field resulted in a significant deficit in insect predation performance. Differences in behavioral variables also suggest that insect predation is a more visually demanding task than fruit foraging. These results support the role of insect predation, but not fruit foraging, in contributing to the selective pressures that led to the evolution of parallel optic axes and a wide binocular field in crown primates.

The relationship between distal trunk morphology and object grasping in the African savannah elephant (Loxodonta africana)

Background

During reach-to-grasp movements, the human hand is preshaped depending on the properties of the object. Preshaping may result from learning, morphology, or motor control variability and can confer a selective advantage on that individual or species. This preshaping ability is known in several mammals (i.e., primates, carnivores and rodents). However, apart from the tongue preshaping of lizards and chameleons, little is known about preshaping of other grasping appendages. In particular, the elephant trunk, a muscular hydrostat, has impressive grasping skills and thus is commonly called a hand. Data on elephant trunk grasping strategies are scarce, and nothing is known about whether elephants preshape their trunk tip according to the properties of their food.

Methods

To determine the influence of food sizes and shapes on the form of the trunk tip, we investigated the morphology of the distal part of the trunk during grasping movements. The influence of food item form on trunk tip shape was quantified in six female African savannah elephants (Loxodonta africana). Three food item types were presented to the elephants (elongated, flat, and cubic), as well as three different sizes of cubic items. A total of 107 ± 10 grips per individual were video recorded, and the related trunk tip shapes were recorded with a 2D geometric morphometric approach.

Results

Half of the individuals adjusted the shape of the distal part of their trunk according to the object type. Of the three elephants that did not preshape their trunk tip, one was blind and another was subadult.

Discussion and perspectives

We found that elephants preshaped their trunk tip, similar to the preshaping of other species' hands or paws during reach-to-grasp movements. This preshaping may be influenced by visual feedback and individual learning. To confirm these results, this study could be replicated with a larger sample of elephants.

Behavioural variability among captive African elephants in the use of the trunk while feeding

The Proboscideans, an order of mammals including elephants, are the largest of the Earth lands animals. One probable consequence of the rapid increase of their body size is the development of the trunk, a multitask highly sensitive organ used in a large repertoire of behaviours. The absence of bones in the trunk allows a substantial degree of freedom for movement in all directions, and this ability could underlie individual-level strategies. We hypothesised a stronger behavioural variability in simple tasks, and a correlation between the employed behaviours and the shape and size of the food. The observations of a captive group of African elephants allowed us to create a complete catalogue of trunk movements in feeding activities. We noted manipulative strategies and impact of food item properties on the performed behaviours. The results show that a given item is manipulated with a small panel of behaviours, and some behaviours are specific to a single shape of items. The study of the five main feeding behaviours emphasises a significant variability between the elephants. Each individual differed from every other individual in the proportion of at least one behaviour, and every behaviour was performed in different proportions by the elephants. Our findings suggest that during their lives elephants develop individual strategies adapted to the manipulated items, which increases their feeding efficiency.

Ontogeny of locomotion in mouse lemurs: Implications for primate evolution

The environment of juvenile primates is very challenging. They have to forage and move on the same substrates as adults do and escape the same predators, despite their immature state. In this study, we explore the developmental strategies that may provide effective locomotor abilities early in life. This could provide new insights into the selective pressures acting on juvenile primates and into evolution of primate locomotion. We conducted an ontogenetic study of 36 arboreal gray mouse lemurs from birth to adulthood (6 months of age). The investigated parameters were, for both limbs, (1) grasping behavior during locomotion (i.e., grip postures), (2) grasping performance (i.e., pull strength), and (3) motor coordination (i.e., rotarod test). Our results show that 8-day-old babies are able to climb substrates of various slopes and diameters outside of their nest. Although juveniles cannot successfully complete a motor coordination test before 30 days of age, young individuals display relative pull strengths that are very high or even on par with adults, guaranteeing stability on narrow substrates. These powerful grasps highlight the importance of the grasping function for these juveniles that are not carried and move independently on arboreal substrates shortly after their first week of life. Moreover, the pedal grasping provides a secure grasp on all substrates across ontogeny; however, manual secure grasps decrease during development, being highly used only shortly after birth on vertical and narrow substrates. These results first suggest different functional roles of the hands and feet, with the hind limbs ensuring body balance on the substrates, freeing the upper limbs for manipulation. They further show vertical and narrow branches to be especially challenging, requiring strong grasps, which suggests that they may drive the evolution of strong grasping abilities in primates.

The Central Role of Small Vertical Substrates for the Origin of Grasping in Early Primates

The manual and pedal grasping abilities of primates, characterized by an opposable hallux, flat nails, and elongated digits, constitute a unique combination of features that likely promoted their characteristic use of arboreal habitats. These hand and foot specificities are central for understanding the origins and early evolution of primates and have long been associated with foraging in a fine-branch milieu. However, other arboreal mammals occupy similar niches, and it remains unclear how substrate type may have exerted a selective pressure on the acquisition of nails and a divergent pollex/hallux in primates or in what sequential order these traits evolved. Here, we video-recorded 14,564 grasps during arboreal locomotion in 11 primate species (6 strepsirrhines and 5 platyrrhines) and 11 non-primate arboreal species (1 scandentian, 3 rodents, 3 carnivorans, and 4 marsupials). We quantified our observations with 19 variables to analyze the effect of substrate orientation and diameter on hand and foot postural repertoire. We found that hand and foot postures correlate with phylogeny. Also, primates exhibited high repertoire diversity, with a strong capability for postural adjustment compared to the other studied groups. Surprisingly, nails do not confer an advantage in negotiating small substrates unless the animal is large, but the possession of a grasping pollex and hallux is crucial for climbing small vertical substrates. We propose that the divergent hallux and pollex may have resulted from a frequent use of vertical plants in early primate ecological scenarios, although nails may not have resulted from a fundamental adaptation to arboreal locomotion.

Food mobility and the evolution of grasping behaviour: a case study in strepsirrhine primates

Manual grasping is widespread among tetrapods but is more prominent and dexterous in primates. Whether the selective pressures that drove the evolution of dexterous hand grasping involved the collection of fruit or predation on mobile insects remains an area of debate. One way to explore this question is to examine preferences for manual versus oral grasping of a moving object. Previous studies on strepsirrhines have shown a preference for oral grasping when grasping static food items and a preference for manual grasping when grasping mobile prey such as insects, but little is known about the factors at play. Using a controlled experiment with a simple and predictable motion of a food item, we tested and compared the grasping behaviours of 53 captive individuals belonging to 17 species of strepsirrhines while grasping swinging food items and static food items. The swinging motion increased the frequency of hand-use for all individuals. Our results provide evidence that the swinging motion of the food is a sufficient parameter to increase hand grasping in a wide variety of strepsirrhine primates. From an evolutionary perspective, this result gives some support to the idea that hand-grasping abilities evolved under selective pressure associated with the predation of food items in motion. Looking at a common grasping pattern across a large set of species, this study provides important insight into comparative approaches to understanding the evolution of the hand grasping of food in primates and potentially other tetrapod taxa.

My branch is your branch: Talar morphology correlates with relative substrate size in platyrrhines at Tiputini Biodiversity Station, Ecuador

Given that most species of primates are predominantly arboreal, maintaining the ability to move among branches of varying sizes has presumably been a common selective force in primate evolution. However, empirical evaluations of the relationships between morphological variation and characteristics of substrate geometry, such as substrate diameter relative to an animal's body mass, have been limited by the lack of quantified substrate usage in the wild. Here we use recently published quantitative data to assess the relationships between relative substrate size and talar morphology in nine New World monkey species at the Tiputini Biodiversity Station, Ecuador. Within this sample, both fibular facet angle (the angle between the fibular facet and the trochlear rims) and body-mass-standardized area of the medial tibial facet decrease as average and maximum relative substrate size increases. Correlations between medial tibial facet area and relative substrate size are driven by the inclusion of callitrichids in this sample. Nevertheless, these findings strengthen the hypothesis that variation in fibular facet orientation and medial tibial facet area are functionally correlated with habitual degrees of pedal inversion. They also strengthen the notion that evolutionarily changing body mass could impact habitat geometry experienced by a lineage and thereby substantially impact major trends in primate morphological evolution. This study highlights the importance of empirical data on substrate use in living primates for inferring functional and evolutionary implications of morphological variation.

The effect of food properties on grasping and manipulation in the aquatic frog Xenopus laevis

The ability to grasp an object is fundamental from an evolutionary perspective. Involved in many daily activities, grasping has been extensively studied in primates and other mammals. Yet other groups of tetrapods, including anurans, have also evolved significant forelimb prehensile capacities that are often thought to have originated in an arboreal context. In addition, grasping is also observed in aquatic species. But how aquatic frogs use their forelimbs to capture and manipulate prey remains largely unknown. The aim of this study is to explore how the grasping and manipulation of food items in aquatic frogs is impacted by food properties such as size and mobility. To do so, we uses the aquatic frog Xenopus laevis and quantified the use of the hands and fingers while processing mobile and stationary prey of different sizes (small, intermediate and large). Our results show that X. laevis is able to individualize the digits and that the mobility and the length of the prey significantly influence the kind of grasping pattern used. Grasping abilities are thus not specific to terrestrial or arboreal species. These results illustrate how prey properties impact grasping and manipulation strategies in an aquatic frog and shed further light on the ecological contexts that may have given rise to the origin of grasping in frogs.

Postcrania of the most primitive euprimate and implications for primate origins

The fossil record of early primates is largely comprised of dentitions. While teeth can indicate phylogenetic relationships and dietary preferences, they say little about hypotheses pertaining to the positional behavior or substrate preference of the ancestral crown primate. Here we report the discovery of a talus bone of the dentally primitive fossil euprimate Donrussellia provincialis. Our comparisons and analyses indicate that this talus is more primitive than that of other euprimates. It lacks features exclusive to strepsirrhines, like a large medial tibial facet and a sloping fibular facet. It also lacks the medially positioned flexor-fibularis groove of extant haplorhines. In these respects, the talus of D. provincialis comes surprisingly close to that of the pen-tailed treeshrew, Ptilocercus lowii, and extinct plesiadapiforms for which tali are known. However, it differs from P. lowii and is more like other early euprimates in exhibiting an expanded posterior trochlear shelf and deep talar body. In overall form, the bone approximates more leaping reliant euprimates. The phylogenetically basal signal from the new fossil is confirmed with cladistic analyses of two different character matrices, which place D. provincialis as the most basal strepsirrhine when the new tarsal data are included. Interpreting our results in the context of other recent discoveries, we conclude that the lineage leading to the ancestral euprimate had already become somewhat leaping specialized, while certain specializations for the small branch niche came after crown primates began to radiate.

Functional associations between support use and forelimb shape in strepsirrhines and their relevance to inferring locomotor behavior in early primates

The evolution of primates is intimately linked to their initial invasion of an arboreal environment. However, moving and foraging in this milieu creates significant mechanical challenges related to the presence of substrates differing in their size and orientation. It is widely assumed that primates are behaviorally and anatomically adapted to movement on specific substrates, but few explicit tests of this relationship in an evolutionary context have been conducted. Without direct tests of form-function relationships in living primates it is impossible to reliably infer behavior in fossil taxa. In this study, we test a hypothesis of co-variation between forelimb morphology and the type of substrates used by strepsirrhines. If associations between anatomy and substrate use exist, these can then be applied to better understand limb anatomy of extinct primates. The co-variation between each forelimb long bone and the type of substrate used was studied in a phylogenetic context. Our results show that despite the presence of significant phylogenetic signal for each long bone of the forelimb, clear support use associations are present. A strong co-variation was found between the type of substrate used and the shape of the radius, with and without taking phylogeny into account, whereas co-variation was significant for the ulna only when taking phylogeny into account. Species that use a thin branch milieu show radii that are gracile and straight and have a distal articular shape that allows for a wide range of movements. In contrast, extant species that commonly use large supports show a relatively robust and curved radius with an increased surface area available for forearm and hand muscles in pronated posture. These results, especially for the radius, support the idea that strepsirrhine primates exhibit specific skeletal adaptations associated with the supports that they habitually move on. With these robust associations in hand it will be possible to explore the same variables in extinct early primates and primate relatives and thus improve the reliability of inferences concerning substrate use in early primates.

Quantification of the position and depth of the flexor hallucis longus groove in euarchontans, with implications for the evolution of primate positional behavior

OBJECTIVE

On the talus, the position and depth of the groove for the flexor hallucis longus tendon have been used to infer phylogenetic affinities and positional behaviors of fossil primates. This study quantifies aspects of the flexor hallucis longus groove (FHLG) to test if: (1) a lateral FHLG is a derived strepsirrhine feature, (2) a lateral FHLG reflects inverted and abducted foot postures, and (3) a deeper FHLG indicates a larger muscle.

METHODS

We used linear measurements of microCT-generated models from a sample of euarchontans (n = 378 specimens, 125 species) to quantify FHLG position and depth. Data are analyzed with ANOVA, Ordinary and Phylogenetic Generalized Least Squares, and Bayesian Ancestral State Reconstruction (ASR).

RESULTS

Extant strepsirrhines, adapiforms, plesiadapiforms, dermopterans, and Ptilocercus exhibit lateral FHLGs. Extant anthropoids, subfossil lemurs, and Tupaia have medial FHLGs. FHLGs of omomyiforms and basal fossil anthropoids are intermediate between those of strepsirrhines and extant anthropoids. FHLG position has few correlations with pedal inversion features. Relative FHLG depth is not significantly correlated with body mass. ASRs support a directional model for FHLG position and a random walk model for FHLG depth.

CONCLUSIONS

The prevalence of lateral FHLGs in many non-euprimates suggests a lateral FHLG is not a derived strepsirrhine feature. The lack of correlations with pedal inversion features suggests a lateral FHLG is not a sufficient indicator of strepsirrhine-like foot postures. Instead, a lateral FHLG may reduce the risk of tendon displacement in abducted foot postures on large diameter supports. A deep FHLG does not indicate a larger muscle, but likely reduces bowstringing during plantarflexion.