Manual function and performance in humans, gorillas, and orangutans during the same tool use task

Humanlike manual activities in Australopithecus

The evolution of the human hand is a topic of great interest in paleoanthropology. As the hand can be involved in a vast array of activities, knowledge regarding how it was used by early hominins can yield crucial information on the factors driving biocultural evolution. Previous research on early hominin hands focused on the overall bone shape. However, while such approaches can inform on mechanical abilities and the evolved efficiency of manipulation, they cannot be used as a definite proxy for individual habitual activity. Accordingly, it is crucial to examine bone structures more responsive to lifetime biomechanical loading, such as muscle attachment sites or internal bone architecture. In this study, we investigate the manual entheseal patterns of Australopithecus afarensis, Australopithecus africanus, and Australopithecus sediba through the application of the validated entheses-based reconstruction of activity method. Using a comparative sample of later Homo and three great ape genera, we analyze the muscle attachment site proportions on the thumb, fifth ray, and third intermediate phalanx to gain insight into the habitual hand use of Australopithecus. We use a novel statistical procedure to account for the effects of interspecies variation in overall size and ray proportions. Our results highlight the importance of certain muscles of the first and fifth digits for humanlike hand use. In humans, these muscles are required for variable in-hand manipulation and are activated during stone-tool production. The entheses of A. sediba suggest muscle activation patterns consistent with a similar suite of habitual manual activities as in later Homo. In contrast, A. africanus and A. afarensis display a mosaic entheseal pattern that combines indications of both humanlike and apelike manipulation. Overall, these findings provide new evidence that some australopith species were already habitually engaging in humanlike manipulation, even if their manual dexterity was likely not as high as in later Homo.

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.

No immediate attentional bias towards or choice bias for male secondary sexual characteristics in Bornean orang-utans (Pongo pygmaeus)

Primate faces provide information about a range of variant and invariant traits, including some that are relevant for mate choice. For example, faces of males may convey information about their health or genetic quality through symmetry or facial masculinity. Because perceiving and processing such information may have bearing on the reproductive success of an individual, cognitive systems are expected to be sensitive to facial cues of mate quality. However, few studies have investigated this topic in non-human primate species. Orang-utans are an interesting species to test mate-relevant cognitive biases, because they are characterised by male bimaturism: some adult males are fully developed and bear conspicuous flanges on the side of their face, while other males look relatively similar to females. Here, we describe two non-invasive computerised experiments with Bornean orang-utans (Pongo pygmaeus), testing (i) immediate attention towards large flanges and symmetrical faces using a dot-probe task (N = 3 individuals; 2F) and (ii) choice bias for pictures of flanged males over unflanged males using a preference test (N = 6 individuals; 4F). In contrast with our expectations, we found no immediate attentional bias towards either large flanges or symmetrical faces. In addition, individuals did not show a choice bias for stimuli of flanged males. We did find exploratory evidence for a colour bias and energy efficiency trade-offs in the preference task. We discuss our null results and exploratory results in the context of the evolutionary history of Bornean orang-utans, and provide suggestions for a more biocentric approach to the study of orang-utan cognition.

Protracted development of stick tool use skills extends into adulthood in wild western chimpanzees

Tool use is considered a driving force behind the evolution of brain expansion and prolonged juvenile dependency in the hominin lineage. However, it remains rare across animals, possibly due to inherent constraints related to manual dexterity and cognitive abilities. In our study, we investigated the ontogeny of tool use in chimpanzees (Pan troglodytes), a species known for its extensive and flexible tool use behavior. We observed 70 wild chimpanzees across all ages and analyzed 1,460 stick use events filmed in the Taï National Park, Côte d'Ivoire during the chimpanzee attempts to retrieve high-nutrient, but difficult-to-access, foods. We found that chimpanzees increasingly utilized hand grips employing more than 1 independent digit as they matured. Such hand grips emerged at the age of 2, became predominant and fully functional at the age of 6, and ubiquitous at the age of 15, enhancing task accuracy. Adults adjusted their hand grip based on the specific task at hand, favoring power grips for pounding actions and intermediate grips that combine power and precision, for others. Highly protracted development of suitable actions to acquire hidden (i.e., larvae) compared to non-hidden (i.e., nut kernel) food was evident, with adult skill levels achieved only after 15 years, suggesting a pronounced cognitive learning component to task success. The prolonged time required for cognitive assimilation compared to neuromotor control points to selection pressure favoring the retention of learning capacities into adulthood.

The deep trabecular structure of first metacarpals in extant hominids

OBJECTIVES

Recent studies have associated subarticular trabecular bone distribution in the extant hominid first metacarpal (Mc1) with observed thumb use, to infer fossil hominin thumb use. Here, we analyze the entire Mc1 to test for interspecific differences in: (1) the absolute volume of trabecular volume fraction, (2) the distribution of the deeper trabecular network, and (3) the distribution of trabeculae in the medullary cavity, especially beneath the Mc1 disto-radial flange.

MATERIALS AND METHODS

Trabecular bone was imaged using micro-computed tomography in a sample of Homo sapiens (n = 11), Pan paniscus (n = 10), Pan troglodytes (n = 11), Gorilla gorilla (n = 10) and Pongo sp., (n = 7). Using Canonical Holistic Morphometric Analysis (cHMA), we tested for interspecific differences in the trabecular bone volume fraction (BV/TV) and its relative distribution (rBV/TV) throughout the Mc1, including within the head, medullary cavity, and base.

RESULTS

P. paniscus had the highest, and H. sapiens the lowest, BV/TV relative to other species. rBV/TV distribution statistically distinguished the radial concentrations and lack of medullary trabecular bone in the H. sapiens Mc1 from all other hominids. H. sapiens and, to a lesser extent, G. gorilla also had a significantly higher trabecular volume beneath the disto-radial flange relative to other hominids.

DISCUSSION

These results are consistent with differences in observed thumb use in these species and may also reflect systemic differences in bone volume fraction. The trabecular bone extension into the medullary cavity and concentrations beneath the disto-radial flange may represent crucial biomechanical signals that will aid in the inference of fossil hominin thumb use.

Captive bonobos (Pan paniscus) apply precision grips when using flaked stone tools

OBJECTIVES

Current evidence suggests that flaked stone tool technologies did not emerge until ~3.3-2.6 million-years-ago (Ma). It is often hypothesized that early hominin (principally Ardipithecus and early Australopithecus) manual anatomy may have prevented an earlier emergence, as the forceful precision grips essential to flake tool-use may have been ineffectively performed by these species. Marzke, Marchant, McGrew, and Reece (2015) observed potentially forceful pad-to-side precision grips being recruited by wild chimpanzees (Pan troglodytes) during feeding behaviors, indicating that Pan-like manual anatomy, and therefore potentially early hominin anatomy, may be capable of effectively securing flake stone tools during their use.

MATERIALS AND METHODS

Here, we report on the grips recruited by four captive, human-trained, bonobos (Pan paniscus) during the use of stone and organic tools, including flake stone tools during cutting behaviors.

RESULTS

It is revealed that pad-to-side precision grips are frequently recruited by these bonobos when securing stone flakes during cutting actions. In some instances, high forces could have been resisted and applied by the thumb and fingers.

DISCUSSION

While our analyzes are preliminary and limited to captive individuals, and Pan is not suggested to secure flakes with the same efficacy as Homo or Australopithecus, it points to early hominins potentially being able to perform the precision grips required to use flake stone tools. In turn, the ability to gain tangible benefits from the effective use of flake tools (i.e., gain energetic returns from processing food resources) may have been - at least anatomically - possible in early Australopithecus and other pre-Early Stone Age hominin species. In turn, hominin manual anatomy may not be a leading restriction on the emergence of the earliest stone tool technologies.

Morphological integration and shape covariation between the trapezium and first metacarpal among extant hominids

OBJECTIVES

The shape of the trapezium and first metacarpal (Mc1) markedly influence thumb mobility, strength, and the manual abilities of extant hominids. Previous research has typically focused solely on trapezium-Mc1 joint shape. Here we investigate how morphological integration and shape covariation between the entire trapezium (articular and non-articular surfaces) and the entire Mc1 reflect known differences in thumb use in extant hominids.

MATERIALS AND METHODS

We analyzed shape covariation in associated trapezia and Mc1s across a large, diverse sample of Homo sapiens (n = 40 individuals) and other extant hominids (Pan troglodytes, n = 16; Pan paniscus, n = 13; Gorilla gorilla gorilla, n = 27; Gorilla beringei, n = 6; Pongo pygmaeus, n = 14; Pongo abelii, n = 9) using a 3D geometric morphometric approach. We tested for interspecific significant differences in degree of morphological integration and patterns of shape covariation between the entire trapezium and Mc1, as well as within the trapezium-Mc1 joint specifically.

RESULTS

Significant morphological integration was only found in the trapezium-Mc1 joint of H. sapiens and G. g. gorilla. Each genus showed a specific pattern of shape covariation between the entire trapezium and Mc1 that was consistent with different intercarpal and carpometacarpal joint postures.

DISCUSSION

Our results are consistent with known differences in habitual thumb use, including a more abducted thumb during forceful precision grips in H. sapiens and a more adducted thumb in other hominids used for diverse grips. These results will help to infer thumb use in fossil hominins.

Cortical bone distribution of the proximal phalanges in great apes: implications for reconstructing manual behaviours

Primate fingers are typically in direct contact with the environment during both locomotion and manipulation, and aspects of external phalangeal morphology are known to reflect differences in hand use. Since bone is a living tissue that can adapt in response to loading through life, the internal bone architecture of the manual phalanges should also reflect differences in manual behaviours. Here, we use the R package Morphomap to analyse high-resolution microCT scans of hominid proximal phalanges of digits 2-5 to determine whether cortical bone structure reflects variation in manual behaviours between bipedal (Homo), knuckle-walking (Gorilla, Pan) and suspensory (Pongo) taxa. We test the hypothesis that relative cortical bone distribution patterns and cross-sectional geometric properties will differ both among extant great apes and across the four digits due to locomotor and postural differences. Results indicate that cortical bone structure reflects the varied hand postures employed by each taxon. The phalangeal cortices of Pongo are significantly thinner and have weaker cross-sectional properties relative to the African apes, yet thick cortical bone under their flexor sheath ridges corresponds with predicted loading during flexed finger grips. Knuckle-walking African apes have even thicker cortical bone under the flexor sheath ridges, as well as in the region proximal to the trochlea, but Pan also has thicker diaphyseal cortices than Gorilla. Humans display a distinct pattern of distodorsal thickening, as well as relatively thin cortices, which may reflect the lack of phalangeal curvature combined with frequent use of flexed fingered hand grips during manipulation. Within each taxon, digits 2-5 have a similar cortical distribution in Pongo, Gorilla and, unexpectedly, Homo, which suggest similar loading of all fingers during habitual locomotion or hand use. In Pan, however, cortical thickness differs between the fingers, potentially reflecting differential loading during knuckle-walking. Inter- and intra-generic variation in phalangeal cortical bone structure reflects differences in manual behaviours, offering a comparative framework for reconstructing hand use in fossil hominins.

Cognitive enrichment in a social setting: assessing the use of a novel food maze in sanctuary-housed chimpanzees

Foraging devices are effective enrichment tools for non-human primates, as they provide both cognitive and manipulative stimulation that may enhance these animals’ welfare. We assessed the behavioral effects of a novel tool-based enrichment on 14 chimpanzees (Pan troglodytes) housed at Fundació Mona (Girona, Spain). The device consisted of a vertical maze filled with food rewards, which chimpanzees could extract by using tools. We conducted behavioral observations in two conditions over an approximately 2.5-month period: when the food maze was loaded (12 enrichment days), and when it was empty (12 baseline days). Data were collected using 2-min scan sampling and untimed-event focal sampling during two daily sessions of 80 min each. We expected that the chimpanzees’ interest in the enrichment would decrease over time, but that its use would be linked to an increase in the occurrence of species-typical behaviors, a reduction in negative indicators of welfare, and changes in social behaviors. We found that participation widely varied among subjects, being higher in females and decreasing through time. Furthermore, participation was linked to an increase in tool use and a decrease in inactivity, but also to an increase in aggression-related behaviors. In contrast, participation had no effect on the occurrence of abnormal behaviors, social proximity or affiliation-related behaviors. Finally, we detected an increase in self-directed behaviors only when subjects actively interacted with the device. We conclude that, in future studies, these types of devices should be evaluated for longer periods of time and more attention should be paid to individuals’ preferences and abilities.

Free hand hitting of stone-like objects in wild gorillas

The earliest stone tool types, sharp flakes knapped from stone cores, are assumed to have played a crucial role in human cognitive evolution. Flaked stone tools have been observed to be accidentally produced when wild monkeys use handheld stones as tools. Holding a stone core in hand and hitting it with another in the absence of flaking, free hand hitting, has been considered a requirement for producing sharp stone flakes by hitting stone on stone, free hand percussion. We report on five observations of free hand hitting behavior in two wild western gorillas, using stone-like objects (pieces of termite mound). Gorillas are therefore the second non-human lineage primate showing free-hand hitting behavior in the wild, and ours is the first report for free hand hitting behavior in wild apes. This study helps to shed light on the morphofunctional and cognitive requirements for the emergence of stone tool production as it shows that a prerequisite for free hand percussion (namely, free hand hitting) is part of the spontaneous behavioral repertoire of one of humans' closest relatives (gorillas). However, the ability to combine free hand hitting with the force, precision, and accuracy needed to facilitate conchoidal fracture in free hand percussion may still have been a critical watershed for hominin evolution.

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.

I Wanna Draw Like You: Inter- and Intra-Individual Differences in Orang-Utan Drawings

This study analyses 749 drawings by five female Bornean orang-utans (Pongo pygmaeus) at Tama Zoological Park in Japan. We searched for differences between individuals but also tried to identify possible temporal changes among the drawings of one individual, Molly, who drew almost 1300 drawings from 2006 to 2011. An analysis of the drawings was carried out after collecting quantitative and qualitative variables. Our findings reveal evidence of differences in the drawing style of the five individuals as well as creative changes in Molly's drawing style throughout her lifetime. Individuals differed in terms of the colours used, the space they filled, and the shapes (fan patterns, circles, or loops) they drew. Molly drew less and less as she grew older, and we found a significant difference between drawings produced in winter, when orang-utans were kept inside and had less activity, and those produced during other seasons. Our results suggest that the drawing behaviour of these five orang-utans is not random and that differences among individuals might reflect differences of styles, states of mind, and motivation to draw.

Captive great apes tend to innovate simple tool behaviors quickly

Recent studies have highlighted the important role that individual learning mechanisms and different forms of enhancenment play in the acquisition of novel behaviors by naïve individuals. A considerable subset of these studies has focused on tool innovation by our closest living relatives, the great apes, to better undestand the evolution of technology in our own lineage. To be able to isolate the role that individual learning plays in great ape tool innovation, researchers usually employ what are known as baseline tests. Although these baselines are commonly used in behavioral studies in captivity, the length of these tests in terms of number of trials and duration remains unstandarized across studies. To address this methodological issue, we conducted a literature review of great ape tool innovation studies conducted in zoological institutions and compiled various methodological data including the timing of innovation. Our literature review revealed an early innovation tendency in great apes, which was particularly pronounced when simple forms of tool use were investigated. In the majority of experiments where tool innovation took place, this occurred within the first trial and/or the first hour of testing. We discuss different possible sources of variation in the latency to innovate such as testing setup, species and task. We hope that our literature review helps researchers design more data-informed, resource-efficient experiments on tool innovation in our closest living relatives.

Dexterity and technique in termite fishing by chimpanzees (Pan troglodytes troglodytes) in the Goualougo Triangle, Republic of Congo

Although the phenomenon of termite fishing by chimpanzees (Pan troglodytes) has historical and theoretical importance for primatology, we still have a limited understanding of how chimpanzees accomplish this activity, and in particular, about details of skilled actions and the nature of individual variation in fishing techniques. We examined movements, hand positions, grips, and other details from remote video footage of seven adult and subadult female chimpanzees using plant probes to extract Macrotermes muelleri termites from epigeal nests. Six chimpanzees used exclusively one hand (left or right) to grip the probe during termite fishing. All chimpanzees used the same repertoire of actions to insert, adjust, and withdraw the probe but differed in the frequency of use of particular actions. Chimpanzees have been described as eating termites in two ways—directly from the probe or by sweeping them from the probe with one hand. We describe a third technique: sliding the probe between the digits of one stationary hand as the probe is extracted from the nest. The sliding technique requires complementary bimanual coordination (extracting with one hand and grasping lightly with the other, at the same time). We highlight the importance of actions with two hands—one gripping, one assisting—in termite fishing and discuss how probing techniques are correlated with performance. Additional research on digital function and on environmental, organismic, and task constraints will further reveal manual dexterity in termite fishing.

‐

Using remote video footage from camera traps in Goualougo Triangle, Republic of Congo, we describe chimpanzees' manual actions, postures, and positions, and movements of the probe while they fished for termites in epigeal termite nests.

‐

Chimpanzees used diverse grips, with and without the thumb, and two hands—one gripping, one assisting—to handle the probe delicately and to move it precisely.

‐

We describe a new technique for recovering termites: sliding the probe between the digits of one stationary hand as the probe is extracted from the nest with the other hand, and a new action: oscillatory movements of the probe while it was inserted in the nest.

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.

The ontogeny of termite gathering among chimpanzees in the Goualougo Triangle, Republic of Congo

Objectives

Acquiring tool‐assisted foraging skills can potentially improve dietary quality and increase fitness for wild chimpanzees (Pan troglodytes). In contrast to chimpanzees in East and West Africa, chimpanzees in the Congo Basin use tool sets and brush‐tipped fishing probes to gather termites. We investigated the ontogeny of these tool skills in chimpanzees of the Goualougo Triangle, Republic of Congo, and compared it to that for chimpanzees at Gombe, Tanzania. We assessed whether chimpanzees acquired simple tool behaviors and single tool use before more complex actions and sequential use of multiple tool types.

Materials and Methods

Using a longitudinal approach, we scored remote video footage to document the acquisition of termite‐gathering critical elements for 25 immature chimpanzees at Goualougo.

Results

All chimpanzees termite fished by 2.9 years but did not manufacture brush‐tipped probes until an average of 4.3 years. Acquisition of sequential tool use extended into juvenility and adolescence. While we did not detect significant sex differences, most critical elements except tool manufacture were acquired slightly earlier by females.

Discussion

These findings contrast with Gombe, where chimpanzees learn to both use and make fishing probes between ages 1.5–3.5 and acquire the complete task by age 5.5. Differences between sites could reflect tool material selectivity and design complexity, the challenge of sequential tool behaviors, and strength requirements of puncturing subterranean termite nests at Goualougo. These results illustrate how task complexity may influence the timing and sequence of skill acquisition, improving models of the ontogeny of tool behavior among early hominins who likely used complex, perishable technologies.

The position of Australopithecus sediba within fossil hominin hand use diversity

The human lineage is marked by a transition in hand use, from locomotion towards increasingly dexterous manipulation, concomitant with bipedalism. The forceful precision grips used by modern humans probably evolved in the context of tool manufacture and use, but when and how many times hominin hands became principally manipulative remains unresolved. We analyse metacarpal trabecular and cortical bone, which provide insight into behaviour during an individual's life, to demonstrate previously unrecognized diversity in hominin hand use. The metacarpals of the palm in Australopithecus sediba have trabecular morphology most like orangutans and consistent with locomotor power-grasping with the fingers, while that of the thumb is consistent with human-like manipulation. This internal morphology is the first record of behaviour consistent with a hominin that used its hand for both arboreal locomotion and human-like manipulation. This hand use is distinct from other fossil hominins in this study, including A. afarensis and A. africanus.

Trabecular variation in the first metacarpal and manipulation in hominids

OBJECTIVES

The dexterity of fossil hominins is often inferred by assessing the comparative manual anatomy and behaviors of extant hominids, with a focus on the thumb. The aim of this study is to test whether trabecular structure is consistent with what is currently known about habitually loaded thumb postures across extant hominids.

MATERIALS AND METHODS

We analyze first metacarpal (Mc1) subarticular trabecular architecture in humans (Homo sapiens, n = 10), bonobos (Pan paniscus, n = 10), chimpanzees (Pan troglodytes, n = 11), as well as for the first time, gorillas (Gorilla gorilla gorilla, n = 10) and orangutans (Pongo sp., n = 1, Pongo abelii, n = 3 and Pongo pygmaeus, n = 5). Using a combination of subarticular and whole-epiphysis approaches, we test for significant differences in relative trabecular bone volume (RBV/TV) and degree of anisotropy (DA) between species.

RESULTS

Humans have significantly greater RBV/TV on the radiopalmar aspects of both the proximal and distal Mc1 subarticular surfaces and greater DA throughout the Mc1 head than other hominids. Nonhuman great apes have greatest RBV/TV on the ulnar aspect of the Mc1 head and the palmar aspect of the Mc1 base. Gorillas possessed significantly lower DA in the Mc1 head than any other taxon in our sample.

DISCUSSION

These results are consistent with abduction of the thumb during forceful "pad-to-pad" precision grips in humans and, in nonhuman great apes, a habitually adducted thumb that is typically used in precision and power grips. This comparative context will help infer habitual manipulative and locomotor grips in fossil hominins.

The unexpected importance of the fifth digit during stone tool production

Unique anatomical features of the human hand facilitate our ability to proficiently and forcefully perform precision grips and in-hand manipulation of objects. Extensive research has been conducted into the role of digits one to three during these manual behaviours, and the origin of the highly derived first digit anatomy that facilitates these capabilities. Stone tool production has long been thought a key influence in this regard. Despite previous research stressing the unique derived morphology of the human fifth digit little work has investigated why humans alone display these features. Here we examine the recruitment frequency, loading magnitude, and loading distribution of all digits on the non-dominant hand of skilled flintknappers during four technologically distinct types of Lower Palaeolithic stone tool production. Our data reveal the fifth digit to be heavily and frequently recruited during all studied behaviours. It occasionally incurred pressures, and was used in frequencies, greater or equal to those of the thumb, and frequently the same or greater than those of the index finger. The fifth digit therefore appears key to >2 million years of stone tool production activities, a behaviour that likely contributed to the derived anatomy observed in the modern human fifth ray.

The impact of hand proportions on tool grip abilities in humans, great apes and fossil hominins: A biomechanical analysis using musculoskeletal simulation

Differences in grip techniques used across primates are usually attributed to variation in thumb-finger proportions and muscular anatomy of the hand. However, this cause-effect relationship is not fully understood because little is known about the biomechanical functioning and mechanical loads (e.g., muscle or joint forces) of the non-human primate hand compared to that of humans during object manipulation. This study aims to understand the importance of hand proportions on the use of different grip strategies used by humans, extant great apes (bonobos, gorillas and orangutans) and, potentially, fossil hominins (Homo naledi and Australopithecus sediba) using a musculoskeletal model of the hand. Results show that certain grips are more challenging for some species, particularly orangutans, than others, such that they require stronger muscle forces for a given range of motion. Assuming a human-like range of motion at each hand joint, simulation results show that H. naledi and A. sediba had the biomechanical potential to use the grip techniques considered important for stone tool-related behaviors in humans. These musculoskeletal simulation results shed light on the functional consequences of the different hand proportions among extant and extinct hominids and the different manipulative abilities found in humans and great apes.