Western lowland gorillas are the largest and most sexually dimorphic ape that habitually exploits arboreal environments. Their size, robust musculature and specialised adaptations in the hands and feet, which are suited for terrestrial quadrupedal locomotion, make them interesting models for understanding how great apes are able to exploit complex arboreal habitats. We present a comprehensive analysis of the arboreal locomotor ecology of western lowland gorillas by studying their behaviour and ecology in the context of their morphology. A group of fully habituated wild western lowland gorillas was followed for 12 months in Loango National Park, Gabon. Statistical analysis applying regression modelling and Akaike's Information Criterion was used to identify the relationships between locomotor behaviours, height, contextual behaviour, support use, hand posture and body size. Our findings suggest that the gorillas were not restricted in their ability to access and move around in tree canopies because of their size or postcranial morphology. Instead, they exhibited considerable behavioural flexibility and engaged in locomotor behaviours that contradicted classic body size predictions for primates. To offset the risks of moving on small supports, the gorillas used hand-assisted bipedal locomotion on multiple small supports, rather than relying on suspensory locomotion. We suggest that this is linked to their hand dimensions, which have been selected to facilitate efficient quadrupedal walking on the ground. The silverback gorilla engaged in less horizontal locomotion in the canopy, spent less time at heights above 20 m, and used large supports more often than the adult females, blackback and adolescents, but the type and number of supports used did not vary between body size groups. We also found that the reproductive status of the females (presence or absence of small infants) may have shaped how they responded to risks when solving the problem of gap-crossing in the trees. Overall, our results highlight that the gorillas likely prioritised risk minimisation in the supports that they used in arboreal environments at the cost of increased energy expenditure.

Background Understanding the needs of animals with physical and cognitive impairments is essential for zoos, rehabilitation centres, and other captive contexts. This case study describes the atypical physical and cognitive development of Jiwa, an adult male Sumatran orangutan (Pongo abelii), to evaluate how these differences may impact Jiwa and to explore how cognitive enrichment can be tailored to his skill level. Methods Keeper records from Jiwa’s birth (1999) to February 2022 were scrutinised to identify key developmental milestones, changes in body weight, and deciduous dental emergence. These were compared with expected milestones for wild and captive orangutans. In addition, a probe feeder apparatus was gradually tailored to an appropriate skill level between January and March 2022. Results Many of Jiwa’s developmental milestones were consistent with expected orangutan development, although there were noticeable differences in body weight, dental emergence, locomotion, and cognition. The welfare domains most likely to be negatively impacted by these differences were physical health, expression of agency, and mental wellbeing. After the enrichment device was tailored to an appropriate level of challenge, Jiwa showed marked improvement in engagement and ability to use the device. Conclusions This study helps to inform future case studies of atypical great apes by detailing the physical and cognitive development of an individual orangutan. Although Jiwa’s differences may impact his own wellbeing in some areas, he has largely been able to overcome them. However, this case study highlights the challenges involved in tailoring enrichment apparatuses and resilience interventions for atypical individuals.

There has been a long debate about the possibility of multiple contemporaneous species of Australopithecus in both eastern and southern Africa, potentially exhibiting different forms of bipedal locomotion. Here, we describe the previously unreported morphology of the os coxae in the 3.67 Ma Australopithecus prometheus StW 573 from Sterkfontein Member 2, comparing it with variation in ossa coxae in living humans and apes as well as other Plio-Pleistocene hominins. Statistical comparisons indicate that StW 573 and 431 resemble humans in their anteroposteriorly great iliac crest breadth compared with many other early australopiths, whereas Homo ergaster KNM WT 15000 surprisingly also has a relatively anterioposteriorly short iliac crest. StW 573 and StW 431 appear to resemble humans in having a long ischium compared with Sts 14 and KNM WT 15000. A Quadratic Discriminant Function Analysis of morphology compared with other Plio-Pleistocene hominins and a dataset of modern humans and hominoids shows that, while Lovejoy's heuristic model of the Ardipithecus ramidus os coxae falls with Pongo or in an indeterminate group, StW 573 and StW 431 from Sterkfontein Member 4 are consistently classified together with modern humans. Although clearly exhibiting the classic "basin shaped" bipedal pelvis, Sts 14 (also from Sterkfontein), AL 288-1 Australopithecus afarensis, MH2 Australopithecus sediba and KNM-WT 15000 occupy a position more peripheral to modern humans, and in some analyses are assigned to an indeterminate outlying group. Our findings strongly support the existence of two species of Australopithecus at Sterkfontein and the variation we observe in os coxae morphology in early hominins is also likely to reflect multiple forms of bipedality.

Wild orangutans (Pongo spp.) rescued from human-wildlife conflict must be adequately rehabilitated before being returned to the wild. It is essential that released orangutans are able to cope with stressful challenges such as food scarcity, navigating unfamiliar environments, and regaining independence from human support. Although practical skills are taught to orangutans in rehabilitation centres, post-release survival rates are low. Psychological resilience, or the ability to ‘bounce back’ from stress, may be a key missing piece of the puzzle. However, there is very little knowledge about species-appropriate interventions which could help captive orangutans increase resilience to stress. This scoping review summarises and critically analyses existing human and non-human animal resilience literature and provides suggestions for the development of interventions for orangutans in rehabilitation. Three scientific databases were searched in 2021 and 2023, resulting in 63 human studies and 266 non-human animal studies. The first section brings together human resilience interventions, identifying common themes and assessing the applicability of human interventions to orangutans in rehabilitation. The second section groups animal interventions into categories of direct stress, separation stress, environmental conditions, social stress, and exercise. In each category, interventions are critically analysed to evaluate their potential for orangutans in rehabilitation. The results show that mild and manageable forms of intervention have the greatest potential benefit with the least amount of risk. The study concludes by emphasising the need for further investigation and experimentation, to develop appropriate interventions and measure their effect on the post-release survival rate of orangutans.

Kinematic differences in the knuckle-walking gaits of chimpanzees and gorillas have provided crucial evidence to support the theory that knuckle-walking evolved independently in the Pan and Gorilla lineages, rather than being inherited from their last common ancestor. This has been used to refute the idea that hominin bipedalism evolved from knuckle-walking. Extended wrist postures during knuckle-walking in chimpanzees compared with columnar wrist postures in gorillas were associated with chimpanzees’ more arboreal lifestyle, and are supposedly constrained by fundamental differences in carpal morphology. However, more recent kinematic data cast doubt on the presence of two distinct types of knuckle-walking in Pan and Gorilla. It is also becoming clear that primates demonstrate substantial flexibility in their locomotor ecology to adapt to habitat conditions, indicating that species-specific stereotyping of locomotion is inappropriate. Furthermore, the potential for plasticity in anatomical development presents challenges to evolutionary predictions that are based on tight links between skeletal form and locomotor function. Given the accumulating evidence showing substantial arboreality in gorillas, this study investigates wrist kinematics during both arboreal and terrestrial knuckle-walking in captive chimpanzees and gorillas, to address the hypothesis that previously reported kinematic differences are phylogenetically constrained, rather than merely a consequence of environment. We find that when the arboreal knuckle-walking behaviour of gorillas is considered, gorillas exhibit more extended wrist postures than chimpanzees, and are therefore capable of the same kinematic responses to arboreal supports. We propose that knuckle-walking epitomises a shared inheritance of behavioural flexibility in chimpanzees and gorillas, rather than either a shared inheritance of knuckle-walking itself or independent evolution of knuckle-walking in response to different environmental pressures. We suggest that future studies of hominoid locomotion consider a species’ full range of environmental contexts, as well as the potential influence of developmental plasticity on locomotor anatomy, in order to make more robust conclusions about the evolution of locomotor behaviour.

The StW 573 skeleton of Australopithecus prometheus from Sterkfontein Member 2 is some 93% complete and thus by far the most complete member of that genus yet found. Firmly dated at 3.67 Ma, it is one of the earliest specimens of its genus. A crucial aspect of interpretation of locomotor behaviour from fossil remains is an understanding of the palaeoenvironment in which the individual lived and the manner in which it would have used it. While the value of this ecomorphological approach is largely accepted, it has not been widely used as a stable framework on which to build evolutionary biomechanical interpretations. Here, we collate the available evidence on StW 573’s anatomy in order, as far as currently possible, to reconstruct what might have been this individual’s realized and potential niche. We explore the concept of a common Australopithecus “bauplan” by comparing the morphology and ecological context of StW 573 to that of paenocontemporaneous australopiths including Australopithecus anamensis and KSD-VP-1/1 Australopithecus afarensis. Each was probably substantially arboreal and woodland-dwelling, relying substantially on arboreal resources. We use a hypothesis-driven approach, tested by: virtual experiments, in the case of extinct species; biomechanical analyses of the locomotor behaviour of living great ape species; and analogical experiments with human subjects. From these, we conclude that the habitual locomotor mode of all australopiths was upright bipedalism, whether on the ground or on branches. Some later australopiths such as Australopithecus sediba undoubtedly became more terrestrial, allowing sacrifice of arboreal stability in favour of manual dexterity. Indeed, modern humans retain arboreal climbing skills but have further sacrificed arboreal effectiveness for enhanced ability to sustain striding terrestrial bipedalism over much greater distances. We compare StW 573’s locomotor adaptations to those of living great apes and protohominins, and agree with those earlier observers who suggest that the common panin-hominin last common ancestor was postcranially more like Gorilla than Pan.

The StW 573 skeleton of Australopithecus prometheus from Sterkfontein Member 2 is some 93% complete and thus by far the most complete member of that genus yet found. Firmly dated at 3.67 Ma it is one of the earliest of its genus. A crucial aspect of interpretation of locomotor behaviour from fossil remains, is an understanding of the palaeoenvironment in which individuals lived and the manner in which they would have used it. While the value of this ecomorphological approach is largely accepted, it has not been widely used, as a stable framework on which to build evolutionary biomechanical interpretations. Here, we collate the available evidence on StW 573’s anatomy in order to reconstruct what might have been this individual’s realised and potential niche. We explore the concept of a common australopithecus bauplan by comparing the morphology and ecological context of StW 573 to that of paenocontemporaneous australopiths including Au. anamensis and KSD-VP-1/1 Au. afarensis. Each was probably substantially arboreal and woodland-dwelling, relying substantially on arboreal resources. Using an hypothesis-driven approach, tested by virtual experiments, in the case of extinct species, biomechanical analyses of the locomotor behaviour of living great ape species, and analogical experiments with human subjects, we conclude that the habitual locomotor mode of all australopiths was upright bipedalism, whether on the ground or on branches. Some later australopiths such as Au. sediba undoubtedly became more terrestrial, allowing sacrifice of arboreal stability in favour of manual dexterity. Indeed, modern humans retain arboreal climbing skills but have further sacrificed arboreal effectiveness for enhanced ability to sustain striding terrestrial bipedalism over much greater distances. We compare StW573’s locomotor adaptations to those of living great apes and protohominins and agree with those earlier observers who suggest that the common panin-hominin Last Common Ancestor was postcranially more like Gorilla then Pan.

Behavior is the interface through which animals interact with their environments, and therefore has potentially cascading impacts on the health of individuals, populations, their habitats, and the humans that share them. Evolution has shaped the interaction between species and their environments. Thus, alterations to the species-typical "wild-type" behavioral repertoire (and the ability of the individual to adapt flexibly which elements of the repertoire it employs) may disrupt the relationship between the organism and its environment, creating cascading One Health effects. A good example is rehabilitant orangutans where, for example, seemingly minor differences from wild conspecifics in the time spent traveling on the ground rather than in the forest canopy can affect an individual's musculoskeletal and nutritional health, as well as social integration. It can also increase two-way transmission of infectious diseases and/or pathogens with local human populations, or potentially with neighboring wild populations if there are no geographical barriers and rehabilitants travel far enough to leave their release area. Primates are well known ecosystem engineers, reshaping plant communities and maintaining biodiversity through seed dispersal, consuming plants, and creating canopy gaps and trails. From the habitat perspective, a rehabilitant orangutan which does not behave like a wild orangutan is unlikely to fulfill these same ecosystem services. Despite the importance of the diversity of an ape's behavioral repertoire, how it compares to that of wild conspecifics and how it alters in response to habitat variation, behavior is an often under-appreciated aspect of One Health. In this review, focusing on orangutans as an example of the kinds of problems faced by all captive great apes, we examine the ways in which understanding and facilitating the expression of wild-type behavior can improve their health, their ability to thrive, and the robustness of local One Health systems.

The well-developed Achilles tendon in humans is generally interpreted as an adaptation for mechanical energy storage and reuse during cyclic locomotion. All other extant great apes have a short tendon and long-fibred triceps surae, which is thought to be beneficial for locomotion in a complex arboreal habitat as this morphology enables a large range of motion. Surprisingly, highly arboreal gibbons show a more human-like triceps surae with a long Achilles tendon. Evidence for a spring-like function similar to humans is not conclusive. We revisit and integrate our anatomical and biomechanical data to calculate the energy that can be recovered from the recoiling Achilles tendon during ankle plantar flexion in bipedal gibbons. Only 7.5% of the required external positive work in a stride can come from tendon recoil, yet it is delivered at an instant when the whole-body energy level drops. Consequently, an additional similar amount of mechanical energy must simultaneously dissipate elsewhere in the system. Altogether, this challenges the concept of an energy-saving function in the gibbon's Achilles tendon. Cercopithecids, sister group of the apes, also have a human-like triceps surae. Therefore, a well-developed Achilles tendon, present in the last common ‘Cercopithecoidea–Hominoidea’ ancestor, seems plausible. If so, the gibbon's anatomy represents an evolutionary relict (no harm–no benefit), and the large Achilles tendon is not the premised key adaptation in humans (although the spring-like function may have further improved during evolution). Moreover, the triceps surae anatomy of extant non-human great apes must be a convergence, related to muscle control and range of motion. This perspective accords with the suggestions put forward in the literature that the last common hominoid ancestor was not necessarily great ape-like, but might have been more similar to the small-bodied catarrhines.

StW 573, from Sterkfontein Member 2, dated ca 3.67 Ma, is by far the most complete skeleton of an australopith to date. Joint morphology is in many cases closely matched in available elements of Australopithecus anamensis ( eg. proximal and distal tibial and humeral joint-surfaces) and there are also close similarities to features of the scapula, in particular, of KSD-VP-1/1 A. afarensis from Woranso-Mille. The closest similarities are, however, to the partial skeleton of StW 431 from Sterkfontein Member 4. When considered together, both StW 573 and StW 431 express an hip joint morphology quite distinct from that of A. africanus Sts14, and a proximal femur of a presumed A. africanus from Jacovec Cavern at Sterkfontein, StW 598. This, and other evidence presented herein, suggests there are two pelvic girdle morphs at Sterkfontein, supporting [Clarke (2013)][1] in his recognition of a second species, A. prometheus, containing StW 573 and StW 431. StW 573 is the first hominid skeleton where limb proportions are known unequivocally. It demonstrates that some early hominins, at the time of formation of the Laetoli footprints (3.6 Ma), were large-bodied. with hindlimbs longer than forelimbs. Modelling studies on extant primates indicate that the intermembral index (IMI) of StW 573, low for a non-human great ape, would have substantially enhanced economy of bipedal walking over medium-to-long distances, but that it was still too high for effective walking while load-carrying. It would, however, have somewhat reduced the economy of horizontal climbing, but made Gorilla- like embracing of large tree-trunks less possible. Consideration of both ethnographic evidence from modern indigenous arboreal foragers and modern degeneracy theory cautions against prescriptive interpretations of hand- and foot-function, by confirming that both human-like upright bipedalism and functional capabilities of the hand and foot can be effective in short-distance arboreal locomotion. [1]: #ref-19

Positional behavior describes an animal's physical activities, and has two components: posture and locomotion. Positional behavior is a vital part of a primate's ecology, as it directly affects foraging and reproductive success, and has strong links with morphology. Primates display hugely diverse and versatile positional repertoires, which allow them to exploit the complex environment of the forest canopy, and which were likely to have been important drivers of primate radiations. This entry develops the ecomorphological and evolutionary frameworks for studying positional behavior, and reviews some of the locomotor strategies that primates have developed in order to overcome environmental challenges.

An animal's size is central to its ecology, yet remarkably little is known about the selective pressures that drive this trait. A particularly compelling example is how ancestral apes evolved large body mass in such a physically and energetically challenging environment as the forest canopy, where weight-bearing branches and lianas are flexible, irregular and discontinuous, and the majority of preferred foods are situated on the most flexible branches at the periphery of tree crowns. To date the issue has been intractable due to a lack of relevant fossil material, the limited capacity of the fossil record to reconstruct an animal's behavioural ecology and the inability to measure energy consumption in freely moving apes. We studied the oxygen consumption of parkour athletes while they traversed an arboreal-like course as an elite model ape, to test the ecomorphological and behavioural mechanisms by which a large-bodied ape could optimize its energetic performance during tree-based locomotion. Our results show that familiarity with the arboreal-like course allowed the athletes to substantially reduce their energy expenditure. Furthermore, athletes with larger arm spans and shorter legs were particularly adept at finding energetic savings. Our results flesh out the scanty fossil record to offer evidence that long, strong arms, broad chests and a strong axial system, combined with the frequent use of uniform branch-to-branch arboreal pathways, were critical to off-setting the mechanical and energetic demands of large mass in ancestral apes.

Arboreal, and in particular suspensory, postures may elicit a preference for the strongest limb to be used in postural support in large bodied primates. However, selection may have favored ambilaterality rather than a preference for a particular hand in chimpanzees (Pan troglodytes) fishing arboreally for ants. To investigate the influence of arboreality on hand preference we recorded handedness in seven captive bonobos (Pan paniscus) manipulating a foraging device during terrestrial and arboreal postures in a symmetrical environment, observing 2726 bouts of manipulation. When accessing the foraging device in the arboreal position the bonobos adopted predominantly suspensory postures. There was no population level hand preference for manipulating the foraging device in either the terrestrial or arboreal positions. However, four of seven individuals that interacted with the foraging devices showed a significant preference for one hand (two were left handed, two were right handed) when manipulating the foraging device in the arboreal position whereas only one individual (left handed) showed a preference in the terrestrial position. This suggests that individuals may have a preferred or strongest limb for postural support in a symmetrical arboreal environment, resulting in a bias to use the opposite hand for manipulation. However, the hand that is preferred for postural support differs between individuals. Although our sample is for two captive groups at the same zoo, our findings suggest that the demand of maintaining arboreal postures and environmental complexity influence hand preference.

Whether tree canopy habitats played a sustained role in the ecology of ancestral bipedal hominins is unresolved. Some argue that arboreal bipedalism was prohibitively risky for hominins whose increasingly modern anatomy prevented them from gripping branches with their feet. Balancing on two legs is indeed challenging for humans under optimal conditions let alone in forest canopy, which is physically and visually highly dynamic. Here we quantify the impact of forest canopy characteristics on postural stability in humans. Viewing a movie of swaying branches while standing on a branch-like bouncy springboard destabilised the participants as much as wearing a blindfold. However “light touch”, a sensorimotor strategy based on light fingertip support, significantly enhanced their balance and lowered their thigh muscle activity by up to 30%. This demonstrates how a light touch strategy could have been central to our ancestor’s ability to avoid falls and reduce the mechanical and metabolic cost of arboreal feeding and movement. Our results may also indicate that some adaptations in the hand that facilitated continued access to forest canopy may have complemented, rather than opposed, adaptations that facilitated precise manipulation and tool use.

This volume is the result of a joint conference for the Anatomical Society and Primate Society of Great Britain (PSGB) on Primate Ecomorphology, which was held at the University of Birmingham, UK, in December 2014. Many of the papers in this volume provide excellent introductions to the ecomorphological framework (see, for example, Elton and colleagues, and Soligo and Smaers), which I shall not attempt to compete with here; suffice to say ecomorphology is broadly the study of the association between an animal's morphology and habitat, and the behavioural responses that mediate it. While ecomorphological relationships are perhaps most richly documented in studies of the diet and locomotion of fish and lizards, the papers in this volume remind us of the many early pioneers of primate ecomorphology, such as Matt Cartmill, Bob Sussman, John Fleagle and Peter Rodman (for references, see Elton et al., and Soligo and Smears, this volume). Their work, among others, laid the foundations for this contribution, which aims to review current understanding of key topics in primate ecomorphology, and ask how modern research can overcome the challenges to understanding the ecomorphology of extant and extinct primates. The ecomorphological framework is grounded on the concept that morphology is an accurate and predictable reflection of an animal's current behaviours and ecology. In an age where technological advances have allowed the generation and analysis of datasets of unimaginable complexity, the apparent simplicity of this framework is appealing. Nevertheless, the recent growth of the field and, particularly, studies that have considered the relationship between microhabitat specialization and morphological specialization in a broad quantitative framework (Brandl et al. 2015) have revealed areas of unanticipated complexity, and shown that many ecomorphological relationships are less clear, much less linear and much more multifaceted than expected. Nowhere is this likely to be more so than in the primates. Many factors might confound the ecomorphological relationship; for example, morphology may be subject to non-functional influences on form, such as genetic drift and phylogenetic inertia, and morphological traits may not keep pace with environmental change, such that observed traits reflect past rather than current environments. For primates, however, the complexity increases. Primates are highly social, and their complex social organizations and hierarchies exert a strong influence on the way different individuals are able to exploit and interact with their habitat. Moreover, their high intelligence and high proportion of muscle to tendon means that they are highly plastic in their behaviour and morphology. Indeed, Elton and colleagues ask in their contribution, can we ever identify a primary ecological determinant of a given primate trait because compromises between different functional pressures may result in a feature being just adequate for any particular task. These issues present considerable challenges for quantifying clear patterns in the ecomorphology of extant primates, and transferring this framework to extinct taxa introduces yet another layer of complexity. All the papers in this issue of Journal of Anatomy were prepared by speakers at the Primate Ecomorphology symposium, together with their research colleagues. Most authors have taken such a holistic view of the opportunities and challenges of applying an ecomorphological framework to quantifying the dynamic interactions between both extant and extinct primates and their physical and social environments that the core themes of plasticity, sociality and interpreting the ecomorphology of extinct species run throughout the volume rather than separating contributions into topical groups. A further repercussion of the holistic nature of the papers is that I cannot possibly do justice to the wealth of information in each paper in this short introductory piece. The description that follows therefore details some of the many highlights I have found in each of the papers. I thank the authors wholeheartedly for their hard work, which has resulted in this amazing, far-reaching and stimulating volume of Journal of Anatomy. The volume begins with Sarah Elton and colleagues who, through a detailed consideration of the ecomorphological framework, pose a holistic suite of questions about the nature of the ecological signals that might be expressed in the generalized skeletons that are typical of many monkeys, how we can interpret these signals and how these factors might influence reconstructions of locomotor behaviours and habitat preferences in extinct species. This is a fascinating and detailed paper, but I was particularly struck by one of their conclusions. They point out that whilst it is heuristically useful to think in terms of ‘terrestrial’ or ‘arboreal’ in palaeobiological research, an animal faced with a predator or a tasty foodstuff is unlikely to be so prescriptive. This alludes to the important concept that we may have been overly narrow in our interpretations of the locomotor behaviour of fossil primates, which has wide-ranging ramifications for our current understanding of primate evolution. In the second paper, Erin Butler and Nate Dominy provide an excellent example of the confounding effect of primate muscular plasticity on reconstructions of the ecomorphology of extinct species. They show that the limited availability of human cadaveric material, which has forced a general assumption that minimal variation exists in human populations and/or that industrialized populations represent the human species as well as any other, has significantly hampered our understanding of early hominin locomotor efficiency. Expanding on their earlier work that demonstrated that exploitation of arboreal resources by small-bodied (pygmy) rainforest hunter-gatherers is largely facilitated by muscular adaptation, they present a more detailed interpretation of their ecomorphology. They show convincingly that despite the volume of research on humans, we still do not fully understand how human muscle–tendon architecture can be tailored to specific ecological or environmental demands. Their conclusion that the human pygmy phenotype is a promising model system for understanding hominin bipedal efficiency links to many of the other papers in the volume that argue arboreality was more important in our ancestry than has been previously thought. In the third paper, Tracy Kivell continues the plasticity theme, with a detailed and candid review of the evidence for whether the plasticity of trabecular bone can provide a strong functional signal of how a bone or joint was used during an individual's lifetime, to supplement the more limited functional information that can be gleaned from external skeletal morphology. Kivell reveals a myriad of unexpected complexities that have become apparent in making functional inferences about locomotor behaviour from variation in trabecular architecture. Nevertheless, she argues that ongoing methodological advances combined with the associated accumulation of sample sizes, and increased understanding of the relationship between ecology, biomechanics, morphology and behaviour in extant species, will mean that analyses of trabecular structure will be able to tease out functional signals in fossil bones in the near future. Anne Burrows and colleagues, in the fourth paper, accepted perhaps the greatest challenge set – to develop the case that a primate's social environment should be considered within the ecomorphogical framework. While linking broad social behaviours to specific morphologies might not always be straightforward, Burrows et al. exploit the fact that mimetic muscle morphology is directly linked to social communication as contraction of the musculature leads directly to the facial display. They present two case studies: (i) comparing gross morphology of the mimetic muscles around the external ear in Rhesus and Sulawesi macaques that are closely related but have very different methods to maintain social cohesion; and (ii) comparative physiology of the orbicularis oris muscle in humans, chimpanzees and siamangs to show the contrasting demands of facial and vocal communications. They conclude that observed morphology might reflect a compromise between the demands of the physical and social environments. This is an exciting and young field, and the authors outline several future directions of research. Clearly the next step is to demonstrate that primate adaptations to social behaviour compromise adaptations to the physical environment and result in less than optimal solutions to the latter. Christophe Soligo and Jeroen Smaers deliver a comprehensive review of current perspectives on the origin and early evolution of primates, coupled with a thought-provoking ecomorphological synthesis of primate origins. Among many other topics, these authors develop the plasticity theme with consideration of the role of the brain in buffering a species’ anatomy against natural selection. They reason that the brain determines how an individual perceives its environment, and the behaviours it uses interact with it. Thus, in the absence of direct evidence of ancestral primate behaviour, they suggest that one of the most significant new sources of information on the adaptive context and behavioural flexibility of early primates could come from application of the increasingly sophisticated comparisons possible on the brain anatomy and phylogenetic mapping of neural characteristics of extant species. In the sixth paper, Kevin Hunt delivers an incredible synthesis of the broad ecomorphology of apes and monkeys to ask ‘why are there apes?’ He brings together a detailed comparison of monkey and ape morphology, locomotion, habitat and social organization to show (among other components) that intense competition between apes and monkeys drove apes to evolve large bodies and suspensory features, and monkeys to evolve the ability to digest unripe fruits. I first referenced this paper the day after it was accepted for publication, it is an important contribution to a topic that has appeared intractable, due to limited fossil evidence. In the seventh paper, Robin Crompton reflects his delivery of the 2014 Osman Hill Memorial Lecture for the PSGB. His paper provides a personal synthesis of early hominin ecomorphology, which has been influenced by many of his own studies during 40 years of truly holistic research on hominin palaeontology, the locomotor biomechanics of extant referential models under lab and field conditions, and the development of powerful predictive-modelling computer simulations. This engaging paper details not just current understanding of early hominin ecomorphology, but also the history and societal influences on research in this field. There is a great deal of synergy in the conclusions of many of the papers in the volume, and Crompton's conclusions on selection for ecological plasticity by early hominins and the continued exploitation of arboreal resources by hominins complement and consolidate those of Elton and Dominy, Hunt and Senut. In the final fascinating paper, Brigitte Senut brings her palaeontological expertise to bear on a holistic approach to interpreting early hominin behaviour from ecomorphological signals. Through case studies of the origins of human bipedalism and the springing adaptations in fossil rodents (Pedetidae), which track the processes of desertification, she demonstrates the efficacy of an integrated approach in which locomotor reconstructions can be used to understand the environment, and knowledge of past environments can help to better reconstruct the behavioural repertoires of fossil species. She shows that to flesh out the fossil record most effectively we need ecomorphological studies not just of the descendents or models of the primates we are interested in, but of the extant species that may be models of the fossil species the primates coexisted with. A key theme in the papers in this volume is that understanding the ecomorphological relationships of extant primates is central to reconstructing the ecomorphology of extinct species. It is true that we cannot expect all fossil forms to be reflected in extant species, but the referential modelling process does not necessarily require that to be true. Of course, as Elton and Dominy show, we need to be careful in our selection of models. But by understanding how living species ‘can’ and ‘do’ interact with their habitat, ‘despite’ skeletal constraints, we can garner a better understanding of the envelope of behaviours a particular fossil species might exhibit without expression in the skeleton, and the range of ways that multiple conflicting selective pressures (from the physical and/or social environment) might be resolved in the skeleton. A living referential model need not necessarily be the whole animal, it might be a single feature or system from one or multiple living species, such as the gastrocnemius muscle–tendon unit of small-bodied rainforest hunter-gatherers, or the brain or trabecular morphology of the femur of a range of extant species. Through this holistic approach, the living world still has much to tell us about the dynamic between morphology and ecology in fossil forms.