The hands and feet of Archaeolemur: metrical affinities and their functional significance

Locomotion of an adult female and juvenile male aye-aye (Daubentonia madagascariensis) in Torotorofotsy, Madagascar

Aye-ayes (Daubentonia madagascariensis) locate and acquire invertebrates from within woody substrates at all levels of the rainforest; yet how their locomotion helps them accommodate this diet has not been explored in detail. We studied the locomotor behavior of an adult female (N = 1,085) and juvenile male (N = 708) aye-aye in the undisturbed forest of Torotorofotsy, Madagascar from May to December 2017. We used bout sampling to record locomotion during foraging and travel of the two radio-collared individuals. We used χ ² tests to compare overall locomotion, travel, and foraging, as well as strata and support use. We performed a correspondence analysis to examine relationships between individual behaviors, strata, and support types. Leaping accounted for 47.9% and 50.1% of all locomotor activity in the adult female and juvenile male, respectively. Leaping was the most common behavior during travel in both individuals (59.2% and 53.9%, respectively), whereas head-first descent was most frequent during foraging (35.0% and 48.0%, respectively). For all three locomotor categories, the main canopy (40.3%-79.6%) was used most frequently and trunks were the most frequently used support type (50.7%-60.0%). There is a strong association between strata and support use overall and during travel. Quadrupedal walking was significantly associated with the main canopy, as was head-first descent with the low canopy. Our analysis demonstrates that aye-ayes use a variety of locomotor behaviors to forage for invertebrates. Aye-ayes' ability to repurpose their positional repertoire to acquire other resources in degraded forests should not obscure the importance of invertebrates to this species.

Functional traits of the world's late Quaternary large-bodied avian and mammalian herbivores

Prehistoric and recent extinctions of large-bodied terrestrial herbivores had significant and lasting impacts on Earth's ecosystems due to the loss of their distinct trait combinations. The world's surviving large-bodied avian and mammalian herbivores remain among the most threatened taxa. As such, a greater understanding of the ecological impacts of large herbivore losses is increasingly important. However, comprehensive and ecologically-relevant trait datasets for extinct and extant herbivores are lacking. Here, we present HerbiTraits, a comprehensive functional trait dataset for all late Quaternary terrestrial avian and mammalian herbivores ≥10 kg (545 species). HerbiTraits includes key traits that influence how herbivores interact with ecosystems, namely body mass, diet, fermentation type, habitat use, and limb morphology. Trait data were compiled from 557 sources and comprise the best available knowledge on late Quaternary large-bodied herbivores. HerbiTraits provides a tool for the analysis of herbivore functional diversity both past and present and its effects on Earth's ecosystems.

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.

Bone indicators of grasping hands in lizards

Grasping is one of a few adaptive mechanisms that, in conjunction with clinging, hooking, arm swinging, adhering, and flying, allowed for incursion into the arboreal eco-space. Little research has been done that addresses grasping as an enhanced manual ability in non-mammalian tetrapods, with the exception of studies comparing the anatomy of muscle and tendon structure. Previous studies showed that grasping abilities allow exploitation for narrow branch habitats and that this adaptation has clear osteological consequences. The objective of this work is to ascertain the existence of morphometric descriptors in the hand skeleton of lizards related to grasping functionality. A morphological matrix was constructed using 51 morphometric variables in 278 specimens, from 24 genera and 13 families of Squamata. To reduce the dimensions of the dataset and to organize the original variables into a simpler system, three PCAs (Principal Component Analyses) were performed using the subsets of (1) carpal variables, (2) metacarpal variables, and (3) phalanges variables. The variables that demonstrated the most significant contributions to the construction of the PCA synthetic variables were then used in subsequent analyses. To explore which morphological variables better explain the variations in the functional setting, we ran Generalized Linear Models for the three different sets. This method allows us to model the morphology that enables a particular functional trait. Grasping was considered the only response variable, taking the value of 0 or 1, while the original variables retained by the PCAs were considered predictor variables. Our analyses yielded six variables associated with grasping abilities: two belong to the carpal bones, two belong to the metacarpals and two belong to the phalanges. Grasping in lizards can be performed with hands exhibiting at least two different independently originated combinations of bones. The first is a combination of a highly elongated centrale bone, reduced palmar sesamoid, divergence angles above 90°, and slender metacarpal V and phalanges, such as exhibited by Anolis sp. and Tropidurus sp. The second includes an elongated centrale bone, lack of a palmar sesamoid, divergence angles above 90°, and narrow metacarpal V and phalanges, as exhibited by geckos. Our data suggest that the morphological distinction between graspers and non-graspers is demonstrating the existence of ranges along the morphological continuum within which a new ability is generated. Our results support the hypothesis of the nested origin of grasping abilities within arboreality. Thus, the manifestation of grasping abilities as a response to locomotive selective pressure in the context of narrow-branch eco-spaces could also enable other grasping-dependent biological roles, such as prey handling.

Dynamic pressure patterns in the hands of olive baboons (Papio anubis) during terrestrial locomotion: implications for cercopithecoid primate hand morphology

Habitually terrestrial monkeys adopt digitigrade hand postures at slow speeds to increase effective forelimb length and reduce distal limb joint moments. As these primates move faster, however, their hands transition to a more palmigrade posture, which is likely associated with the inability of wrist and hand joints to resist higher ground reaction forces (GRF) associated with faster speeds. Transitioning to a palmigrade posture may serve to distribute GRFs over a larger surface area (i.e., increased palmar contact), ultimately reducing stresses in fragile hand bones. To test this hypothesis, dynamic palmar pressure data were collected on two adult baboons (Papio anubis) walking, running, and galloping across a runway integrated with a dynamic pressure mat (20 steps each; speed range: 0.46-4.0 m/s). Peak GRF, contact area, peak pressure, and pressure-time integral were quantified in two regions of the hand: fingers and palms (including metacarpal heads). At slower speeds with lower GRFs, the baboons use digitigrade postures resulting in small palmar contact area (largely across the metacarpal heads). At faster speeds with higher GRFs, they used less digitigrade hand postures resulting in increased palmar contact area. Finger contact area did not change across speeds. Despite higher GRFs at faster speeds, metacarpal pressure was moderated across speeds due to increased palmar contact area as animals transitioned from digitigrady to palmigrady. In contrast, the pressure in the fingers increased with faster speeds. Results indicate that the transition from digitigrady to palmigrady distributes increased forces over a larger palmar surface area. Such dynamic changes in palmar pressure likely moderate strain in the gracile bones of the hand, a structure that is integral not only for locomotion, but also feeding and social behaviors in primates.

The mechanics of the gibbon foot and its potential for elastic energy storage during bipedalism

The mechanics of the modern human foot and its specialization for habitual bipedalism are well understood. The windlass mechanism gives it the required stability for propulsion generation, and flattening of the arch and stretching of the plantar aponeurosis leads to energy saving. What is less well understood is how an essentially flat and mobile foot, as found in protohominins and extant apes, functions during bipedalism. This study evaluates the hypothesis that an energy-saving mechanism, by stretch and recoil of plantar connective tissues, is present in the mobile gibbon foot and provides a two-dimensional analysis of the internal joint mechanics of the foot during spontaneous bipedalism of gibbons using a four-link segment foot model. Available force and pressure data are combined with detailed foot kinematics, recorded with a high-speed camera at 250 Hz, to calculate the external joint moments at the metatarsophalangeal (MP), tarsometatarsal (TM)and talocrural (TC) joints. In addition, instantaneous joint powers are estimated to obtain insight into the propulsion-generating capacities of the internal foot joints. It is found that, next to a wide range of motion at the TC joint, substantial motion is observed at the TM and MP joint, underlining the importance of using a multi-segment foot model in primate gait analyses. More importantly, however, this study shows that although a compliant foot is less mechanically effective for push-off than a `rigid' arched foot, it can contribute to the generation of propulsion in bipedal locomotion viastretch and recoil of the plantarflexor tendons and plantar ligaments.

Foot morphology and locomotor behaviour of squirrels in Gabon rainforests

Equatorial rain forests of Gabon contain nine species of squirrels that exploit different environments along a vertical gradient from the ground to the forest canopy. This study aims at identifying morphological traits of the hind foot that are related to environment preferences, using seven of the nine species. The morphology of the foot was defined by the coordinates in three dimensions of twenty five landmarks on the foot. These were analysed using Procrustes analysis and principal component analysis (PCA). The first axis of the PCA corresponded to a gradient of adaptation to tree-dwelling, in relation to foot morphology. Species living on the ground had a longer, narrower and thinner foot, adapted to horizontal locomotion on the ground. Species with an arboreal habitat had a shorter, wider and thicker foot, adapted to vertical locomotion. The second axis of the PCA could be interpreted in terms of phylogeny.

New hand bones of Hadropithecus stenognathus: implications for the paleobiology of the Archaeolemuridae

A partial, associated skeleton of Hadropithecus stenognathus (AHA-I) was discovered in 2003 at Andrahomana Cave in southeastern Madagascar. Among the postcranial elements found were the first hand bones (right scaphoid, right hamate, left first metacarpal, and right and left fifth metacarpals) attributed to this rare subfossil lemur. These hand bones were compared to those of extant strepsirrhines and catarrhines in order to infer the positional adaptations of Hadropithecus, and they were compared to those of Archaeolemur in order to assess variation in hand morphology among archaeolemurids. The scaphoid tubercle does not project palmarly as in suspensory and climbing taxa, and the hamate has no hook at all (just a small tubercle), which also points to a poorly developed carpal tunnel. There is a distinctive, radioulnarly directed "spiral" facet for articulation with the triquetrum that is most similar in orientation to that of more terrestrial primates (i.e., Lemur catta, Papio, and Gorilla). The first metacarpal is very reduced and represents only 48% of the length of metacarpal V, as in Archaeolemur, which suggests that pollical grasping of arboreal supports was not important. Compared to Archaeolemur, the shaft of metacarpal V is gracile, and the head has no dorsal ridge and lacks characteristics functionally associated with digitigrade, extended metacarpophalangeal joint postures. Proximally, the articular facet for the hamate is oriented more dorsally. Thus, the carpometacarpal joint V appears to have a distinctive hyperextended set, which has no analog among living or extinct primates. The carpals of Hadropithecus are diagnostic of a pronograde, arboreal and terrestrial (although not digitigrade) locomotor repertoire that typifies Lemur catta and some Old World monkeys. No clinging, suspensory, or climbing specializations that characterize indriids or lorises can be found in the hand of this subfossil lemur. The hand of Hadropithecus likely had similar ranges of movement at the radiocarpal and midcarpal joints as of those of pronograde primates, such as lemurids, for which the hand is held in a more extended, pronated, and neutral (i.e., showing less ulnar deviation) position during locomotion in comparison to that of vertical clingers or slow climbers. Although highly autapomorphic, the hand of Hadropithecus resembles that of its sister taxon, Archaeolemur, in having a very reduced pollex and an articular facet on the scaphoid for a sizeable prepollex. These unusual hand features reinforce the monophyly of the Archaeolemuridae.

Intrinsic hand proportions of euarchontans and other mammals: implications for the locomotor behavior of plesiadapiforms

Arboreal primates have distinctive intrinsic hand proportions compared with many other mammals. Within Euarchonta, platyrrhines and strepsirrhines have longer manual proximal phalanges relative to metacarpal length than colugos and terrestrial tree shrews. This trait is part of a complex of features allowing primates to grasp small-diameter arboreal substrates. In addition to many living and Eocene primates, relative elongation of proximal manual phalanges is also present in most plesiadapiforms. In order to evaluate the functional and evolutionary implications of manual similarities between crown primates and plesiadapiforms, we measured the lengths of the metacarpal, proximal phalanx, and intermediate phalanx of manual ray III for 132 extant mammal species (n=702 individuals). These data were compared with measurements of hands in six plesiadapiform species using ternary diagrams and phalangeal indices. Our analyses reveal that many arboreal mammals (including some tree shrews, rodents, marsupials, and carnivorans) have manual ray III proportions similar to those of various arboreal primates. By contrast, terrestrial tree shrews have hand proportions most similar to those of other terrestrial mammals, and colugos are highly derived in having relatively long intermediate phalanges. Phalangeal indices of arboreal species are significantly greater than those of the terrestrial species in our sample, reflecting the utility of having relatively long digits in an arboreal context. Although mammals known to be capable of prehensile grips demonstrate long digits relative to palm length, this feature is not uniquely associated with manual prehension and should be interpreted with caution in fossil taxa. Among plesiadapiforms, Carpolestes, Nannodectes, Ignacius, and Dryomomys have manual ray III proportions that are unlike those of most terrestrial species and most similar to those of various arboreal species of primates, tree shrews, and rodents. Within Euarchonta, Ignacius and Carpolestes have intrinsic hand proportions most comparable to those of living arboreal primates, while Nannodectes is very similar to the arboreal tree shrew Tupaia minor. These results provide additional evidence that plesiadapiforms were arboreal and support the hypothesis that Euarchonta originated in an arboreal milieu.

DNA from extinct giant lemurs links archaeolemurids to extant indriids

BACKGROUND

Although today 15% of living primates are endemic to Madagascar, their diversity was even greater in the recent past since dozens of extinct species have been recovered from Holocene excavation sites. Among them were the so-called "giant lemurs" some of which weighed up to 160 kg. Although extensively studied, the phylogenetic relationships between extinct and extant lemurs are still difficult to decipher, mainly due to morphological specializations that reflect ecology more than phylogeny, resulting in rampant homoplasy.

RESULTS

Ancient DNA recovered from subfossils recently supported a sister relationship between giant "sloth" lemurs and extant indriids and helped to revise the phylogenetic position of Megaladapis edwardsi among lemuriformes, but several taxa - such as the Archaeolemuridae - still await analysis. We therefore used ancient DNA technology to address the phylogenetic status of the two archaeolemurid genera (Archaeolemur and Hadropithecus). Despite poor DNA preservation conditions in subtropical environments, we managed to recover 94- to 539-bp sequences for two mitochondrial genes among 5 subfossil samples.

CONCLUSION

This new sequence information provides evidence for the proximity of Archaeolemur and Hadropithecus to extant indriids, in agreement with earlier assessments of their taxonomic status (Primates, Indrioidea) and in contrast to recent suggestions of a closer relationship to the Lemuridae made on the basis of analyses of dental developmental and postcranial characters. These data provide new insights into the evolution of the locomotor apparatus among lemurids and indriids.

Comparative morphometrics of the primate apical tuft

The relationship between the structure and function of the primate apical tuft is poorly understood. This study addresses several hypotheses about apical tuft morphology using a large modern primate comparative sample. Two indices of tuft size are employed: expansion and robusticity. First, comparisons of relative apical tuft size were drawn among extant nonhuman primate groups in terms of locomotion and phylogenetic category. Both of these factors appear to play a role in apical tuft size among nonhuman primates. Suspensory primates and all platyrrhines had the smallest apical tufts, while terrestrial quadrupeds and all strepsirrhines (regardless of locomotor category) had the largest tufts. Similarly, hypotheses regarding the apical tufts of hominins, especially the large tufts of Neandertals were addressed using a comparison of modern warm- and cold-adapted humans. The results showed that cold-adapted populations possessed smaller apical tufts than did warm-adapted groups. Therefore, the cold-adaptation hypothesis for Neandertal distal phalangeal morphology is not supported. Also, early modern and Early Upper Paleolithic humans had apical tufts that were significantly less expanded and less robust than those of Neandertals. The hypothesis that a large apical tuft serves as support for an expanded digital pulp is supported by radiographic analysis of modern humans in that a significant correlation was discovered between the width of the apical tuft and the width of the pulp. The implications of these findings for hypotheses about the association of apical tuft size and tool making in the hominin fossil record are discussed.

Body size and scaling of the hands and feet of prosimian primates

The hands and feet of primates fulfill a variety of biological roles linked with food acquisition and positional behavior. Current explanations of shape differences in cheiridial morphology among prosimians are closely tied to body size differences. Although numerous studies have examined the relationships between body mass and limb morphology in prosimians, no scaling analysis has specifically considered hand and foot dimensions and intrinsic proportions. In this study, we present such an analysis for a sample of 270 skeletal specimens distributed over eight prosimian families. The degree of association between size and shape was assessed using nonparametric correlational techniques, while the relationship between each ray element length and body mass (from published data and a body mass surrogate) was tested for allometric scaling. Since tarsiers and strepsirrhines encompass many taxa of varying degrees of phylogenetic relatedness, effective degrees of freedom were calculated, and comparisons between families were performed to partially address the problem of statistical nonindependence and "phylogenetic inertia." Correlational analyses indicate negative allometry between relative phalangeal length (as reflected by phalangeal indices) and body mass, except for the pollex and hallux. Thus, as size increases, there is a significant decrease in the relative length of the digits when considering all prosimian taxa sampled. Regression analyses show that while the digital portion of the rays scales isometrically with body mass, the palmar/plantar portion of the rays often scales with positive allometry. Some but not all of these broadly interspecific allometric patterns remain statistically significant when effective degrees of freedom are taken into account. As is often the case in interspecific scaling, comparisons within families show different scaling trends in the cheiridia than those seen across families (i.e., lorisids, indriids, and lemurids exhibit rather different allometries). The interspecific pattern of positive allometry that appears to best characterize the metapodials of prosimians, especially those of the foot, parallels differences found in the morphology of the volar skin. Indeed, relatively longer metapodials appear to covary with flatter and more coalesced volar pads, which in turn slightly improve frictional force for animals that are at a comparative disadvantage while climbing because of their larger mass. Despite the essentially isometric relationship found between digit length and body mass across prosimians, examination of the residual variation reveals that tarsiers and Daubentonia possess, relative to their body sizes, remarkably long fingers. Such marked departures between body size and finger length observed in these particular primates are closely linked with specialized modes of prey acquisition and manipulation involving the hands.

New discoveries of skeletal elements of Hadropithecus stenognathus from Andrahomana Cave, southeastern Madagascar

Remains of what appears to be a single, subadult Hadropithecus stenognathus were recovered from a previously unexcavated site at Andrahomana Cave (southeastern Madagascar). Specimens found comprise isolated teeth and cranial fragments (including the frontal processes of the orbits), as well as a partial postcranial skeleton. They include the first associated fore- and hind-limb bones, confirming the hind-limb attributions made by Godfrey and co-workers in 1997, and refuting earlier attributions by Lamberton in 1937/1938. Of particular interest here are the previously unknown elements, including a sacrum, other vertebrae and ribs, some hand bones, and the distal epiphysis of a femur. We briefly discuss the functional implications of previously unknown elements. Hadropithecus displayed a combination of characters reminiscent of lemurids, others more like those of the larger-bodied Old World monkeys, and still others more like those of African apes. Yet other characteristics appear unique. Lemurid-like postcranial characteristics may be primitive for the Archaeolemuridae. Hadropithecus diverges from the Lemuridae in the direction of Archaeolemur, but more extremely so. Thus, for example, it exhibits a stronger reduction in the size of the hamulus of the hamate, greater anteroposterior compression of the femoral shaft, and greater asymmetry of the femoral condyles. Nothing in its postcranial anatomy signals a close relationship to either the Indriidae or the Palaeopropithecidae.

Evidence at hand: Diversity, functional implications, and locomotor prediction in intrinsic hand proportions of diprotodontian marsupials

Knowledge about the diversity, locomotor adaptations, and evolution of the marsupial forelimb is limited, resulting in an underrepresentation of marsupials in comparative anatomical literature on mammalian forelimb anatomy. This study investigated hand proportions in the diverse marsupial order Diprotodontia. Fifty-two measurements of 95 specimens representing 47 species, as well as 6 non-diprotodontian specimens, were explored using principal components analysis (PCA). Bootstrapping was used to assess the reliability of the loadings. Phylogenetically independent contrasts and phylogenetic ANOVA were used to test for correlation with size and functional adaptation of forelimbs for locomotor habit, scored as arboreal vs. terrestrial. Analysis of first principal component (PC1) scores revealed significant differences between arboreal and terrestrial species, and was related to relative slenderness of their phalangeal elements. Both locomotor groups displayed allometry along PC1 scores, but with different intercepts such that PC1 discriminated between the two locomotor habits almost completely. PC2 separated some higher-level clades and burrowing species. Analysis of locomotor predictors commonly applied by palaeontologists indicates that ratios between proximal and intermediate phalanges were unsuitable as predictors of arboreality/terrestriality, but the phalangeal index was more effective. From PCA results, a phalangeal slenderness ratio was developed which proved to be a useful discriminator, suggesting that a single unallocated phalanx can be used for an impression of locomotor mode in fossils. Most Diprotodontia are laterally paraxonic or ectaxonic, with the exception of digging species whose hands are medially paraxonic. Our results complement those of studies on placental mammals, suggesting that the demands of arboreality, terrestriality, or frequent digging on intrinsic hand proportions are met with similar anatomical adaptations in marsupials.