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In everyday life during terrestrial locomotion our body interacts with two media opposing the forward movement of the body: the ground and the air. Whereas the work done to overcome the ground reaction force has been extensively studied, the work done to overcome still air resistance has been only indirectly estimated by means of theoretical studies and by measurements of the force exerted on puppets simulating the geometry of the human body. In this study, we directly measured the force exerted by still air resistance on eight male subjects during walking and running on an instrumented treadmill with a belt moving at the same speed of a flow of laminar air facing the subject. Overall, the coefficient of proportionality between drag and velocity squared (Aeff) was smaller during running than walking. During running Aeff decreased progressively with increasing average velocity up to an apparently constant, velocity independent value, similar to that predicted in the literature using indirect methods. A predictive equation to estimate drag as a function of the speed and the height of the running subject is provided.

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Graded substrates require legged animals to modulate their limb mechanics to meet locomotor demands. Previous work has elucidated strategies used by cursorial animals with upright limb posture, but it remains unclear how sprawling species such as alligators transition between grades. We measured individual limb forces and 3D kinematics as alligators walked steadily across level, 15 deg incline and 15 deg decline conditions. We compared our results with the literature to determine how limb posture alters strategies for managing the energetic variation that accompanies shifts in grade. We found that juvenile alligators maintain spatiotemporal characteristics of gait and locomotor speed while selectively modulating craniocaudal impulses (relative to level) when transitioning between grades. Alligators seem to accomplish this using a variety of kinematic strategies, but consistently sprawl both limb pairs outside of the parasagittal plane during decline walking. This latter result suggests alligators and other sprawling species may use movements outside of the parasagittal plane as an axis of variation to modulate limb mechanics when transitioning between graded substrates. We conclude that limb mechanics during graded locomotion are fairly predictable across quadrupedal species, regardless of body plan and limb posture, with hindlimbs playing a more propulsive role and forelimbs functioning to dissipate energy. Future work will elucidate how shifts in muscle properties or function underlie such shifts in limb kinematics.

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Guenons are the most diverse clade of African primates, and many species living within the core of the Congo Basin rainforest are still understudied. The recently described guenon species, Cercopithecus lomamiensis, known as lesula, is a cryptic, semi-terrestrial species endemic to the central Congo Basin in the Democratic Republic of the Congo. The recent IUCN Red List Assessment recognizes lesula's risk of extinction in the wild as Vulnerable. The objective of our study was to use camera traps to expand knowledge on the behavioral ecology of lesula. We conducted three systematic, terrestrial camera trap (CT) surveys within Lomami National Park and buffer zone (Okulu: 2013; Losekola: 2014; E15: 2015). We accumulated 598 independent events of lesula over 5960 CT days from 92 CTs. Typical of Cercopithecus species, camera trap videos reveal that lesula has a diurnal activity pattern, birth seasonality, a group size of up to 32 individuals, and social organization with female philopatry and male dispersal. Results also suggest that lesula are highly terrestrial, distinguishing them from other Cercopithecus species, which are mostly arboreal. Our study provides new information about the behavioral ecology of this little-studied primate, generating species-specific knowledge of a threatened species for successful conservation planning.

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We investigated how baboons transition from quadrupedal to bipedal walking without any significant interruption in their forward movement (i.e. transition 'on the fly'). Building on basic mechanical principles (momentum only changes when external forces/moments act on the body), insights into possible strategies for such a dynamical mode transition are provided and applied first to the recorded planar kinematics of an example walking sequence (including several continuous quadrupedal, transition and subsequent bipedal steps). Body dynamics are calculated from the kinematics. The strategy used in this worked example boils down to: crouch the hind parts and sprint them underneath the rising body centre of mass. Forward accelerations are not in play. Key characteristics of this transition strategy were extracted: progression speed, hip height, step duration (frequency), foot positioning at touchdown with respect to the hip and the body centre of mass (BCoM), and congruity between the moments of the ground reaction force about the BCoM and the rate of change of the total angular moment. Statistical analyses across the full sample (15 transitions of 10 individuals) confirm this strategy is always used and is shared across individuals. Finally, the costs (in J kg-1 m-1) linked to on the fly transitions were estimated. The costs are approximately double those of both the preceding quadrupedal and subsequent bipedal walking. Given the short duration of the transition as such (<1 s), it is argued that the energetic costs to change walking posture on the fly are negligible when considered in the context of the locomotor repertoire.

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Amphibious fishes moving from water to land experience continuous changes in environmental forces. How these subtle changes impact behavioural transitions cannot be resolved by comparisons of aquatic and terrestrial locomotion. For example, aquatic and terrestrial locomotion appear distinct in the actinopterygian fish Polypterus senegalus; however, it is unclear how gradual water level changes influence the transition between these locomotor behaviours. We tested the hypothesis in P. senegalus that swimming and walking are part of an incremental continuum of behaviour and muscle activity across the environmental transition from water to land rather than two discrete behaviours, as proposed by previous literature. We exposed P. senegalus to discrete environments from fully aquatic to fully terrestrial while recording body and pectoral fin kinematics and muscle activity. Anterior axial red muscle effort increases as water depth decreases; however, a typical swimming-like anterior-to-posterior wave of axial red muscle activity is always present, even during terrestrial locomotion, indicating gradual motor control changes. Thus, walking appears to be based on swimming-like axial muscle activity whereas kinematic differences between swimming and walking appear to be due to mechanical constraints. A discrete change in left-right pectoral fin coordination from in-phase to out-of-phase at 0.7 body depths relies on adductor muscle activity with a similar duty factor and adductor muscle effort that increases gradually as water depth decreases. Thus, despite distinct changes in kinematic timing, neuromuscular patterning is similar across the water depth continuum. As the observed, gradual increases in axial muscle effort reflect muscle activity changes between aquatic and terrestrial environments observed in other elongate fishes, a modified, swimming-like axial muscle activity pattern for terrestrial locomotion may be common among elongate amphibious fishes.

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Walking can be defined broadly as a slow-speed movement produced when appendages interact with the ground to generate forward propulsion. Until recently, most studies of walking have focused on humans and a handful of domesticated vertebrates moving at a steady rate over highly simplified, static surfaces, which may bias our understanding of the unifying principles that underlie vertebrate locomotion. In the last few decades, studies have expanded to include a range of environmental contexts (e.g., uneven terrain, perturbations, deformable substrates) and greater phylogenetic breadth (e.g., non-domesticated species, small and/or ectothermic tetrapods and fishes); these studies have revealed that even a gait as superficially simple as walking is far more complex than previously thought. In addition, technological advances and accessibility of imaging systems and computational power have recently expanded our capabilities to test hypotheses about the locomotor movements of extant and extinct organisms in silico. In this symposium, scientists showcased diverse taxa (from extant fishes to extinct dinosaurs) moving through a range of variable conditions (speed perturbations, inclines, and deformable substrates) to address the causes and consequences of functional diversity in locomotor systems and discuss nascent research areas and techniques. From the symposium contributions, several themes emerged: (1) slow-speed, appendage-based movements in fishes are best described as walking-like movements rather than true walking gaits, (2) environmental variation (e.g., deformable substrates) and dynamic stimuli (e.g., perturbations) trigger kinematic and neuromuscular changes in animals that make defining a single gait or the transition between gaits more complicated than originally thought, and (3) computational advances have increased the ability to process large data sets, emulate the 3D motions of extant and extinct taxa, and even model species interactions in ancient ecosystems. Although this symposium allowed us to make great strides forward in our understanding of vertebrate walking, much ground remains to be covered. First, there is a much greater range of vertebrate appendage-based locomotor behaviors than has been previously recognized and existing terminology fails to accurately capture and describe this diversity. Second, despite recent efforts, the mechanisms that vertebrates use modify locomotor behaviors in response to predictable and unpredictable locomotor challenges are still poorly understood. Third, while computer-based models and simulations facilitate a greater understanding of the kinetics and kinematics of movement in both extant and extinct animals, a universal, one-size-fits-all, predictive model of appendage-based movement in vertebrates remains elusive.

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Many ecological factors influence animal movement, including properties of the media that they move on or through. Animals moving in terrestrial environments encounter conditions that can be challenging for generating propulsion and maintaining stability, such as inclines and deformable substrates that can cause slipping and sinking. In response, tetrapods tend to adopt a more crouched posture and lower their center of mass on inclines and increase the surface area of contact on deformable substrates, such as sand. Many amphibious fishes encounter the same challenges when moving on land, but how these finned animals modulate their locomotion with respect to different environmental conditions and how these modifications compare with those seen within tetrapods is relatively understudied. Mudskippers (Gobiidae: Oxudercinae) are a particularly noteworthy group of amphibious fishes in this context given that they navigate a wide range of environmental conditions, from flat mud to inclined mangrove trees. They use a unique form of terrestrial locomotion called 'crutching', where their pectoral fins synchronously lift and vault the front half of the body forward before landing on their pelvic fins while the lower half of the body and tail are kept straight. However, recent work has shown that mudskippers modify some aspects of their locomotion when crutching on deformable surfaces, particularly those at an incline. For example, on inclined dry sand, mudskippers bent their bodies laterally and curled and extended their tails to potentially act as a secondary propulsor and/or anti-slip device. In order to gain a more comprehensive understanding of the functional diversity and context-dependency of mudskipper crutching, we compared their kinematics on different combinations of substrate types (solid, mud, dry sand) and inclines (0°, 10°, 20°). In addition to increasing lateral bending on deformable and inclined substrates, we found that mudskippers increased the relative contact time and contact area of their paired fins while becoming more crouched, responses comparable to those seen in tetrapods and other amphibious fishes. Mudskippers on these substrates also exhibited previously undocumented behaviors, such as extending and adpressing the distal portions of their pectoral fins more anteriorly, dorsoventrally bending their trunk, "belly-flopping" on sand, and "gripping" the mud substrate with their pectoral fin rays. Our study highlights potential compensatory mechanisms shared among vertebrates in terrestrial environments while also illustrating that locomotor flexibility and even novelty can emerge when animals are challenged with environmental variation.

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To traverse complex terrain, animals often transition between locomotor modes. It is well known that locomotor transitions can be induced by switching in neural control circuits or driven by a need to minimize metabolic energetic cost. Recent work revealed that locomotor transitions in complex 3D terrain cluttered with large obstacles can emerge from physical interaction with the environment controlled by the nervous system. For example, to traverse cluttered, stiff grass-like beams, the discoid cockroach often transitions from using a strenuous pitch mode pushing across the beams to using a less strenuous roll mode rolling into and through the gaps. This transition can save mechanical energetic cost substantially (∼100-101 mJ) but requires overcoming a potential energy barrier (∼10-3-10-2 mJ). Previous robotic physical modeling demonstrated that kinetic energy fluctuation of body oscillation from self-propulsion can help overcome the barrier and facilitate this transition. However, the animal was observed to transition even when the barrier still exceeded kinetic energy fluctuation. Here, we further studied whether and how the cockroach makes active adjustments to facilitate this transition to traverse cluttered beams. The animal repeatedly flexed its head and abdomen, reduced hindleg sprawl, and depressed one hindleg and elevated the other during the pitch-to-roll transition, adjustments which were absent when running on a flat ground. Using a refined potential energy landscape with additional degrees of freedom to model these adjustments, we found that head flexion did not substantially reduce the transition barrier (by ∼10-3 mJ), whereas leg sprawl reduction did so dramatically (by ∼10-2 mJ). We speculate that head flexion is for sensing the terrain to guide the transition via sensory feedback control.

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Amphibious fishes have many adaptations that make them successful in a wide variety of conditions, including air-breathing, terrestrial locomotor capabilities, and extreme tolerance of poor water quality. However, the traits that make them highly adaptable may allow these fishes to successfully establish themselves outside of their native regions. In particular, the terrestrial capabilities of invasive amphibious fishes allow them to disperse overland, unlike fully aquatic invasive fishes, making their management more complicated. Despite numerous amphibious fish introductions around the world, ecological risk assessments and management plans often fail to adequately account for their terrestrial behaviors. In this review, I discuss the diversity of invasive amphibious fishes and what we currently know about why they emerge onto land, how they move around terrestrial environments, and how they orient while on land. In doing so, I use case studies of the performance and motivations of nonnative amphibious fishes in terrestrial environments to propose management solutions that factor in their complete natural history. Because of their terrestrial capabilities, we may need to manage amphibious fishes more like amphibians than fully aquatic fishes, but to do so, we need to learn more about how these species perform in a wide range of terrestrial environments and conditions.

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Terrestrial and aquatic environments present drastically different challenges, yet amphibious behaviors evolved multiple times in vertebrates. Terrestrial salamanders are often used to model the locomotion of crownward stem tetrapods, but amphibious fishes may model earlier evolutionary stages as vertebrates became terrestrial. For instance, some early tetrapods may have moved on land with a mudskipper-like gait. Previously published kinetic data found that the ground reaction forces produced by the pectoral fins of mudskippers (Periophthalmus barbarus) were more medial than the limbs of tiger salamanders (Ambystoma tigrinum), which might elevate bending stresses in the fins. However, kinematic data are needed to explain these kinetic differences. Therefore, we quantified the three-dimensional kinematics of mudskipper pectoral fins and compared these to published data on tiger salamander limbs. We found that mudskipper pectoral fins generally remained more retracted, extended, and adducted compared to salamander limbs. Kinematic patterns in mudskipper pectoral fins were aligned with published kinetic data and shared a restricted range of motion found in early tetrapods. Our findings demonstrate that mudskipper pectoral fins provide weight support and propulsion but have lower mobility in the proximal versus distal elements, for which greater flexibility in the latter might compensate. Broadly, these data provide new insights into the biomechanics of using fins versus limbs for moving over land and factors that may favor the evolution of different terrestrial gaits.

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Amphibious fishes and salamanders are valuable functional analogs for vertebrates that spanned the water-land transition. However, investigations of walking mechanics have focused on terrestrial salamanders and, thus, may better reflect the capabilities of stem tetrapods that were already terrestrial. The earliest tetrapods were likely aquatic, so salamanders that are not primarily terrestrial may yield more appropriate data for modeling the incipient stages of terrestrial locomotion. In the present study, locomotor biomechanics were quantified from semi-aquatic Pleurodeles waltl, a salamander that spends most of its adult life in water, and then compared with those of a primarily terrestrial salamander (Ambystoma tigrinum) and a semi-aquatic fish (Periophthalmus barbarus) to evaluate whether terrestrial locomotion was more comparable between species with ecological versus phylogenetic similarities. Ground reaction forces (GRFs) from individual limbs or fins indicated that the pectoral appendages of each taxon had distinct patterns of force production, but GRFs from the hindlimbs were comparable between the salamander species. The rate at which force is produced can affect musculoskeletal function, so we also calculated 'yank' (first time derivative of force) to quantify the dynamics of GRF production. Yank was sometimes slower in P. waltl but there were some similarities between the three species. Finally, the semi-aquatic taxa (P. waltl and P. barbarus) had a more medial inclination of the GRF compared to terrestrial salamanders, potentially elevating bone stresses among more aquatic taxa and limiting their excursions onto land.

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Clarias batrachus (walking catfish) is an invasive species in Florida, renowned for its air-breathing and terrestrial locomotor capabilities. However, it is unknown how this species orients in terrestrial environments. Furthermore, while anecdotal life history information is widespread for this species in its nonnative range, little of this information exists in the literature. The goals of this study were to identify sensory modalities that C. batrachus use to orient on land, and to describe the natural history of this species in its nonnative range. Fish (n = 150) were collected from around Ruskin, FL, and housed in a greenhouse, where experiments took place. Individual catfish were placed in the center of a terrestrial arena and were exposed to nine treatments: two controls, L-alanine, quinine, allyl isothiocynate, sucrose, volatile hydrogen sulphide, pond water and aluminium foil. These fish exhibited significantly positive chemotaxis toward alanine and pond water, and negative chemotaxis away from volatile hydrogen sulphide, suggesting chemoreception - both through direct contact and through the air - is important to their terrestrial orientation. Additionally, 88 people from Florida wildlife-related Facebook groups who have personal observations of C. batrachus on land were interviewed for information regarding their terrestrial natural history. These data were combined with observations from 38 YouTube videos. C. batrachus appear to emerge most frequently during or just after heavy summer rains, particularly from stormwater drains in urban areas, where they may feed on terrestrial invertebrates. By better understanding the full life history of C. batrachus, we can improve management of this species.

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An arboreal lifestyle is thought to be central to primate origins, and most extant primate species still live in the trees. Nonetheless, terrestrial locomotion is a widespread adaptation that has arisen repeatedly within the primate lineage. The absence of terrestriality among the New World monkeys (Platyrrhini) is thus notable and raises questions about the ecological pressures that constrain the expansion of platyrrhines into terrestrial niches. Here, we report the results of a natural experiment, comparing patterns of terrestrial behavior in white-faced capuchin monkeys (Cebus capucinus imitator) living on two islands off the Pacific coast of Panama that lack mammalian predators (island sites) with the behavior of capuchins at three sites in central Panama with more intact predator communities (mainland sites). Surveys with camera traps revealed increased terrestriality in island vs. mainland sites. Capuchin detection rates were higher, the range of party sizes observed was larger, and individuals engaged in a wider range of terrestrial behaviors on the islands lacking mammalian predators. Furthermore, females carrying infants were frequently photographed on the ground at the island sites, but never at the mainland sites. These findings support the long-standing hypothesis that predators constrain the exploitation of terrestrial niches by primates. These results are also consistent with the hypothesis that arboreal locomotion imposes costs that primates will avoid by walking on the ground when predation risk is low.

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Many teleost fishes with no apparent modifications for life on land are able to produce effective terrestrial locomotor behaviors, including a ballistic behavior called the "tail-flip" jump. Cyprinodontiformes (killifishes, Teleostei: Atherinomorpha) that live at the water's edge vary in morphology and inclination to emerge onto land. Do fish with an amphibious predisposition have extensive modification of the propulsive region of the body when compared to fully aquatic relatives? We quantified body shape and anatomy of the caudal peduncle and tail (the propulsive organ on land and in water) in 11 cyprinodontiform species and two outgroup taxa (Atherinomorpha). We hypothesized that amphibious species would have longer, "shallower" bodies (larger body fineness ratios), deeper (proportionally larger) caudal peduncles, and more robust bones in the tail fin (larger ossified area of the hypural/epural bones) to facilitate locomotor movements on land. We found no evidence of convergence in body shape or skeletal anatomy among species known to make voluntary sojourns onto land. In fact, deep-bodied species, shallow-bodied species, and species with intermediate morphologies all are able to emerge from the water and move on land. It is possible that there are as-yet-undocumented subtle soft-tissue (muscle, tendon, and ligament) modifications that enhance terrestrial locomotor performance in species known to spend large periods of time on land. However, it is also possible that extreme anatomical changes are not required for aquatic cyprinodontiform species to produce effective locomotor movements when they emerge out of the water and move across the land. Anat Rec, 2019. © 2019 American Association for Anatomy.

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The goal of the present study was to determine which sensory cues the mangrove rivulus Kryptolebias marmoratus, a quasi-amphibious, hermaphroditic fish, uses to orient in an unfamiliar terrestrial environment. In a laboratory setting, K. marmoratus were placed on a terrestrial test arena and were provided the opportunity to move toward reflective surfaces, water, dark colours v. light colours, and orange colouration. Compared with hermaphrodites, males moved more often toward an orange section of the test arena, suggesting that the response may be associated with camouflage or male-male competition, since only males display orange colouration. Younger individuals also moved more often toward the orange quadrant than older individuals, suggesting age-dependent orientation performance or behaviour. Sloped terrain also had a significant effect on orientation, with more movement downhill, suggesting the importance of the otolith-vestibular system in terrestrial orientation of K. marmoratus. By understanding the orientation of extant amphibious fishes, we may be able to infer how sensory biology and behaviour might have evolved to facilitate invasion of land by amphibious vertebrates millions of years ago.

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Tidepool sculpins (Oligocottus maculosus) have been observed moving overland in the rocky intertidal, and we documented the terrestrial walking behavior that they use to accomplish this. We quantified the terrestrial movements of O. maculosus and compared them to (1) their aquatic locomotion, (2) terrestrial locomotion of closely-related subtidal species (Leptocottus armatus and Icelinus borealis), and (3) terrestrial movements of walking catfishes (Clarias spp.). We recorded sculpin movements (210 fps) on a terrestrial platform and in a water tank and tracked body landmarks for kinematic analysis. The axial-appendage-based terrestrial locomotion of O. maculosus is driven by cyclic lateral oscillations of the tail, synchronized with alternating rotations about the base of the pectoral fins, a behavior that appears similar to a military "army crawl." The pectoral fins do not provide propulsion, but act as stable points for the body to rotate around. In contrast, individuals of O. maculosus use primarily axial undulation during slow-speed swimming. The army crawl is a more effective terrestrial behavior (greater distance ratio) than the movements produced by L. armatus and I. borealis, which use rapid, cyclic oscillations of the tail, without coordinated pectoral fin movements. Relative to Clarias spp., O. maculosus rotated the body about the base of the pectoral fin, rather than the tip of the fin, which may cause O. maculosus to have a lower distance ratio. Since O. maculosus lack major morphological adaptations for terrestrial locomotion, instead relying on behavioral adaptations, we propose behavioral adaptations may evolutionarily predate morphological adaptations for terrestrial locomotion in vertebrates.

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OBJECTIVES: Gorillas, along with chimpanzees and bonobos, are ubiquitously described as 'knuckle-walkers.' Consequently, knuckle-walking (KW) has been featured pre-eminently in hypotheses of the pre-bipedal locomotor behavior of hominins and in the evolution of locomotor behavior in apes. However, anecdotal and behavioral accounts suggest that mountain gorillas may utilize a more complex repertoire of hand postures, which could alter current interpretations of African ape locomotion and its role in the emergence of human bipedalism. Here we documented hand postures during terrestrial locomotion in wild mountain gorillas to investigate the frequency with which KW and other hand postures are utilized in the wild. MATERIALS AND METHODS: Multiple high-speed cameras were used to record bouts of terrestrial locomotion of 77 habituated mountain gorillas at Bwindi Impenetrable National Park (Uganda) and Volcanoes National Park (Rwanda). RESULTS: We captured high-speed video of hand contacts in 8% of the world's population of mountain gorillas. Our results reveal that nearly 40% of these gorillas used "non-KW" hand postures, and these hand postures constituted 15% of all hand contacts. Some of these "non-KW" hand postures have never been documented in gorillas, yet match hand postures previously identified in orangutans. DISCUSSION: These results highlight a previously unrecognized level of hand postural diversity in gorillas, and perhaps great apes generally. Although present at lower frequencies than KW, we suggest that the possession of multiple, versatile hand postures present in wild mountain gorillas may represent a shared feature of the African ape and human clade (or even great ape clade) rather than KW per se.

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The hindlimbs in bats are functionally adapted to serve as a hook to attach to the mother from birth, and to roost during independent life. Although bats exhibit different terrestrial locomotion capabilities involving hindlimbs, hindlimb morphology and postnatal development have been poorly studied. We describe in detail the postnatal development and bone morphology of hindlimbs of the nimble walker vampire bat, Desmodus rotundus, and compare adult characters with the insectivorous Molossus molossus (erratic walker) and the frugivorous Artibeus lituratus (non-walker). The advanced ossification of most hindlimb elements of D. rotundus at the newborn stage is consistent with the functional role of this structure at birth in bats. The development completion events of hindlimb bone elements and bone processes in D. rotundus coincide with the cranial bone processes completion and suture closure events. Those events occur when individuals begin to feed by themselves. There are differences in the number and position of bone processes and sesamoids in adults among the compared species, most of which are described for the first time, and in the case of D. rotundus and M. molossus mostly related to a greater and tight articulation between elements. These facts seem to be closely associated with the different terrestrial locomotion capabilities, and in the case of the exclusively sanguivorous D. rotundus with specializations for obtaining food. Anat Rec, 300:2150-2165, 2017. © 2017 Wiley Periodicals, Inc.

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Establishing links between morphology and performance is important for understanding the functional, ecological, and evolutionary implications of morphological diversity. Relationships between morphology and performance are expected to be age dependent if, at different points during ontogeny, animals must perform in different capacities to achieve high fitness returns. Few studies have examined how the relationship between form and function changes across ontogeny. Here, we assess this relationship in the amphibious mangrove rivulus (Kryptolebias marmoratus) fish, a species that is both capable of and reliant on "tail-flip jumping" for terrestrial locomotion. Tail-flip jumping entails an individual transferring its weight to the caudal region of the body, launching itself from the substrate to navigate to new aquatic or semi-aquatic habitats. By combining repeated trials of jumping performance in 237 individuals from distinct age classes with a clearing and staining procedure to visualize bones in the caudal region, we test the hypotheses that as age increases (i) average jumping performance (body lengths jumped) will increase, (ii) the amount of variation for each trait will change, and (iii) the patterns of covariation/correlation among traits, which tell us about the integration of form with function, will also change. We find a significant increase in size-adjusted jumping performance with age, and modification to the correlation structure among traits across ontogeny. However, we also find that significant links between form and function evident in young animals disappear at later ontogenetic stages. Our study suggests that different functional mechanisms may be associated with high performance at different stages of development.

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Birds and humans are successful bipedal runners, who have individually evolved bipedalism, but the extent of the similarities and differences of their bipedal locomotion is unknown. In turn, the anatomical differences of their locomotor systems complicate direct comparisons. However, a simplifying mechanical model, such as the conservative spring-mass model, can be used to describe both avian and human running and thus, provides a way to compare the locomotor strategies that birds and humans use when running on level and uneven ground. Although humans run with significantly steeper leg angles at touchdown and stiffer legs when compared with cursorial ground birds, swing-leg adaptations (leg angle and leg length kinematics) used by birds and humans while running appear similar across all types of uneven ground. Nevertheless, owing to morphological restrictions, the crouched avian leg has a greater range of leg angle and leg length adaptations when coping with drops and downward steps than the straight human leg. On the other hand, the straight human leg seems to use leg stiffness adaptation when coping with obstacles and upward steps unlike the crouched avian leg posture.