Beyond power limits: the kinetic energy capacity of skeletal muscle

Muscle is the universal agent of animal movement, and limits to muscle performance are therefore an integral aspect of animal behaviour, ecology and evolution. A mechanical perspective on movement makes it amenable to analysis from first principles, and so brings the seeming certitude of simple physical laws to the challenging comparative study of complex biological systems. Early contributions on movement biomechanics considered muscle energy output to be limited by muscle work capacity, Wmax; triggered by seminal work in the late 1960s, it is now held broadly that a complete analysis of muscle energy output must also consider muscle power capacity, for no unit of work can be delivered in arbitrarily brief time. Here, we adopt a critical stance towards this paradigmatic notion of a power limit, and argue that the alternative constraint to muscle energy output is imposed instead by a characteristic kinetic energy capacity, Kmax, dictated by the maximum speed with which the actuating muscle can shorten. The two critical energies can now be directly compared, and define the physiological similarity index, Γ=Kmax/Wmax. It is the explanatory power of this comparison that lends weight to a shift in perspective from muscle power to kinetic energy capacity, as is argued through a series of illustrative examples. Γ emerges as an important dimensionless number in musculoskeletal dynamics, and sparks novel hypotheses on functional adaptations in musculoskeletal 'design' that depart from the parsimonious evolutionary null hypothesis of geometric similarity.

Deciphering the influence of evolutionary legacy and functional constraints on the patella: an example in modern rhinoceroses amongst perissodactyls

In mammals, the patella is the biggest sesamoid bone of the skeleton and is of crucial importance in posture and locomotion, ensuring the role of a pulley for leg extensors while protecting and stabilizing the knee joint. Despite its central biomechanical role, the relation between the shape of the patella and functional factors, such as body mass or locomotor habit, in the light of evolutionary legacy are poorly known. Here, we propose a morphofunctional investigation of the shape variation of the patella among modern rhinoceroses and more generally among perissodactyls, this order of ungulates displaying a broad range of body plan, body mass and locomotor habits, to understand how the shape of this sesamoid bone varies between species and relatively to these functional factors. Our investigation, relying on three dimensional geometric morphometrics and comparative analyses, reveals that, within Rhinocerotidae and between the three perissodactyl families, the shape of the patella strongly follows the phylogenetic affinities rather than variations in body mass. The patellar shape is more conservative than initially expected both within and between rhinoceroses, equids and tapirs. The development of a medial angle, engendering a strong mediolateral asymmetry of the patella, appears convergent in rhinoceroses and equids, while tapirs retain a symmetric bone close to the plesiomorphic condition of the order. This asymmetric patella is likely associated with the presence of a "knee locking" mechanism in both equids and rhinos. The emergence of this condition may be related to a shared locomotor habit (transverse gallop) in both groups. Our investigation underlines unexcepted evolutionary constraints on the shape of a sesamoid bone usually considered as mostly driven by functional factors.

Predictive musculoskeletal simulations reveal the mechanistic link between speed, posture and energetics among extant mammals

An unusual pattern among the scaling laws in nature is that the fastest animals are neither the largest, nor the smallest, but rather intermediately sized. Because of the enormous diversity in animal shape, the mechanisms underlying this have long been difficult to determine. To address this, we challenge predictive human musculoskeletal simulations, scaled in mass from the size of a mouse (0.1 kg) to the size of an elephant (2000 kg), to move as fast as possible. Our models replicate patterns observed across extant animals including: (i) an intermediate optimal body mass for speed; (ii) a reduction in the cost of transport with increasing size; and (iii) crouched postures at smaller body masses and upright postures at larger body masses. Finally, we use our models to determine the mechanical limitations of speed with size, showing larger animals may be limited by their ability to produce muscular force while smaller animals are likely limited by their ability to produce larger ground reaction forces. Despite their bipedal gait, our models replicate patterns observed across quadrupedal animals, suggesting these biological phenomena likely represent general rules and are not the result of phylogenetic or other ecological factors that typically hinder comparative studies.

Understanding Trophic Interactions in a Warming World by Bridging Foraging Ecology and Biomechanics with Network Science

Climate change will disrupt biological processes at every scale. Ecosystem functions and services vital to ecological resilience are set to shift, with consequences for how we manage land, natural resources, and food systems. Increasing temperatures cause morphological shifts, with concomitant implications for biomechanical performance metrics crucial to trophic interactions. Biomechanical performance, such as maximum bite force or running speed, determines the breadth of resources accessible to consumers, the outcome of interspecific interactions, and thus the structure of ecological networks. Climate change-induced impacts to ecosystem services and resilience are therefore on the horizon, mediated by disruptions of biomechanical performance and, consequently, trophic interactions across whole ecosystems. Here, we argue that there is an urgent need to investigate the complex interactions between climate change, biomechanical traits, and foraging ecology to help predict changes to ecological networks and ecosystem functioning. We discuss how these seemingly disparate disciplines can be connected through network science. Using an ant-plant network as an example, we illustrate how different data types could be integrated to investigate the interaction between warming, bite force, and trophic interactions, and discuss what such an integration will achieve. It is our hope that this integrative framework will help to identify a viable means to elucidate previously intractable impacts of climate change, with effective predictive potential to guide management and mitigation.

Biomechanical and morphological determinants of maximal jumping performance in callitrichine monkeys

Jumping is a crucial behavior in fitness-critical activities including locomotion, resource acquisition, courtship displays and predator avoidance. In primates, paleontological evidence suggests selection for enhanced jumping ability during their early evolution. However, our interpretation of the fossil record remains limited, as no studies have explicitly linked levels of jumping performance with interspecific skeletal variation. We used force platform analyses to generate biomechanical data on maximal jumping performance in three genera of callitrichine monkeys falling along a continuum of jumping propensity: Callimico (relatively high propensity jumper), Saguinus (intermediate jumping propensity) and Callithrix (relatively low propensity jumper). Individuals performed vertical jumps to perches of increasing height within a custom-built tower. We coupled performance data with high-resolution micro-CT data quantifying bony features thought to reflect jumping ability. Levels of maximal performance between species - e.g. maximal take-off velocity of the center of mass (CoM) - parallel established gradients of jumping propensity. Both biomechanical analysis of jumping performance determinants (e.g. CoM displacement, maximal force production and peak mechanical power during push-off) and multivariate analyses of bony hindlimb morphology highlight different mechanical strategies among taxa. For instance, Callimico, which has relatively long hindlimbs, followed a strategy of fully extending of the limbs to maximize CoM displacement - rather than force production - during push-off. In contrast, relatively shorter-limbed Callithrix depended mostly on relatively high push-off forces. Overall, these results suggest that leaping performance is at least partially associated with correlated anatomical and behavioral adaptations, suggesting the possibility of improving inferences about performance in the fossil record.

Footfall patterns and stride parameters of Common hippopotamus (Hippopotamus amphibius) on land

Common hippopotamuses (hippos) are among the largest extant land mammals. They thus offer potential further insight into how giant body size on land influences locomotor patterns and abilities. Furthermore, as they have semi-aquatic habits and unusual morphology, they prompt important questions about how locomotion evolved in Hippopotamidae. However, basic information about how hippos move is limited and sometimes contradictory. We aimed to test if hippos trot at all speeds and if they ever use an aerial (suspended) phase, and to quantify how their locomotor patterns (footfalls and stride parameters) change with approximate speed. We surveyed videos available online and collected new video data from two zoo hippos in order to calculate the data needed to achieve our aims; gathering a sample of 169 strides from 32 hippos. No hippos studied used other than trotting (or near-trotting) footfall patterns, but at the fastest relative speeds hippos used brief aerial phases, apparently a new discovery. Hippos exhibit relatively greater athletic capacity than elephants in several ways, but perhaps not greater than rhinoceroses. Our data help form a baseline for assessing if other hippos use normal locomotion; relevant to clinical veterinary assessments of lameness; and for reconstructing the evolutionary biomechanics of hippo lineages.

Why animals can outrun robots

Animals are much better at running than robots. The difference in performance arises in the important dimensions of agility, range, and robustness. To understand the underlying causes for this performance gap, we compare natural and artificial technologies in the five subsystems critical for running: power, frame, actuation, sensing, and control. With few exceptions, engineering technologies meet or exceed the performance of their biological counterparts. We conclude that biology's advantage over engineering arises from better integration of subsystems, and we identify four fundamental obstacles that roboticists must overcome. Toward this goal, we highlight promising research directions that have outsized potential to help future running robots achieve animal-level performance.

The air conditioning in the nose of mammals depends on their mass and on their maximal running speed

The nose of the mammals is responsible for filtering, humidifying, and heating the air before entering the lower respiratory tract. This conditioning avoids, notably, dehydration of the bronchial and alveolar mucosa. However, since this conditioning is not perfect, exercising in cold air can induce lung inflammation, both for human and non-human mammals. This work aims to compare the air conditioning in the noses of various mammals during inspiration. We build our study on computational fluid dynamics simulations of the heat exchanges in the lumen of the upper respiratory tract of these mammals. These simulations show that the efficiency of the air conditioning in the nose during inspiration does not relate only to the mass m of the mammal but also to its maximal running speed v. More precisely, the results allow establishing a scaling law relating the efficiency of air conditioning in the nose of mammals to the ratio v / log 10 ( m ) . The simulations also correlate the resistance to the flow in the nose to the efficiency of this air conditioning. The obtained scaling law allows predicting the air temperature at the top of the trachea during inspiration for nasal-breathing mammals, and thus notably for humans of various ages.

Dynamic similarity and the peculiar allometry of maximum running speed

Animal performance fundamentally influences behaviour, ecology, and evolution. It typically varies monotonously with size. A notable exception is maximum running speed; the fastest animals are of intermediate size. Here we show that this peculiar allometry results from the competition between two musculoskeletal constraints: the kinetic energy capacity, which dominates in small animals, and the work capacity, which reigns supreme in large animals. The ratio of both capacities defines the physiological similarity index Γ, a dimensionless number akin to the Reynolds number in fluid mechanics. The scaling of Γ indicates a transition from a dominance of muscle forces to a dominance of inertial forces as animals grow in size; its magnitude defines conditions of "dynamic similarity" that enable comparison and estimates of locomotor performance across extant and extinct animals; and the physical parameters that define it highlight opportunities for adaptations in musculoskeletal "design" that depart from the eternal null hypothesis of geometric similarity. The physiological similarity index challenges the Froude number as prevailing dynamic similarity condition, reveals that the differential growth of muscle and weight forces central to classic scaling theory is of secondary importance for the majority of terrestrial animals, and suggests avenues for comparative analyses of locomotor systems.

Neuropil Variation in the Prefrontal, Motor, and Visual Cortex of Six Felids

INTRODUCTION

Felids have evolved a specialized suite of morphological adaptations for obligate carnivory. Although the musculoskeletal anatomy of the Felidae has been studied extensively, the comparative neuroanatomy of felids is relatively unexplored. Little is known about how variation in the cerebral anatomy of felids relates to species-specific differences in sociality, hunting strategy, or activity patterns.

METHODS

We quantitatively analyzed neuropil variation in the prefrontal, primary motor, and primary visual cortices of six species of Felidae (Panthera leo, Panthera uncia, Panthera tigris, Panthera leopardus, Acinonyx jubatus, Felis sylvestris domesticus) to investigate relationships with brain size, neuronal cell parameters, and select behavioral and ecological factors. Neuropil is the dense, intricate network of axons, dendrites, and synapses in the brain, playing a critical role in information processing and communication between neurons.

RESULTS

There were significant species and regional differences in neuropil proportions, with African lion, cheetah, and tiger having more neuropil in all three cortical regions in comparison to the other species. Based on regression analyses, we find that the increased neuropil fraction in the prefrontal cortex supports social and behavioral flexibility, while in the primary motor cortex, this facilitates the neural activity needed for hunting movements. Greater neuropil fraction in the primary visual cortex may contribute to visual requirements associated with diel activity patterns.

CONCLUSION

These results provide a cross-species comparison of neuropil fraction variation in the Felidae, particularly the understudied Panthera, and provide evidence for convergence of the neuroanatomy of Panthera and cheetahs.

A century of exercise physiology: concepts that ignited the study of human thermoregulation. Part 4: evolution, thermal adaptation and unsupported theories of thermoregulation

This review is the final contribution to a four-part, historical series on human exercise physiology in thermally stressful conditions. The series opened with reminders of the principles governing heat exchange and an overview of our contemporary understanding of thermoregulation (Part 1). We then reviewed the development of physiological measurements (Part 2) used to reveal the autonomic processes at work during heat and cold stresses. Next, we re-examined thermal-stress tolerance and intolerance, and critiqued the indices of thermal stress and strain (Part 3). Herein, we describe the evolutionary steps that endowed humans with a unique potential to tolerate endurance activity in the heat, and we examine how those attributes can be enhanced during thermal adaptation. The first of our ancestors to qualify as an athlete was Homo erectus, who were hairless, sweating specialists with eccrine sweat glands covering almost their entire body surface. Homo sapiens were skilful behavioural thermoregulators, which preserved their resource-wasteful, autonomic thermoeffectors (shivering and sweating) for more stressful encounters. Following emigration, they regularly experienced heat and cold stress, to which they acclimatised and developed less powerful (habituated) effector responses when those stresses were re-encountered. We critique hypotheses that linked thermoregulatory differences to ancestry. By exploring short-term heat and cold acclimation, we reveal sweat hypersecretion and powerful shivering to be protective, transitional stages en route to more complete thermal adaptation (habituation). To conclude this historical series, we examine some of the concepts and hypotheses of thermoregulation during exercise that did not withstand the tests of time.

Investigating the quadrupedal abilities of Scutellosaurus lawleri and its implications for locomotor behavior evolution among dinosaurs

A reversion to secondary quadrupedality is exceptionally rare in nature, yet the convergent re-evolution of this locomotor style occurred at least four separate times within Dinosauria. Facultative quadrupedality, an intermediate state between obligate bipedality and obligate quadrupedality, may have been an important transitional step in this locomotor shift, and is proposed for a range of basal ornithischians and sauropodomorphs. Advances in virtual biomechanical modeling and simulation have allowed for the investigation of limb anatomy and function in a range of extinct dinosaurian species, yet this technique has not been widely applied to explore facultatively quadrupedal gait generation. This study places its focus on Scutellosaurus, a basal thyreophoran that has previously been described as both an obligate biped and a facultative quadruped. The functional anatomy of the musculoskeletal system (myology, mass properties, and joint ranges of motion) has been reconstructed using extant phylogenetic bracketing and comparative anatomical datasets. This information was used to create a multi-body dynamic locomotor simulation that demonstrates that whil quadrupedal gaits were physically possible, they did not outperform bipedal gaits is any tested metric. Scutellosaurus cannot therefore be described as an obligate biped, but we would predict its use of quadrupedality would be very rare, and perhaps restricted to specific activities such as foraging. This finding suggests that basal thyreophorans are still overwhelmingly bipedal but is perhaps indicative of an adaptive pathway for later evolution of quadrupedality.

Mechanisms of group-hunting in vertebrates

Group-hunting is ubiquitous across animal taxa and has received considerable attention in the context of its functions. By contrast much less is known about the mechanisms by which grouping predators hunt their prey. This is primarily due to a lack of experimental manipulation alongside logistical difficulties quantifying the behaviour of multiple predators at high spatiotemporal resolution as they search, select, and capture wild prey. However, the use of new remote-sensing technologies and a broadening of the focal taxa beyond apex predators provides researchers with a great opportunity to discern accurately how multiple predators hunt together and not just whether doing so provides hunters with a per capita benefit. We incorporate many ideas from collective behaviour and locomotion throughout this review to make testable predictions for future researchers and pay particular attention to the role that computer simulation can play in a feedback loop with empirical data collection. Our review of the literature showed that the breadth of predator:prey size ratios among the taxa that can be considered to hunt as a group is very large (<100 to >102 ). We therefore synthesised the literature with respect to these predator:prey ratios and found that they promoted different hunting mechanisms. Additionally, these different hunting mechanisms are also related to particular stages of the hunt (search, selection, capture) and thus we structure our review in accordance with these two factors (stage of the hunt and predator:prey size ratio). We identify several novel group-hunting mechanisms which are largely untested, particularly under field conditions, and we also highlight a range of potential study organisms that are amenable to experimental testing of these mechanisms in connection with tracking technology. We believe that a combination of new hypotheses, study systems and methodological approaches should help push the field of group-hunting in new directions.

Visual feedback influences the consistency of the locomotor pattern in Asian elephants (Elephas maximus)

Elephants are atypical of most quadrupeds in that they maintain the same lateral sequence footfall pattern across all locomotor speeds. It has been speculated that the preservation of the footfall patterns is necessary to maintain a statically stable support polygon. This should be a particularly important constraint in large, relatively slow animals. This suggests that elephants must rely on available sensory feedback mechanisms to actively control their massive pillar-like limbs for proper foot placement and sequencing. How the nervous system of elephants integrates the available sensory information for a stable gait is unknown. Here we explored the role that visual feedback plays in the control of the locomotor pattern in Asian elephants. Four Asian elephants (Elephas maximus) walked with and without a blindfold as we measured their stride time intervals. Coefficient of variation was used to assess changes in the overall variability of the stride time intervals, while approximate entropy was used to measure the stride-to-stride consistency of the time intervals. We show that visual feedback plays a role in the stride-to-stride consistency of the locomotor pattern in Asian elephants. These results suggest that elephants use visual feedback to correct and maintain proper sequencing of the limbs during locomotion.

New genicular joint angle criteria for flexor muscle (Musculus Semimembranosus) during the terrestrial mammals walking

Background

The genicular or knee joint angles of terrestrial mammals remain constant during the stance phase of walking; however, the angles differ among taxa. The knee joint angle is known to correlate with taxa and body mass among extant mammals, yet several extinct mammals, such as desmostylians, do not have closely related descendants. Furthermore, fossils lose their soft tissues by the time they are unearthed, making body mass estimates difficult. These factors cause significant problems when reconstructing the proper postures of extinct mammals. Terrestrial mammals use potential and kinetic energy for locomotion; particularly, an inverted pendulum mechanism is used for walking. This mechanism requires maintaining the rod length constant, therefore, terrestrial mammals maintain their joint angle in a small range. A muscle reaction referred to as co-contraction is known to increase joint stiffness; both the agonist and antagonist muscles work simultaneously on the same joint at the same time. The musculus semimembranosus flexes the knee joint and acts as an antagonist to muscles that extend it.

Methods

Twenty-one species of terrestrial mammals were examined to identify the elements that constitute the angle between the m. semimembranosus and the tibia based on the period between the hindlimb touching down and taking off from the ground. Measurements were captured from videos in high-speed mode (420 fps), selecting 13 pictures from the first 75% of each video while the animals were walking. The angles between the main force line of the m. semimembranosus and the tibia, which were defined as θ_sm-t, were measured.

Results

The maximum and minimum angles between the m. semimembranosus and the tibia (θ_sm-t) of the stance instance (SI) were successfully determined for more than 80% of the target animals (17 out of 21 species) during SI-1 to SI-13 within ±10° from the mean. The difference between each successive SI was small and, therefore, the θ_sm-t transition was smooth. According to the results of the total stance differences among the target animals, θ_sm-t was relatively constant during a stance and, therefore, average θ_sm-t (θ_ave) can represent each animal. Only Carnivora had a significant difference in the correlation between body mass and θ_ave. In addition, there were significant differences in θ_ave between plantigrade and unguligrade locomotion.

Conclusion

Our measurements show that θ_ave was 100 ± 10° regardless taxon, body mass, and locomotor mode. Thus, only three points on skeletons need to be measured to determine θ_ave. This offers a new approximation approach for understanding hindlimb posture that could be applied to the study of the hindlimbs of extinct mammals with no closely related extant descendants.

Ecological rules for global species distribution also predict performance variation in Ironman triathletes

Bergmann's and Allen's rules predict changes in body size and appendage length across temperature gradients for species with broad geographic distributions. Larger bodies and longer limbs facilitate cooling whereas smaller bodies and compact limbs limit heat loss. Although these patterns are highly repeatable (hence "rules" of ecology) the patterns and underlying mechanisms are less-well understood in humans. Here I show that variation in running performance among human male triathletes is consistent with both Bergmann's and Allen's rules. Males (but not females) with relatively larger body size and longer limbs performed better at hot compared to cold race venues and vice-versa. Consistent with results in other taxa, sex-specificity may reflect selection for sexual dimorphism. Results suggest that ecological patterns detected over large-spatial scales may arise from fine-scale variation in locomotor performance.

Equid nutritional physiology and behavior: an evolutionary perspective

Like other members of the even-toed ungulates (the perissodactyls), equids once had a higher species diversity in the fossil record than they have today. This is generally explained in comparison to the enormous diversity of bovid ruminants. Theories on putative competitive disadvantages of equids include the use of a single toe as opposed to two toes per leg, the lack of a specific brain cooling (and hence water-saving) mechanism, longer gestation periods that delay reproductive output, and in particular digestive physiology. To date, there is no empirical support for the theory that equids fare better on low-quality forage than ruminants. In contrast to the traditional juxtaposition of hindgut and foregut fermenters, we suggest that it is more insightful to sketch the evolution of equid and ruminant digestive physiology as a case of convergence: both evolved a particularly high chewing efficacy in their respective groups, which facilitates comparatively high feed and hence energy intakes. But because the ruminant system, less based on tooth anatomy but more on a forestomach sorting mechanism, is more effective, equids depend more on high feed intakes than ruminants and may well be more susceptible to feed shortages. Arguably, the most under-emphasized characteristic of equids may be that in contrast to many other herbivores including ruminants and coprophageous hindgut fermenters, equids do not use the microbial biomass growing in their gastrointestinal tract. Equids display behavioral and morphophysiological adaptations to high feed intakes, and their cranial anatomy that facilitates the cropping of forage while performing grinding chewing at the same time might be unique. Rather than looking for explanations how equids are better adapted to their present niches than other organisms, considering them remnants of a different morphophysiological solution may be more appropriate.

Carpal allometry of African apes among mammals

OBJECTIVES

Morphological variation in African ape carpals has been used to support the idea that Pan and Gorilla evolved knuckle-walking independently. Little work, however, has focused on the effect of body mass on carpal morphology. Here, we compare carpal allometry in Pan and Gorilla to that of other quadrupedal mammals with similar body mass differences. If allometric trends in Pan and Gorilla carpals mirror those of other mammals with similar body mass variation, then body mass differences may provide a more parsimonious explanation for African ape carpal variation than the independent evolution of knuckle-walking.

MATERIALS AND METHODS

Three linear measurements were collected on the capitate, hamate, lunate, and scaphoid (or scapholunate) of 39 quadrupedal species from six mammalian families/subfamilies. Relationships between linear measurements and estimated body mass were analyzed using reduced major axis regression. Slopes were compared to 0.33 for isometry.

RESULTS

Within Hominidae, higher body mass taxa (Gorilla) have relatively anteroposteriorly wider, mediolaterally wider, and/or proximodistally shorter capitates, hamates, and scaphoids than low body mass taxa (Pan). These allometric relationships are mirrored in most, but not all, mammalian families/subfamilies included in the analysis.

CONCLUSIONS

Within most mammalian families/subfamilies, carpals of high body mass taxa are proximodistally shorter, anteroposteriorly wider, and mediolaterally wider than those of low body mass taxa. These distinctions may be caused by the need to accommodate relatively higher forelimb loading associated with greater body mass. Because these trends occur within multiple mammalian families/subfamilies, some carpal variation in Pan and Gorilla is consistent with body mass differences.

Anatomical Correlates of Cursoriality are Compromised by Body Size and Propensity to Burrow in a Group of Small Mammals (Lagomorpha)

Highly cursorial animals are specialised for fast, sustained running via specific morphological adaptations, notably including changes in limb segment length and mechanical advantage. Members of the order Lagomorpha (hares, rabbits and pikas) vary in cursorial ability; hares are generally highly cursorial, rabbits more frequently saltate, and pikas predominantly trot. Previous investigations of lagomorphs have identified anatomical trends correlated with this ‘cursoriality gradient’, however, the phylogenetic sampling of such investigations has been limited to three American species, namely the American pika (Ochotona princeps), brush rabbit (Sylvilagus bachmani), and black-tailed jackrabbit (Lepus californicus). Here, we expand the phylogenetic sample and body size range by including novel data from Australian samples of the European rabbit (Oryctolagus cuniculus) and European hare (L. europaeus), alongside unpublished data on the Eastern cottontail (S. floridanus). X-ray Computed Tomography and digital landmarking were used to capture proportions within the appendicular skeleton of ~ 40 specimens of each European species. In doubling the number of species studied, we find the previously-identified morphological gradients associated with cursorial behaviour are complicated when evaluated in the larger sample. The relative length and joint velocity of limbs was found to be lower than predicted in European rabbits and hares. Furthermore, we present a novel assessment of morphological integration in the lagomorph appendicular skeleton, finding between-limb covariation patterns that are generally similar to those of other mammals. Broadly, these results suggest cursoriality is only one of many selective forces driving lagomorph skeletal evolution, with variations in body size and fossoriality potentially having measurable impacts.

Down a Rabbit Hole: Burrowing Behaviour and Larger Home Ranges are Related to Larger Brains in Leporids

Studies on the evolution of brain size variation usually focus on large clades encompassing broad phylogenetic groups. This risks introducing ‘noise’ in the results, often obscuring effects that might be detected in less inclusive clades. Here, we focus on a sample of endocranial volumes (endocasts) of 18 species of rabbits and hares (Lagomorpha: Leporidae), which are a discrete radiation of mammals with a suitably large range of body sizes. Using 60 individuals, we test five popular hypotheses on brain size and olfactory bulb evolution in mammals. We also address the pervasive issue of missing data, using multiple phylogenetic imputations as to conserve the full sample size for all analyses. Our analyses show that home range and burrowing behaviour are the only predictors of leporid brain size variation. Litter size, which is one of the most widely reported constraints on brain size, was unexpectedly not associated with brain size. However, a constraining effect may be masked by a strong association of litter size with temperature seasonality, warranting further study. Lastly, we show that unreasonable estimations of phylogenetic signal (Pagel’s lamba) warrant additional caution when using small sample sizes, such as ours, in comparative studies.

Low effective mechanical advantage of giraffes' limbs during walking reveals trade-off between limb length and locomotor performance

Giraffes (Giraffa camelopardalis) possess specialized locomotor morphology, namely elongate and gracile distal limbs. While this contributes to their overall height and enhances feeding behavior, we propose that the combination of long limb segments and modest muscle lever arms results in low effective mechanical advantage (EMA, the ratio of in-lever to out-lever moment arms), when compared with other cursorial mammals. To test this, we used a combination of experimentally measured kinematics and ground reaction forces (GRFs), musculoskeletal modeling, and inverse dynamics to calculate giraffe forelimb EMA during walking. Giraffes walk with an EMA of 0.34 (±0.05 SD), with no evident association with speed within their walking gait. Giraffe EMA was about four times lower than expectations extrapolated from other mammals, ranging from 0.03 to 297 kg, and this provides further evidence that EMA plateaus or even diminishes in mammals exceeding horse size. We further tested the idea that limb segment length is a factor which determines EMA, by modeling the GRF and muscle moment arms in the extinct giraffid Sivatherium giganteum and the other extant giraffid, Okapia johnstoni. Giraffa and Okapia shared similar EMA, despite a four to sixfold difference in body mass (Okapia EMA = 0.38). In contrast, Sivatherium, sharing a similar body mass with Giraffa, had greater EMA (0.59), which we propose reflects behavioral differences, such as a somewhat increased capability for athletic performance. Our modeling approach suggests that limb length is a determinant of GRF moment arm magnitude and that unless muscle moment arms scale isometrically with limb length, tall mammals are prone to low EMA.

Understanding Australia’s unique hopping species: a comparative review of the musculoskeletal system and locomotor biomechanics in Macropodoidea

Kangaroos and other macropodoids stand out among mammals for their unusual hopping locomotion and body shape. This review examines the scaling of hind- and forelimb bones, and the primary ankle extensor muscles and tendons. We find that the scaling of the musculoskeletal system is sensitive to the phylogenetic context. Tibia length increases with positive allometry among most macropodoids, but negative allometry in eastern grey kangaroos and isometry in red kangaroos. Femur length decreases with stronger negative allometry in eastern grey and red kangaroos than among other macropodoids. Muscle masses scale with negative allometry in western grey kangaroos and with isometry in red kangaroos, compared to positive allometry in other macropodoids. We further summarise the work on the hopping gait, energetics in macropodoids, and stresses in the musculoskeletal system in an evolutionary context, to determine what trade-offs may limit locomotor performance in macropodoids. When large kangaroos hop, they do not increase oxygen consumption with speed, unlike most mammals, including small hopping species. We conclude that there is not enough information to isolate the biomechanical factors that make large kangaroos so energy efficient. We identify key areas for further research to fill these gaps.

Characterizing the performance of human leg external force control

Our legs act as our primary contact with the surrounding environment, generating external forces that enable agile motion. To be agile, the nervous system has to control both the magnitude of the force that the feet apply to the ground and the point of application of this force. The purpose of this study was to characterize the performance of the healthy human neuromechanical system in controlling the force-magnitude and position of an externally applied force. To accomplish this, we built an apparatus that immobilized participants but allowed them to exert variable but controlled external forces with a single leg onto a ground embedded force plate. We provided real-time visual feedback of either the leg force-magnitude or force-position that participants were exerting against the force platform and instructed participants to best match their real-time signal to prescribed target step functions. We tested target step functions of a range of sizes and quantified the responsiveness and accuracy of the control. For the control of force-magnitude and for intermediate step sizes of 0.45 bodyweights, we found a bandwidth of 1.8 ± 0.5 Hz, a steady-state error of 2.6 ± 0.9%, and a steady-state variability of 2.7 ± 0.9%. We found similar control performance in terms of responsiveness and accuracy across step sizes and between force-magnitude and position control. Increases in responsiveness correlated with reductions in other measures of control performance, such as a greater magnitude of overshooting. We modelled the observed control performance and found that a second-order model was a good predictor of external leg force control. We discuss how benchmarking force control performance in young healthy humans aids in understanding differences in agility between humans, between humans and other animals, and between humans and engineered systems.

Only the Good Die Old? Ontogenetic Determinants of Locomotor Performance in Eastern Cottontail Rabbits (Sylvilagus floridanus)

For many animals, the juvenile stage of life can be particularly perilous. Once independent, immature animals must often complete the same basic survival functions as adults despite smaller body size and other growth-related limits on performance. Because, by definition, juveniles have yet to reproduce, we should expect strong selection for mechanisms to offset these ontogenetic limitations, allowing individuals to reach reproductive adulthood and maintain Darwinian fitness. We use an integrated ontogenetic dataset on morphology, locomotor performance, and longevity in wild cottontail rabbits (Sylvilagus floridanus, Allen 1848) to test the hypothesis that prey animals are under selective pressure to maximize juvenile performance. We predicted that (1) juveniles would accelerate more quickly than adults, allowing them to reach adult-like escape speeds, and (2) juveniles with greater levels of performance should survive for longer durations in the wild, thus increasing their reproductive potential. Using high-speed video and force platform measurements, we quantified burst acceleration, escape speed, and mechanical power production in 38 wild-caught S. floridanus (26 juveniles, 12 adults; all rabbits >1 kg in body mass were designated to be adults, based on published growth curves and evidence of epiphyseal fusion). A subsample of 22 rabbits (15 juveniles, 7 adults) was fitted with radio-telemetry collars for documenting survivorship in the wild. We found that acceleration and escape speed peaked in the late juvenile period in S. floridanus, at an age range that coincides with a period of pronounced demographic attrition in wild populations. Differences in mass-specific mechanical power production explained ∼75% of the variation in acceleration across the dataset, indicating that juvenile rabbits outpace adults by producing more power per unit body mass. We found a positive, though non-significant, association between peak escape speed and survivorship duration in the wild, suggesting a complex relationship between locomotor performance and fitness in growing S. floridanus.

Lagomorpha as a Model Morphological System

Due to their global distribution, invasive history, and unique characteristics, European rabbits are recognizable almost anywhere on our planet. Although they are members of a much larger group of living and extinct mammals [Mammalia, Lagomorpha (rabbits, hares, and pikas)], the group is often characterized by several well-known genera (e.g., Oryctolagus, Sylvilagus, Lepus, and Ochotona). This representation does not capture the extraordinary diversity of behavior and form found throughout the order. Model organisms are commonly used as exemplars for biological research, but there are a limited number of model clades or lineages that have been used to study evolutionary morphology in a more explicitly comparative way. We present this review paper to show that lagomorphs are a strong system in which to study macro- and micro-scale patterns of morphological change within a clade that offers underappreciated levels of diversity. To this end, we offer a summary of the status of relevant aspects of lagomorph biology.

The evolutionary biomechanics of locomotor function in giant land animals

Giant land vertebrates have evolved more than 30 times, notably in dinosaurs and mammals. The evolutionary and biomechanical perspectives considered here unify data from extant and extinct species, assessing current theory regarding how the locomotor biomechanics of giants has evolved. In terrestrial tetrapods, isometric and allometric scaling patterns of bones are evident throughout evolutionary history, reflecting general trends and lineage-specific divergences as animals evolve giant size. Added to data on the scaling of other supportive tissues and neuromuscular control, these patterns illuminate how lineages of giant tetrapods each evolved into robust forms adapted to the constraints of gigantism, but with some morphological variation. Insights from scaling of the leverage of limbs and trends in maximal speed reinforce the idea that, beyond 100-300 kg of body mass, tetrapods reduce their locomotor abilities, and eventually may lose entire behaviours such as galloping or even running. Compared with prehistory, extant megafaunas are depauperate in diversity and morphological disparity; therefore, turning to the fossil record can tell us more about the evolutionary biomechanics of giant tetrapods. Interspecific variation and uncertainty about unknown aspects of form and function in living and extinct taxa still render it impossible to use first principles of theoretical biomechanics to tightly bound the limits of gigantism. Yet sauropod dinosaurs demonstrate that >50 tonne masses repeatedly evolved, with body plans quite different from those of mammalian giants. Considering the largest bipedal dinosaurs, and the disparity in locomotor function of modern megafauna, this shows that even in terrestrial giants there is flexibility allowing divergent locomotor specialisations.

The allometry of locomotion

Organismal locomotion mediates ecological interactions and shapes community dynamics. Locomotion is constrained by intrinsic and environmental factors and integrating these factors should clarify how locomotion affects ecology across scales. We extended general theory based on metabolic scaling and biomechanics to predict the scaling of five locomotor performance traits: routine speed, maximum speed, maximum acceleration, minimum powered turn radius, and angular speed. To test these predictions, we used phylogenetically informed analyses of a new database with 884 species and found support for our quantitative predictions. Larger organisms were faster but less maneuverable than smaller organisms. Routine and maximum speeds scaled with body mass to 0.20 and 0.17 powers, respectively, and plateaued at higher body masses, especially for maximum speed. Acceleration was unaffected by body mass. Minimum turn radius scaled to a 0.19 power, and the 95% CI included our theoretical prediction, as we predicted. Maximum angular speed scaled higher than predicted but in the same direction. We observed universal scaling among locomotor modes for routine and maximum speeds but the intercepts varied; flying organisms were faster than those that swam or ran. Acceleration was independent of size in flying and aquatic taxa but decreased with body mass in land animals, possibly due to the risk of injury large, terrestrial organisms face at high speeds and accelerations. Terrestrial mammals inhabiting structurally simple habitats tended to be faster than those in complex habitats. Despite effects of body size, locomotor mode, and habitat complexity, universal scaling of locomotory performance reveals the general ways organisms move across Earth's complex environments.

Limb myology and muscle architecture of the Indian rhinoceros Rhinoceros unicornis and the white rhinoceros Ceratotherium simum (Mammalia: Rhinocerotidae)

Land mammals support and move their body using their musculoskeletal system. Their musculature usually presents varying adaptations with body mass or mode of locomotion. Rhinocerotidae is an interesting clade in this regard, as they are heavy animals potentially reaching three tons but are still capable of adopting a galloping gait. However, their musculature has been poorly studied. Here we report the dissection of both forelimb and hindlimb of one neonate and one adult each for two species of rhinoceroses, the Indian rhinoceros (Rhinoceros unicornis) and the white rhinoceros (Ceratotherium simum). We show that their muscular organisation is similar to that of their relatives, equids and tapirs, and that few evolutionary convergences with other heavy mammals (e.g. elephants and hippopotamuses) are present. Nevertheless, they show clear adaptations to their large body mass, such as more distal insertions for the protractor and adductor muscles of the limbs, giving them longer lever arms. The quantitative architecture of rhino muscles is again reminiscent of that of horses and tapirs, although contrary to horses, the forelimb is much stronger than the hindlimb, which is likely due to its great role in body mass support. Muscles involved mainly in counteracting gravity (e.g. serratus ventralis thoracis, infraspinatus, gastrocnemius, flexores digitorum) are usually highly pennate with short fascicles facilitating strong joint extension. Muscles involved in propulsion (e.g. gluteal muscles, gluteobiceps, quadriceps femoris) seem to represent a compromise between a high maximal isometric force and long fascicles, allowing a reasonably fast and wide working range. Neonates present higher normalized maximal isometric force than the adults for almost every muscle, except sometimes for the extensor and propulsor muscles, which presumably acquire their great force-generating capacity during the growth of the animal. Our study clarifies the way the muscles of animals of cursorial ancestry can adapt to support a greater body mass and calls for further investigations in other clades of large body mass.

From Telson to Attack in Mantis Shrimp: Bridging Biomechanics and Behavior in Crustacean Contests

In the spirit of this symposium on the physical mechanisms of behavior, we review mantis shrimp ritualized fighting, from the telson to the attack, as an inspiring example of how the integration of biomechanics and behavioral research can yield a penetrating narrative for how animals accomplish important activities, including agonistic actions. Resolving conflicts with conspecifics over valuable resources is an essential task for animals, and this takes an unusual form in mantis shrimp due to their powerful raptorial appendages. Decades of field and laboratory research have provided key insights into the natural agonistic interactions of diverse mantis shrimp species, including how they use their raptorial weapons against one another in telson sparring matches over cavities. These insights provided the foundation for functional morphologists, biomechanists, and engineers to work through different levels of organization: from the kinematics of how the appendages move to the elastic mechanisms that power the strike, and down to the structure, composition, and material properties that transmit and protect against high-impact forces. Completing this narrative are studies on the defensive telson and how this structure is biomechanically matched to the weapon and the role it plays in ritualized fighting. The biomechanical understanding of the weapon and defense in mantis shrimp has, in turn, enabled a better understanding of whether mantis shrimp assess one another during contests and encouraged questions of evolutionary drivers on both the arsenal and behavior. Altogether, the body of research focused on mantis shrimp has presented perhaps the most comprehensive understanding of fighting, weapons, and defenses among crustaceans, from morphology and biomechanics to behavior and evolution. While this multi-level analysis of ritualized fighting in mantis shrimp is comprehensive, we implore the need to include additional levels of analysis to obtain a truly holistic understanding of this and other crustacean agonistic interactions. Specifically, both molting and environmental conditions are often missing from the narrative, yet they greatly affect crustacean weapons, defenses, and behavior. Applying this approach more broadly would generate a similarly profound understanding of how crustaceans carry out a variety of important tasks in diverse habitats.

Rules of nature's Formula Run: Muscle mechanics during late stance is the key to explaining maximum running speed

The maximum running speed of legged animals is one evident factor for evolutionary selection-for predators and prey. Therefore, it has been studied across the entire size range of animals, from the smallest mites to the largest elephants, and even beyond to extinct dinosaurs. A recent analysis of the relation between animal mass (size) and maximum running speed showed that there seems to be an optimal range of body masses in which the highest terrestrial running speeds occur. However, the conclusion drawn from that analysis-namely, that maximum speed is limited by the fatigue of white muscle fibres in the acceleration of the body mass to some theoretically possible maximum speed-was based on coarse reasoning on metabolic grounds, which neglected important biomechanical factors and basic muscle-metabolic parameters. Here, we propose a generic biomechanical model to investigate the allometry of the maximum speed of legged running. The model incorporates biomechanically important concepts: the ground reaction force being counteracted by air drag, the leg with its gearing of both a muscle into a leg length change and the muscle into the ground reaction force, as well as the maximum muscle contraction velocity, which includes muscle-tendon dynamics, and the muscle inertia-with all of them scaling with body mass. Put together, these concepts' characteristics and their interactions provide a mechanistic explanation for the allometry of maximum legged running speed. This accompanies the offering of an explanation for the empirically found, overall maximum in speed: In animals bigger than a cheetah or pronghorn, the time that any leg-extending muscle needs to settle, starting from being isometric at about midstance, at the concentric contraction speed required for running at highest speeds becomes too long to be attainable within the time period of a leg moving from midstance to lift-off. Based on our biomechanical model, we, thus, suggest considering the overall speed maximum to indicate muscle inertia being functionally significant in animal locomotion. Furthermore, the model renders possible insights into biological design principles such as differences in the leg concept between cats and spiders, and the relevance of multi-leg (mammals: four, insects: six, spiders: eight) body designs and emerging gaits. Moreover, we expose a completely new consideration regarding the muscles' metabolic energy consumption, both during acceleration to maximum speed and in steady-state locomotion.

Bigger is not always better: Viability selection on body mass varies across life stages in a hibernating mammal

Body mass is often viewed as a proxy of past access to resources and of future survival and reproductive success. Links between body mass and survival or reproduction are, however, likely to differ between age classes and sexes. Remarkably, this is rarely taken into account in selection analyses. Selection on body mass is likely to be the primary target accounting for juvenile survival until reproduction but may weaken after recruitment. Males and females also often differ in how they use resources for reproduction and survival. Using a long-term study on body mass and annual survival in yellow-bellied marmots (Marmota flaviventer), we show that body mass was under stabilizing viability selection in the first years of life, before recruitment, which changed to positive directional selection as age increased and animals matured. We found no evidence that viability selection across age classes on body mass differed between sexes. By investigating the link between running speed and body mass, we show that the capacity to escape predators was not consistent across age classes and followed a quadratic relationship at young ages only. Overall, our results indicate that mature age classes exhibit traditional patterns of positive viability selection on body mass, as expected in a hibernating mammal, but that mass in the first years of life is subjected to stabilizing selection which may come from additional predation pressures that negate the benefits of the largest body masses. Our study highlights the importance to disentangle selection pressures on traits across critical age (or life) classes.

Why aren't rabbits and hares larger?

Macroevolutionary consequences of competition among large clades have long been sought in patterns of lineage diversification. However, mechanistically clear examples of such effects remain elusive. Here, we postulated that the limited phenotypic diversity and insular gigantism in lagomorphs could be explained at least in part by an evolutionary constraint placed on them by potentially competing ungulate-type herbivores (UTHs). Our analyses yielded three independent lines of evidence supporting this hypothesis: (1) the minimum UTH body mass is the most influential predictor of the maximum lagomorph body mass in modern ecoregions; (2) the scaling patterns of local-population energy use suggest universal competitive disadvantage of lagomorphs weighing over approximately 6.3 kg against artiodactyls, closely matching their observed upper size limit in continental settings; and (3) the trajectory of maximum lagomorph body mass in North America from the late Eocene to the Pleistocene (37.5-1.5 million years ago) was best modeled by the body mass ceiling placed by the smallest contemporary perissodactyl or artiodactyl. Body size evolution in lagomorphs has likely been regulated by the forces of competition within the clade, increased predation in open habitats, and importantly, competition from other ungulate-type herbivores. Our findings suggest conditionally-coupled dynamics of phenotypic boundaries among multiple clades within an adaptive zone, and highlight the synergy of biotic and abiotic drivers of diversity.

Understanding the limits to the hydraulic leg mechanism: the effects of speed and size on limb kinematics in vagrant arachnids

Among invertebrates, spiders (order Araneae) may be unique in their relationship between speed and mass as they use a combination of direct muscular contractions to flex their appendages, and internally controlled hydraulic pressure to extend them. To explore this, we measured maximal running speeds in 128 individual lycosids and sparassids, which varied in mass between 0.0054 and 3.01 g. We show maximum speed scaled with M^0.353, while mean running speed scaled much lower as M^0.197. We show no strong limitation of the hydraulic mechanism, with leg extension speed being equal to or greater than leg flexion speed. The reduction in leg flexion speed, only apparent in the distal most joint of the limb, might be a result of the requirement for flexor muscles to act against the hydraulic system. We explored the role of the limbs and found an alternating pattern of joint use among limbs, which may represent a strategy to avoid interference with adjacent limbs during running. Furthermore, we observed a reduced movement speed (increased leg dragging) in the rearward facing fourth limb with size. This may be linked to the increased size of the abdomen in larger spiders and may suggest a speed limitation in larger individuals.

How fast do mobile organisms respond to stimuli? Response times from bacteria to elephants and whales

Quick responses to fast changes in the environment are crucial in animal behaviour and survival, for example to seize prey, escape predators, or negotiate obstacles. Here, we study the 'simple response time' that is the time elapsed between receptor stimulation and motor activation as typically shown in escape responses, for mobile organisms of various taxa ranging from bacteria to large vertebrates. We show that 95% of these simple response times lie within one order of magnitude of the overall geometric mean of about 25 ms, which is similar to that of a well-studied sensory time scale, the inverse of the critical flicker fusion frequency in vision, also lying within close bounds for all the organisms studied. We find that this time scale is a few times smaller than the minimum time to move by one body length, which is known to lie also within a relatively narrow range for all moving organisms. The remarkably small 10²-fold range of the simple response time among so disparate life forms varying over 10²⁰-fold in body mass suggests that it is determined by basic physicochemical constraints, independently on the structure and scale of the organism. We thus propose first-principle estimates of the simple response and sensory time scales in terms of physical constants and a few basic biological properties common to mobile organisms and constraining their responses.

Functional diversity of small-mammal postcrania is linked to both substrate preference and body size

Selective pressures favor morphologies that are adapted to distinct ecologies, resulting in trait partitioning among ecomorphotypes. However, the effects of these selective pressures vary across taxa, especially because morphology is also influenced by factors such as phylogeny, body size, and functional trade-offs. In this study, we examine how these factors impact functional diversification in mammals. It has been proposed that trait partitioning among mammalian ecomorphotypes is less pronounced at small body sizes due to biomechanical, energetic, and environmental factors that favor a “generalist” body plan, whereas larger taxa exhibit more substantial functional adaptations. We title this the Divergence Hypothesis (DH) because it predicts greater morphological divergence among ecomorphotypes at larger body sizes. We test DH by using phylogenetic comparative methods to examine the postcranial skeletons of 129 species of taxonomically diverse, small-to-medium-sized (<15 kg) mammals, which we categorize as either “tree-dwellers” or “ground-dwellers.” In some analyses, the morphologies of ground-dwellers and tree-dwellers suggest greater between-group differentiation at larger sizes, providing some evidence for DH. However, this trend is neither particularly strong nor supported by all analyses. Instead, a more pronounced pattern emerges that is distinct from the predictions of DH: within-group phenotypic disparity increases with body size in both ground-dwellers and tree-dwellers, driven by morphological outliers among “medium”-sized mammals. Thus, evolutionary increases in body size are more closely linked to increases in within-locomotor-group disparity than to increases in between-group disparity. We discuss biomechanical and ecological factors that may drive these evolutionary patterns, and we emphasize the significant evolutionary influences of ecology and body size on phenotypic diversity.

Rethinking trophic niches: Speed and body mass colimit prey space of mammalian predators

Realized trophic niches of predators are often characterized along a one-dimensional range in predator-prey body mass ratios. This prey range is constrained by an "energy limit" and a "subdue limit" toward small and large prey, respectively. Besides these body mass ratios, maximum speed is an additional key component in most predator-prey interactions.Here, we extend the concept of a one-dimensional prey range to a two-dimensional prey space by incorporating a hump-shaped speed-body mass relation. This new "speed limit" additionally constrains trophic niches of predators toward fast prey.To test this concept of two-dimensional prey spaces for different hunting strategies (pursuit, group, and ambush predation), we synthesized data on 63 terrestrial mammalian predator-prey interactions, their body masses, and maximum speeds.We found that pursuit predators hunt smaller and slower prey, whereas group hunters focus on larger but mostly slower prey and ambushers are more flexible. Group hunters and ambushers have evolved different strategies to occupy a similar trophic niche that avoids competition with pursuit predators. Moreover, our concept suggests energetic optima of these hunting strategies along a body mass axis and thereby provides mechanistic explanations for why there are no small group hunters (referred to as "micro-lions") or mega-carnivores (referred to as "mega-cheetahs").Our results demonstrate that advancing the concept of prey ranges to prey spaces by adding the new dimension of speed will foster a new and mechanistic understanding of predator trophic niches and improve our predictions of predator-prey interactions, food web structure, and ecosystem functions.

Modeling escape success in terrestrial predator–prey interactions

Prey species often modify their foraging and reproductive behaviors to avoid encounters with predators; yet once they are detected, survival depends on out-running, out-maneuvering, or fighting off the predator. Though predation attempts involve at least two individuals—namely, a predator and its prey—studies of escape performance typically measure a single trait (e.g., sprint speed) in the prey species only. Here, we develop a theoretical model in which the likelihood of escape is determined by the prey animal’s tactics (i.e., path trajectory) and its acceleration, top speed, agility, and deceleration relative to the performance capabilities of a predator. The model shows that acceleration, top speed, and agility are all important determinants of escape performance, and because speed and agility are biomechanically related to size, smaller prey with higher agility should force larger predators to run along curved paths that do not allow them to use their superior speeds. Our simulations provide clear predictions for the path and speed a prey animal should choose when escaping from predators of different sizes (thus, biomechanical constraints) and could be used to explore the dynamics between predators and prey.

The fast and the frugal: Divergent locomotory strategies drive limb lengthening in theropod dinosaurs

Limb length, cursoriality and speed have long been areas of significant interest in theropod paleobiology, since locomotory capacity, especially running ability, is critical in the pursuit of prey and to avoid becoming prey. The impact of allometry on running ability, and the limiting effect of large body size, are aspects that are traditionally overlooked. Since several different non-avian theropod lineages have each independently evolved body sizes greater than any known terrestrial carnivorous mammal, ~1000kg or more, the effect that such large mass has on movement ability and energetics is an area with significant implications for Mesozoic paleoecology. Here, using expansive datasets that incorporate several different metrics to estimate body size, limb length and running speed, we calculate the effects of allometry on running ability. We test traditional metrics used to evaluate cursoriality in non-avian theropods such as distal limb length, relative hindlimb length, and compare the energetic cost savings of relative hindlimb elongation between members of the Tyrannosauridae and more basal megacarnivores such as Allosauroidea or Ceratosauridae. We find that once the limiting effects of body size increase is incorporated there is no significant correlation to top speed between any of the commonly used metrics, including the newly suggested distal limb index (Tibia + Metatarsus/ Femur length). The data also shows a significant split between large and small bodied theropods in terms of maximizing running potential suggesting two distinct strategies for promoting limb elongation based on the organisms’ size. For small and medium sized theropods increased leg length seems to correlate with a desire to increase top speed while amongst larger taxa it corresponds more closely to energetic efficiency and reducing foraging costs. We also find, using 3D volumetric mass estimates, that the Tyrannosauridae show significant cost of transport savings compared to more basal clades, indicating reduced energy expenditures during foraging and likely reduced need for hunting forays. This suggests that amongst theropods, hindlimb evolution was not dictated by one particular strategy. Amongst smaller bodied taxa the competing pressures of being both a predator and a prey item dominant while larger ones, freed from predation pressure, seek to maximize foraging ability. We also discuss the implications both for interactions amongst specific clades and Mesozoic paleobiology and paleoecological reconstructions as a whole.

Can skeletal surface area predict in vivo foot surface area?

The surface area of feet in contact with the ground is a key morphological feature that influences animal locomotion. Underfoot pressures (and consequently stresses experienced by the foot), as well as stability of an animal during locomotion, depend on the size and shape of this area. Here we tested whether the area of a skeletal foot could predict in vivo soft tissue foot surface area. Computed tomography scans of 29 extant tetrapods (covering mammals, reptiles, birds and amphibians) were used to produce models of both the soft tissues and the bones of their feet. Soft tissue models were oriented to a horizontal plane, and their outlines projected onto a surface to produce two-dimensional silhouettes. Silhouettes of skeletal models were generated either from bones in CT pose or with all autopodial bones aligned to the horizontal plane. Areas of these projections were calculated using alpha shapes (mathematical tight-fitting outline). Underfoot area of soft tissue was approximately 1.67 times that of skeletal tissue area (~ 2 times for manus, ~ 1.6 times for pes, if analysed separately). This relationship between skeletal foot area and soft tissue area, while variable in some of our study taxa, could provide information about the size of the organisms responsible for fossil trackways, suggest what size of tracks might be expected from potential trackmakers known only from skeletal remains, and aid in soft tissue reconstruction of skeletal remains for biomechanical modelling.

Divergent evolution of terrestrial locomotor abilities in extant Crocodylia

Extant Crocodylia are exceptional because they employ almost the full range of quadrupedal footfall patterns ("gaits") used by mammals; including asymmetrical gaits such as galloping and bounding. Perhaps this capacity evolved in stem Crocodylomorpha, during the Triassic when taxa were smaller, terrestrial, and long-legged. However, confusion about which Crocodylia use asymmetrical gaits and why persists, impeding reconstructions of locomotor evolution. Our experimental gait analysis of locomotor kinematics across 42 individuals from 15 species of Crocodylia obtained 184 data points for a wide velocity range (0.15-4.35 ms-1). Our results suggest either that asymmetrical gaits are ancestral for Crocodylia and lost in the alligator lineage, or that asymmetrical gaits evolved within Crocodylia at the base of the crocodile line. Regardless, we recorded usage of asymmetrical gaits in 7 species of Crocodyloidea (crocodiles); including novel documentation of these behaviours in 5 species (3 critically endangered). Larger Crocodylia use relatively less extreme gait kinematics consistent with steeply decreasing athletic ability with size. We found differences between asymmetrical and symmetrical gaits in Crocodylia: asymmetrical gaits involved greater size-normalized stride frequencies and smaller duty factors (relative ground contact times), consistent with increased mechanical demands. Remarkably, these gaits did not differ in maximal velocities obtained: whether in Alligatoroidea or Crocodyloidea, trotting or bounding achieved similar velocities, revealing that the alligator lineage is capable of hitherto unappreciated extreme locomotor performance despite a lack of asymmetrical gait usage. Hence asymmetrical gaits have benefits other than velocity capacity that explain their prevalence in Crocodyloidea and absence in Alligatoroidea-and their broader evolution.

Quantifying koala locomotion strategies: implications for the evolution of arborealism in marsupials

The morphology and locomotor performance of a species can determine their inherent fitness within a habitat type. Koalas have an unusual morphology for marsupials, with several key adaptations suggested to increase stability in arboreal environments. We quantified the kinematics of their movement over ground and along narrow arboreal trackways to determine the extent to which their locomotion resembled that of primates, occupying similar niches, or basal marsupials from which they evolved. On the ground, the locomotion of koalas resembled a combination of marsupial behaviours and primate-like mechanics. For example, their fastest strides were bounding type gaits with a top speed of 2.78 m s^-1 (mean 1.20 m s^-1), resembling marsupials, while the relatively longer stride length was reflective of primate locomotion. Speed was increased using equal modification of stride length and frequency. On narrow substrates, koalas took longer but slower strides (mean 0.42 m s^-1), adopting diagonally coupled gaits including both lateral and diagonal sequence gaits, the latter being a strategy distinctive among arboreal primates. The use of diagonally coupled gaits in the arboreal environment is likely only possible because of the unique gripping hand morphology of both the fore and hind feet of koalas. These results suggest that during ground locomotion, they use marsupial-like strategies but alternate to primate-like strategies when moving amongst branches, maximising stability in these environments. The locomotion strategies of koalas provide key insights into an independent evolutionary branch for an arboreal specialist, highlighting how locomotor strategies can convergently evolve between distant lineages.

Terrestrial locomotion of the Svalbard rock ptarmigan: comparing field and laboratory treadmill studies

Research into the terrestrial locomotion of birds is often based upon laboratory treadmill experiments. However, it is unclear how transposable these results are for birds moving in the wild. Here, using video recordings, we compared the kinematics of locomotion (stride frequency, stride length, stance phase, swing phase, duty factor) and speed range of Svalbard rock ptarmigan (Lagopus muta hyperborea) under field and laboratory treadmill conditions. Our findings indicate that the kinematics of walking and aerial running are conserved when moving on the treadmill and in the field. Differences, however, were found when grounded running under the two conditions, linked to substrate. Substrate effects were confirmed by analysing trials only moving over very hard snow. In line with laboratory treadmill energetic predictions, wild ptarmigan have a preferred speed during walking and to a lesser extent when aerial running but not when moving with a grounded running gait. The birds were also capable of a higher top speed in the field than that observed during treadmill studies. Our findings demonstrate that laboratory treadmill research provides meaningful information relevant to wild birds while highlighting the importance of understanding the substrate the animals are moving over.