A field study of the effects of incline on the escape locomotion of a bipedal lizard, Callisaurus draconoides

Ecomechanics and the Rules of Life: a Critical Conduit Between the Physical and Natural Sciences

Nature provides the parameters, or boundaries, within which organisms must cope in order to survive. Therefore, ecological conditions have an unequivocal influence on the ability of organisms to perform the necessary functions for survival. Biomechanics brings together physics and biology to understand how an organism will function under a suite of conditions. Despite a relatively rich recent history linking physiology and morphology with ecology, less attention has been paid to the linkage between biomechanics and ecology. This linkage, however, could provide key insights into patterns and processes of evolution. Ecomechanics, also known as ecological biomechanics or mechanical ecology, is not necessarily new, but has received far less attention than ecophysiology or ecomorphology. Here, we briefly review the history of ecomechanics, and then identify what we believe are grand challenges for the discipline and how they can inform some of the most pressing questions in science today, such as how organisms will cope with global change.

The spring-mass model and other reductionist models of bipedal locomotion on inclines

The spring-mass model is a model of locomotion aimed at giving the essential mathematical laws of the trajectory of the center of mass of an animal during bouncing gaits, such as hopping (one-dimensional) and running (two-dimensional). This reductionist mechanical system has been extensively investigated for locomotion over horizontal surfaces, whereas it has been largely neglected on other ecologically relevant surfaces, including inclines. For example, how the degree of inclination impacts the dynamics of the center of mass of the spring-mass model has not been investigated thoroughly. In this work, we derive a mathematical model which extends the spring-mass model to inclined surfaces. Among our results, we derive an approximate solution of the system, assuming a small angular sweep of the limb and a small spring compression during stance, and show that this approximation is very accurate, especially for small inclinations of the ground. Furthermore, we derive theoretical bounds on the difference between the Lagrangian and Lagrange equations of the true and approximate system, and discuss locomotor stability questions of the approximate solutions. We test our models through a sensitivity analysis using parameters relevant to the locomotion of bipedal animals (quail, pheasant, guinea fowl, turkey, ostrich, and humans) and compare our approximate solution to the numerically derived solution of the exact system. We compare the two-dimensional spring-mass model on inclines with the one-dimensional spring-mass model to which it reduces under the limit of no horizontal velocity; we compare the two-dimensional spring-mass model on inclines with the inverted-pendulum model on inclines towards which it converges in the case of high stiffness-to-mass ratio. We include comparisons with historically prevalent no-gravity approximations of these models, as well. The insights we have gleaned through all these comparisons and the ability of our approximation to replicate some of the kinematic changes observed in animals moving on different inclines (e.g. reduction in vertical oscillation of the center of mass and decreased stride length) underlines the valuable and reasonable contributions that very simple, reductionist models, like the spring-mass model, can provide.

A bio-inspired robotic climbing robot to understand kinematic and morphological determinants for an optimal climbing gait

Robotic systems for complex tasks, such as search and rescue or exploration, are limited for wheeled designs, thus the study of legged locomotion for robotic applications has become increasingly important. To successfully navigate in regions with rough terrain, a robot must not only be able to negotiate obstacles, but also climb steep inclines. Following the principles of biomimetics, we developed a modular bio-inspired climbing robot, named X4, which mimics the lizard's bauplan including an actuated spine, shoulders, and feet which interlock with the surface via claws. We included the ability to modify gait and hardware parameters and simultaneously collect data with the robot's sensors on climbed distance, slip occurrence and efficiency. We first explored the speed-stability trade-off and its interaction with limb swing phase dynamics, finding a sigmoidal pattern of limb movement resulted in the greatest distance travelled. By modifying foot orientation, we found two optima for both speed and stability, suggesting multiple stable configurations. We varied spine and limb range of motion, again showing two possible optimum configurations, and finally varied the centre of pro- and retraction on climbing performance, showing an advantage for protracted limbs during the stride. We then stacked optimal regions of performance and show that combining optimal dynamic patterns with either foot angles or ROM configurations have the greatest performance, but further optima stacking resulted in a decrease in performance, suggesting complex interactions between kinematic parameters. The search of optimal parameter configurations might not only be beneficial to improve robotic in-field operations but may also further the study of the locomotive evolution of climbing of animals, like lizards or insects.

Escape behaviour varies with distance from safe refuge

Locomotor performance and behaviour are important for escape from predators, yet the intersection of these strategies is poorly studied. Escape behaviour is context dependent, and optimal escape theory predicts that animals that are farther from a safe refuge will generally use faster running speeds but might choose to use more variable escape paths. We studied locomotor performance and behaviour of six-lined racerunner lizards (Aspidoscelis sexlineata) escaping on natural surface runways that were varied experimentally to be either 5 or 10 m from a safe refuge. On the 5 m runway, lizards usually escaped directly towards the refuge, attained a slower maximal running speed (3.2 m s−1) at ~3 m from the start, and reached the target refuge in most of the trials (80%). On the 10 m runway, lizards used more variable behaviour, including reversals and turns, attained a faster maximal running speed (3.7 m s−1) at ~6 m from the start, and reached the final refuge in only 43% of trials. Free-ranging racerunners were rarely > 5 m from their nearest refuge and used escape paths that were typically < 5 m. Our findings align with predictions from optimal escape theory, in that the perceived risk of a predator–prey encounter can drive adjustments in locomotor behaviour and performance. Additionally, we show that the escape behaviour of free-ranging lizards closely matches their escape behaviour and performance during controlled escape trials.

Geckos cling best to, and prefer to use, rough surfaces

BACKGROUND

Fitness is strongly related to locomotor performance, which can determine success in foraging, mating, and other critical activities. Locomotor performance on different substrates is likely to require different abilities, so we expect alignment between species' locomotor performance and the habitats they use in nature. In addition, we expect behaviour to enhance performance, such that animals will use substrates on which they perform well.

METHODS

We examined the associations between habitat selection and performance in three species of Oedura geckos, including two specialists, (one arboreal, and one saxicolous), and one generalist species, which used both rocks and trees. First, we described their microhabitat use in nature (tree and rock type) for these species, examined the surface roughnesses they encountered, and selected materials with comparable surface microtopographies (roughness measured as peak-to-valley heights) to use as substrates in lab experiments quantifying behavioural substrate preferences and clinging performance.

RESULTS

The three Oedura species occupied different ecological niches and used different microhabitats in nature, and the two specialist species used a narrower range of surface roughnesses compared to the generalist. In the lab, Oedura geckos preferred substrates (coarse sandpaper) with roughness characteristics similar to substrates they use in nature. Further, all three species exhibited greater clinging performance on preferred (coarse sandpaper) substrates, although the generalist used fine substrates in nature and had good performance capabilities on fine substrates as well.

CONCLUSION

We found a relationship between habitat use and performance, such that geckos selected microhabitats on which their performance was high. In addition, our findings highlight the extensive variation in surface roughnesses that occur in nature, both among and within microhabitats.

Functional and Environmental Constraints on Prey Capture Speed in a Lizard

Movement is an important component of animal behavior and determines how an organism interacts with its environment. The speed at which an animal moves through its environment can be constrained by internal (e.g., physiological state) and external factors (e.g., habitat complexity). When foraging, animals should move at speeds that maximize prey capture while minimizing mistakes (i.e., missing prey, slipping). We used experimental arenas containing obstacles spaced in different arrays to test how variation in habitat complexity influenced attack distance, prey capture speed, and foraging success in the Prairie Lizard. Obstacles spaced uniformly across arenas resulted in 15% slower prey capture speed and 30–38% shorter attack distance compared to arenas with no obstacles or with obstacles clustered in opposite corners of the arena. Prey capture probability was not influenced by arena type or capture speed, but declined with increasing attack distance. Similarly, the probability of prey consumption declined with attack distance across arena types. However, prey consumption probability declined with increasing prey capture speed in more open arenas but not in the cluttered arena. Foraging accuracy declined with increasing speed in more open arenas, and remained relatively constant when obstacles were in closer proximity. Foraging success was primarily constrained by intrinsic properties (speed-maneuverability tradeoff) when ample space was available, but environmental conditions had a greater impact on foraging success in “cluttered” habitats. This empirical test of theoretical predictions about optimal movement speeds in animals provides a step forward in understanding how animals select speeds in nature.

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 pelvis as an anatomical indicator for facultative bipedality and substrate use in lepidosaurs

Facultative bipedality is regarded as an enigmatic middle ground in the evolution of obligate bipedality and is associated with high mechanical demands in extant lepidosaurs. Traits linked with this phenomenon are largely associated with the caudal end of the animal: hindlimbs and tail. The articulation of the pelvis with both of these structures suggests a morphofunctional role in the use of a facultative locomotor mode. Using a three-dimensional geometric morphometric approach, we examine the pelvic osteology and associated functional implications for 34 species of extant lepidosaur. Anatomical trends associated with the use of a bipedal locomotor mode and substrate preferences are correlated and functionally interpreted based on musculoskeletal descriptions. Changes in pelvic osteology associated with a facultatively bipedal locomotor mode are similar to those observed in species preferring arboreal substrates, indicating shared functionality between these ecologies.

The Physiology of Exercise in Free-Living Vertebrates: What Can We Learn from Current Model Systems?

SYNOPSIS

Many behaviors crucial for survival and reproductive success in free-living animals, including migration, foraging, and escaping from predators, involve elevated levels of physical activity. However, although there has been considerable interest in the physiological and biomechanical mechanisms that underpin individual variation in exercise performance, to date, much work on the physiology of exercise has been conducted in laboratory settings that are often quite removed from the animal's ecology. Here we review current, laboratory-based model systems for exercise (wind or swim tunnels for migration studies in birds and fishes, manipulation of exercise associated with non-migratory activity in birds, locomotion in lizards, and wheel running in rodents) to identify common physiological markers of individual variation in exercise capacity and/or costs of increased activity. Secondly, we consider how physiological responses to exercise might be influenced by (1) the nature of the activity (i.e., voluntary or involuntary, intensity, and duration), and (2) resource acquisition and food availability, in the context of routine activities in free-living animals. Finally, we consider evidence that the physiological effects of experimentally-elevated activity directly affect components of fitness such as reproduction and survival. We suggest that developing more ecologically realistic laboratory systems, incorporating resource-acquisition, functional studies across multiple physiological systems, and a life-history framework, with reproduction and survival end-points, will help reveal the mechanisms underlying the consequences of exercise, and will complement studies in free-living animals taking advantage of new developments in wildlife-tracking.

Lizards ran bipedally 110 million years ago

Four heteropod lizard trackways discovered in the Hasandong Formation (Aptian-early Albian), South Korea assigned to Sauripes hadongensis, n. ichnogen., n. ichnosp., which represents the oldest lizard tracks in the world. Most tracks are pes tracks (N = 25) that are very small, average 22.29 mm long and 12.46 mm wide. The pes tracks show "typical" lizard morphology as having curved digit imprints that progressively increase in length from digits I to IV, a smaller digit V that is separated from the other digits by a large interdigital angle. The manus track is 19.18 mm long and 19.23 mm wide, and shows a different morphology from the pes. The predominant pes tracks, the long stride length of pes, narrow trackway width, digitigrade manus and pes prints, and anteriorly oriented long axis of the fourth pedal digit indicate that these trackways were made by lizards running bipedally, suggesting that bipedality was possible early in lizard evolution.

Terrestrial movement energetics: current knowledge and its application to the optimising animal

The energetic cost of locomotion can be a substantial proportion of an animal's daily energy budget and thus key to its ecology. Studies on myriad species have added to our knowledge about the general cost of animal movement, including the effects of variations in the environment such as terrain angle. However, further such studies might provide diminishing returns on the development of a deeper understanding of how animals trade-off the cost of movement with other energy costs, and other ecological currencies such as time. Here, I propose the ‘individual energy landscape’ as an approach to conceptualising the choices facing the optimising animal. In this Commentary, first I outline previous broad findings about animal walking and running locomotion, focusing in particular on the use of net cost of transport as a metric of comparison between species, and then considering the effects of environmental perturbations and other extrinsic factors on movement costs. I then introduce and explore the idea that these factors combine with the behaviour of the animal in seeking short-term optimality to create that animal's individual energy landscape – the result of the geographical landscape and environmental factors combined with the animal's selected trade-offs. Considering an animal's locomotion energy expenditure within this context enables hard-won empirical data on transport costs to be applied to questions about how an animal can and does move through its environment to maximise its fitness, and the relative importance, or otherwise, of locomotion energy economy.

Geckos decouple fore- and hind limb kinematics in response to changes in incline

BACKGROUND

Terrestrial animals regularly move up and down surfaces in their natural habitat, and the impacts of moving uphill on locomotion are commonly examined. However, if an animal goes up, it must go down. Many morphological features enhance locomotion on inclined surfaces, including adhesive systems among geckos. Despite this, it is not known whether the employment of the adhesive system results in altered locomotor kinematics due to the stereotyped motions that are necessary to engage and disengage the system. Using a generalist pad-bearing gecko, Chondrodactylus bibronii, we determined whether changes in slope impact body and limb kinematics.

RESULTS

Despite the change in demand, geckos did not change speed on any incline. This constant speed was achieved by adjusting stride frequency, step length and swing time. Hind limb, but not forelimb, kinematics were altered on steep downhill conditions, thus resulting in significant de-coupling of the limbs.

CONCLUSIONS

Unlike other animals on non-level conditions, the geckos in our study only minimally alter the movements of distal limb elements, which is likely due to the constraints associated with the need for rapid attachment and detachment of the adhesive system. This suggests that geckos may experience a trade-off between successful adhesion and the ability to respond dynamically to locomotor perturbations.

The effects of multiple obstacles on the locomotor behavior and performance of a terrestrial lizard

Negotiation of variable terrain is important for many small terrestrial vertebrates. Variation in the running surface due to obstacles (woody debris, vegetation, rocks) can alter escape paths and running performance. The ability to navigate obstacles likely influences survivorship via predator evasion success, and other key ecological tasks (finding mates, acquiring food). Earlier work established that running posture and sprint performance are altered when organisms face an obstacle, and yet studies involving multiple obstacles are limited. Indeed, some habitats are cluttered with obstacles, while others are not. For many species, obstacle density may be important in predator escape and/or colonization potential by conspecifics. This study examines how multiple obstacles influence running behavior and locomotor posture in lizards. We predict that an increasing number of obstacles will increase the frequency of pausing and decrease sprint velocity. Furthermore, bipedal running over multiple obstacles is predicted to maintain greater mean sprint velocity compared to quadrupedal running, thereby revealing a potential advantage of bipedalism. Lizards were filmed (300 fps) running through a racetrack with zero, one, or two obstacles. Bipedal running posture over one obstacle was significantly faster than quadrupedal posture. Bipedal running trials contained fewer total strides than quadrupedal ones. But as obstacle number increased, the number of bipedal strides decreased. Increasing obstacle number led to slower and more intermittent locomotion. Bipedalism provided clear advantages for one obstacle, but was not associated with further benefits on additional obstacles. Hence, bipedalism helps mitigate obstacle negotiation, but not when numerous obstacles are encountered in succession.

Introduction to the Symposium: Towards a General Framework for Predicting Animal Movement Speeds in Nature

Speed of movement is fundamental to animal behavior-defining the intensity of a task, the time needed to complete it, and the likelihood of success-but how does an animal decide how fast to move? Most studies of animal performance measure maximum capabilities, but animals rarely move at their maximum in the wild. It was the goal of our symposium to develop a conceptual framework to explore the choices of speed in nature. A major difference between our approach and previous work is our move toward understanding optimal rather than maximal speeds. In the following series of papers, we provide a starting point for future work on animal movement speeds, including a conceptual framework, a simple optimality model, an evolutionary context, and an exploration of the various biomechanical and energetic constraints on speed. By applying a cross-disciplinary approach to the study of the choice of speed-as we have done here-we can reveal much about the way animals use habitats, interact with conspecifics, avoid predators, obtain food, and negotiate human-modified landscapes.

The effect of substrate diameter and incline on locomotion in an arboreal frog

Frogs are characterized by a unique morphology associated with their saltatory lifestyle. Yet, arboreal species show morphological specializations relative to other ecological specialists allowing them to hold on to narrow substrates. However, almost nothing is known about the effects of substrate characteristics on locomotion in frogs. Here, we quantified the 3D kinematics of forelimb movement for frogs moving across branches of different diameters (1 and 40 mm) and two different inclines (horizontal and 45 deg uphill). Our results show that grip types differ while moving across substrates of different diameters and inclines. The kinematics of the wrist, elbow and shoulder as well as the body position relative to the substrate also showed significant effects of individual, diameter and incline. Kinematic differences involved duration, velocity of movement and angular excursions. Differences were most pronounced for the proximal joints of the forelimb and effects for substrate diameter were greater than for incline. Interestingly, the effects of diameter and incline on both grip type and kinematics are similar to what has been observed for lizards and primates, suggesting that the mechanics of narrow substrate locomotion drive the kinematics of movement independent of morphology and phylogeny.

Exercising for food: bringing the laboratory closer to nature

Traditionally, exercise physiology experiments have borne little resemblance to how animals express physical activity in the wild. In this experiment, 15 adult male rats were divided into three equal-sized groups: exercise contingent (CON), non-exercise contingent (NON) and sedentary (SED). The CON group was placed in a cage with a running wheel, where the acquisition of food was contingent upon the distance run. Every three days the distance required to run to maintain food intake at free feeding levels was increased by 90% in comparison to the previous 3 days. The NON group were housed identically to the CON group, but food acquisition was not dependent upon running in the wheel. Finally, the SED group were kept in small cages with no opportunity to perform exercise. A two-way ANOVA with repeated measures was used to determine significant differences in responses between the experimental phases and treatment groups and ANCOVA to analyse growth and tissue mass variables with body length and body mass used separately as covariates. A post hoc Tukey's test was used to indicate significant differences. A Pearson's correlation was used to test the relationship between the distance travelled by the animal and the distance/food ratio. The level of significance was set at p<0.05 for all tests. The CON group showed the hypothesized correlation between distance required to run to obtain food and their mean distance travelled (p<0.001), during 45 days in contingency phase. The CON group showed a decrease in body mass, rather than an increase as shown by NON and SED groups. The CON group had a significantly lower body temperature (p<0.05) and adiposity (p<0.05) when compared to the other two groups for the same body size. The present experimental model based on animals choosing the characteristics of their physical exercise to acquire food (i.e., distance travelled, speed and duration) clearly induced physiological effects (body characteristics and internal temperature), which are useful for investigating relevant topics in exercise physiology such as the link between exercise, food and body weight.

Energy landscapes shape animal movement ecology

The metabolic costs of animal movement have been studied extensively under laboratory conditions, although frequently these are a poor approximation of the costs of operating in the natural, heterogeneous environment. Construction of "energy landscapes," which relate animal locality to the cost of transport, can clarify whether, to what extent, and how movement properties are attributable to environmental heterogeneity. Although behavioral responses to aspects of the energy landscape are well documented in some fields (notably, the selection of tailwinds by aerial migrants) and scales (typically large), the principles of the energy landscape extend across habitat types and spatial scales. We provide a brief synthesis of the mechanisms by which environmentally driven changes in the cost of transport can modulate the behavioral ecology of animal movement in different media, develop example cost functions for movement in heterogeneous environments, present methods for visualizing these energy landscapes, and derive specific predictions of expected outcomes from individual- to population- and species-level processes. Animals modulate a suite of movement parameters (e.g., route, speed, timing of movement, and tortuosity) in relation to the energy landscape, with the nature of their response being related to the energy savings available. Overall, variation in movement costs influences the quality of habitat patches and causes nonrandom movement of individuals between them. This can provide spatial and/or temporal structure to a range of population- and species-level processes, ultimately including gene flow. Advances in animal-attached technology and geographic information systems are opening up new avenues for measuring and mapping energy landscapes that are likely to provide new insight into their influence in animal ecology.

How forelimb and hindlimb function changes with incline and perch diameter in the green anole, Anolis carolinensis

The range of inclines and perch diameters in arboreal habitats poses a number of functional challenges for locomotion. To effectively overcome these challenges, arboreal lizards execute complex locomotor behaviors involving both the forelimbs and the hindlimbs. However, few studies have examined the role of forelimbs in lizard locomotion. To characterize how the forelimbs and hindlimbs differentially respond to changes in substrate diameter and incline, we obtained three-dimensional high-speed video of green anoles (Anolis carolinensis) running on flat (9 cm wide) and narrow (1.3 cm) perches inclined at 0, 45 and 90 deg. Changes in perch diameter had a greater effect on kinematics than changes in incline, and proximal limb variables were primarily responsible for these kinematic changes. In addition, a number of joint angles exhibited greater excursions on the 45 deg incline compared with the other inclines. Anolis carolinensis adopted strategies to maintain stability similar to those of other arboreal vertebrates, increasing limb flexion, stride frequency and duty factor. However, the humerus and femur exhibited several opposite kinematic trends with changes in perch diameter. Further, the humerus exhibited a greater range of motion than the femur. A combination of anatomy and behavior resulted in differential kinematics between the forelimb and the hindlimb, and also a potential shift in the propulsive mechanism with changes in external demand. This suggests that a better understanding of single limb function comes from an assessment of both forelimbs and hindlimbs. Characterizing forelimb and hindlimb movements may reveal interesting functional differences between Anolis ecomorphs. Investigations into the physiological mechanisms underlying the functional differences between the forelimb and the hindlimb are needed to fully understand how arboreal animals move in complex habitats.

Perch diameter and branching patterns have interactive effects on the locomotion and path choice of anole lizards

Natural branches vary conspicuously in their diameter, density and orientation, but how these latter two factors affect animal locomotion is poorly understood. Thus, for three species of arboreal anole lizards found on different size branches and with different limb lengths, we tested sprinting performance on cylinders with five diameters (5–100 mm) and five patterns of pegs, which simulated different branch orientations and spacing. We also tested whether the lizards preferred surfaces that enhanced their performance. The overall responses to different surfaces were similar among the three species, although the magnitude of the effects differed. All species were faster on cylinders with larger diameter and no pegs along the top. The short-limbed species was the slowest on all surfaces. Much of the variation in performance resulted from variable amounts of pausing among different surfaces and species. Lizards preferred to run along the top of cylinders, but pegs along the top of the narrow cylinders interfered with this. Pegs on top of the 100-mm diameter cylinder, however, had little effect on speed as the lizards ran quite a straight path alongside pegs without bumping into them. All three species usually chose surfaces with greater diameters and fewer pegs, but very large diameters with pegs were preferred to much smaller diameter cylinders without pegs. Our results suggest that preferring larger diameters in natural vegetation has a direct benefit for speed and an added benefit of allowing detouring around branches with little adverse effect on speed.

Performance and three-dimensional kinematics of bipedal lizards during obstacle negotiation

Bipedal running is common among lizard species, but although the kinematics and performance of this gait have been well characterized, the advantages in biologically relevant situations are still unclear. Obstacle negotiation is a task that is ecologically relevant to many animals while moving at high speeds, such as during bipedal running, yet little is known about how obstacles impact locomotion and performance. We examined the effects of obstacle negotiation on the kinematics and performance of lizards during bipedal locomotion. We quantified three-dimensional kinematics from high-speed video (500 Hz) of six-lined racerunners (Aspidoscelis sexlineata) running on a 3 m racetrack both with and without an obstacle spanning the width of the track. The lizards did not alter their kinematics prior to contacting the obstacle. Although contact with the obstacle caused changes to the hindlimb kinematics, mean forward speed did not differ between treatments. The deviation of the vertical position of the body center of mass did not differ between treatments, suggesting that in the absence of a cost to overall performance, lizards forgo maintaining normal kinematics while negotiating obstacles in favor of a steady body center of mass height to avoid destabilizing locomotion.

Multi-functional foot use during running in the zebra-tailed lizard (Callisaurus draconoides)

A diversity of animals that run on solid, level, flat, non-slip surfaces appear to bounce on their legs; elastic elements in the limbs can store and return energy during each step. The mechanics and energetics of running in natural terrain, particularly on surfaces that can yield and flow under stress, is less understood. The zebra-tailed lizard (Callisaurus draconoides), a small desert generalist with a large, elongate, tendinous hind foot, runs rapidly across a variety of natural substrates. We use high speed video to obtain detailed three-dimensional running kinematics on solid and granular surfaces to reveal how leg, foot, and substrate mechanics contribute to its high locomotor performance. Running at ~10 body length/s (~1 m/s), the center of mass oscillates like a spring-mass system on both substrates, with only 15% reduction in stride length on the granular surface. On the solid surface, a strut-spring model of the hind limb reveals that the hind foot saves about 40% of the mechanical work needed per step, significant for the lizard's small size. On the granular surface, a penetration force model and hypothesized subsurface foot rotation indicates that the hind foot paddles through fluidized granular medium, and that the energy lost during irreversible deformation of the substrate does not differ from the reduction in the mechanical energy of the center of mass. The upper hind leg muscles must perform three times as much mechanical work on the granular surface as on the solid surface to compensate for the greater energy lost within the foot and to the substrate.

Environmental differences in substrate mechanics do not affect sprinting performance in sand lizards (Uma scoparia and Callisaurus draconoides)

Running performance depends on a mechanical interaction between the feet of an animal and the substrate. This interaction may differ between two species of sand lizard from the Mojave Desert that have different locomotor morphologies and habitat distributions. Uma scorparia possesses toe fringes and inhabits dunes, whereas the closely related Callisaurus draconoides lacks fringes and is found on dune and wash habitats. The present study evaluated whether these distribution patterns are related to differential locomotor performance on the fine sand of the dunes and the course sand of the wash habitat. We measured the kinematics of sprinting and characterized differences in grain size distribution and surface strength of the soil in both habitats. Although wash sand had a surface strength (15.4±6.2 kPa) that was more than three times that of dune sand (4.7±2.1 kPa), both species ran with similar sprinting performance on the two types of soil. The broadly distributed C. draconoides ran with a slightly (22%) faster maximum speed (2.2±0.2 m s–1) than the dune-dwelling U. scorparia (1.8±0.2 m s–1) on dune sand, but not on wash sand. Furthermore, there were no significant differences in maximum acceleration or the time to attain maximum speed between species or between substrates. These results suggest that differences in habitat distribution between these species are not related to locomotor performance and that sprinting ability is dominated neither by environmental differences in substrate nor the presence of toe fringes.

Why go bipedal? Locomotion and morphology in Australian agamid lizards

Bipedal locomotion by lizards has previously been considered to provide a locomotory advantage. We examined this premise for a group of quadrupedal Australian agamid lizards, which vary in the extent to which they will become bipedal. The percentage of strides that each species ran bipedally, recorded using high speed video cameras, was positively related to body size and the proximity of the body centre of mass to the hip, and negatively related to running endurance. Speed was not higher for bipedal strides, compared with quadrupedal strides, in any of the four species, but acceleration during bipedal strides was significantly higher in three of four species. Furthermore, a distinct threshold between quadrupedal and bipedal strides, was more evident for acceleration than speed, with a threshold in acceleration above which strides became bipedal. We calculated these thresholds using probit analysis, and compared these to the predicted threshold based on the model of Aerts et al. Although there was a general agreement in order, the acceleration thresholds for lizards were often lower than that predicted by the model. We suggest that bipedalism, in Australian agamid lizards, may have evolved as a simple consequence of acceleration, and does not confer any locomotory advantage for increasing speed or endurance. However, both behavioural and threshold data suggest that some lizards actively attempt to run bipedally, implying some unknown advantage to bipedal locomotion.

Posture, gait and the ecological relevance of locomotor costs and energy-saving mechanisms in tetrapods

A reanalysis of locomotor data from functional, energetic, mechanical and ecological perspectives reveals that limb posture has major effects on limb biomechanics, energy-saving mechanisms and the costs of locomotion. Regressions of data coded by posture (crouched vs. erect) reveal nonlinear patterns in metabolic cost, limb muscle mass, effective mechanical advantage, and stride characteristics. In small crouched animals energy savings from spring and pendular mechanisms are inconsequential and thus the metabolic cost of locomotion is driven by muscle activation costs. Stride frequency appears to be the principal functional parameter related to the decreasing cost of locomotion in crouched animals. By contrast, the shift to erect limb postures invoked a series of correlated effects on the metabolic cost of locomotion: effective mechanical advantage increases, relative muscle masses decrease, metapodial limb segments elongate dramatically (as limbs shift from digitigrade to unguligrade designs) and biological springs increase in size and effectiveness. Each of these factors leads to decreases in the metabolic cost of locomotion in erect forms resulting from real and increasing contributions of pendular savings and spring savings. Comparisons of the relative costs and ecological relevance of different gaits reveal that running is cheaper than walking in smaller animals up to the size of dogs but running is more expensive than walking in horses. Animals do not necessarily use their cheapest gaits for their predominant locomotor activity. Therefore, locomotor costs are driven more by ecological relevance than by the need to optimize locomotor economy.