Rock-dwelling lizards exhibit less sensitivity of sprint speed to increases in substrate rugosity

Kinematics and coordination of moth flies walking on smooth and rough surfaces

The moth fly, Clogmia albipunctata, is a common synanthropic insect with a worldwide range that lives in nearly any area with moist, decaying organic matter. These habitats comprise both smooth, slippery substrates (e.g., bathroom drains) and heterogeneous, bumpy ground (e.g., soil in plant pots). By using terrain of varying levels of roughness, we focus specifically on how substrate roughness at the approximate size scale of the organism affects kinematics and coordination in adult moth flies. Finally, we compare and contrast our characterizations of locomotion in C. albipunctata with previous work of insect walking in naturalistic environments.

Locomotor kinematics on sand versus vinyl flooring in the sidewinder rattlesnake Crotalus cerastes

For terrestrial locomotion of animals and machines, physical characteristics of the substrate can strongly impact kinematics and performance. Snakes are an especially interesting system for studying substrate effects because their gait depends more on the environment than on their speed. We tested sidewinder rattlesnakes (Crotalus cerastes) on two surfaces: sand collected from their natural environment and vinyl tile flooring, an artificial surface often used to elicit sidewinding in laboratory settings. Of ten kinematic variables examined, two differed significantly between the substrates: the body's waveform had an average of ∼17% longer wavelength on vinyl flooring (measured in body lengths), and snakes lifted their bodies an average of ∼40% higher on sand (measured in body lengths). Sidewinding may also differ among substrates in ways we did not measure (e.g. ground reaction forces and energetics), leaving open clear directions for future study.

The bumpy road ahead: the role of substrate roughness on animal walking and a proposed comparative metric

Outside laboratory conditions and human-made structures, animals rarely encounter flat surfaces. Instead, natural substrates are uneven surfaces with height variation that ranges from the microscopic scale to the macroscopic scale. For walking animals (which we define as encompassing any form of legged movement across the ground, such as walking, running, galloping, etc.), such substrate 'roughness' influences locomotion in a multitude of ways across scales, from roughness that influences how each toe or foot contacts the ground, to larger obstacles that animals must move over or navigate around. Historically, the unpredictability and variability of natural environments has limited the ability to collect data on animal walking biomechanics. However, recent technical advances, such as more sensitive and portable cameras, biologgers, laboratory tools to fabricate rough terrain, as well as the ability to efficiently store and analyze large variable datasets, have expanded the opportunity to study how animals move under naturalistic conditions. As more researchers endeavor to assess walking over rough terrain, we lack a consistent approach to quantifying roughness and contextualizing these findings. This Review summarizes existing literature that examines non-human animals walking on rough terrain and presents a metric for characterizing the relative substrate roughness compared with animal size. This framework can be applied across terrain and body scales, facilitating direct comparisons of walking over rough surfaces in animals ranging in size from ants to elephants.

Phylogenomic data resolve the historical biogeography and ecomorphs of Neotropical forest lizards (Squamata, Diploglossidae)

Few studies have been conducted on the biogeography and phylogenetic relationships of Neotropical forest lizards (Diploglossidae) because of incomplete taxon sampling, conflicting datasets, and low statistical support at phylogenetic nodes. Here, we enhance a recent nine-gene dataset with a genomic dataset of 3,232 loci and 642,775 aligned base pairs. The resulting phylogeny includes 30 diploglossid species, 10 of the 11 genera, and the three subfamilies. It shows significant support for all supra-specific taxa in either maximum likelihood or Bayesian analyses or both. With this well-supported phylogeny, we further investigate the historical biogeography of the group and how diploglossids reached the Caribbean islands. Our analyses indicate that Antillean diploglossid lizards originated from at least two overwater dispersals from South America. Our tests for the strength of convergent evolution between morphologically similar taxa support the recognition of a soil and a tree ecomorph. In addition, we propose grass, ground, rock, and swamp ecomorphs for species in this family based on ecological and morphological data and analyses.

A terrain treadmill to study animal locomotion through large obstacles

A challenge to understanding locomotion in complex 3-D terrain with large obstacles is to create tools for controlled, systematic experiments. Recent terrain arenas allow observations at small spatiotemporal scales (∼10 body length or cycles). Here, we create a terrain treadmill to enable high-resolution observation of animal locomotion through large obstacles over large spatiotemporal scales. An animal moves through modular obstacles on an inner sphere, while a rigidly-attached, concentric, transparent outer sphere rotates with the opposite velocity via closed-loop feedback to keep the animal atop. During sustained locomotion, a discoid cockroach moved through pillar obstacles for up to 25 minutes (2500 cycles) over 67 m (1500 body lengths). Over 12 trials totaling∼1 hour, the animal was maintained within a radius of 1 body length (4.5 cm) on top of the sphere 90% of the time. The high-resolution observation enables study of diverse locomotor behaviors and quantification of animal-obstacle interaction.

Performance and Kinematic Differences Between Terrestrial and Aquatic Running in Anolis Sagrei

Many animals frequently transition between different media while navigating their heterogeneous environments. These media vary in compliance, moisture content, and other characteristics that affect their physical properties. As a result, animals may need to alter their kinematics to adapt to potential changes in media while maintaining performance during predator escape and foraging. Due to its fluid nature, water is highly compliant, and although usually associated with swimming, water running has evolved in a variety of animals ranging from insects to mammals. While the best studied large water runners are the bipedal basilisk lizards (Basiliscus spp.), other lizards have also been observed to run across the surface of water, namely Hemidactylus platyurus, a house gecko, and in this study, Anolis sagrei, the brown anole. Unlike the basilisk lizard, the primarily arboreal Anolis sagrei is not adapted for water running. Moreover, water running in A. sagrei, similar to that of the house gecko, was primarily quadrupedal. Here, we tested for performance and kinematic differences between aquatic and terrestrial running and if the variance in performance and kinematic variables differed between the two media. We found no difference in average and maximum velocity between running on land and water. We also found that Anolis sagrei had higher hindlimb stride frequencies, decreased duty factor, and shorter stride lengths on water, as well as more erect postures. Finally, we found that most kinematics did not differ in variance between the two media, but of those that were different, almost all were more variable during terrestrial running. Our findings show that animals may be capable of specialized modes of locomotion, even if they are not obviously adapted for them, and that they may do this by modulating their kinematics to facilitate locomotion through novel environments.

How head shape and substrate particle size affect fossorial locomotion in lizards

Granular substrates ranging from silt to gravel cover much of the Earth's land area, providing an important habitat for fossorial animals. Many of these animals use their heads to penetrate the substrate. Although there is considerable variation in head shape, how head shape affects fossorial locomotor performance in different granular substrates is poorly understood. Here, head shape variation for 152 species of fossorial lizards was quantified for head diameter, slope and pointiness of the snout. The force needed to penetrate different substrates was measured using 28 physical models spanning this evolved variation. Ten substrates were considered, ranging in particle size from 0.025 to 4 mm in diameter and consisting of spherical or angular particles. Head shape evolved in a weakly correlated manner, with snouts that were gently sloped being blunter. There were also significant clade differences in head shape among fossorial lizards. Experiments with physical models showed that as head diameter increased, absolute penetration force increased but force normalized by cross-sectional area decreased. Penetration force decreased for snouts that tapered more gradually and were pointier. Larger and angular particles required higher penetration forces, although intermediate size spherical particles, consistent with coarse sand, required the lowest force. Particle size and head diameter effect were largest, indicating that fossorial burrowers should evolve narrow heads and bodies, and select relatively fine particles. However, variation in evolved head shapes and recorded penetration forces suggests that kinematics of fossorial movement are likely an important factor in explaining evolved diversity.

Side-impact collision: mechanics of obstacle negotiation in sidewinding snakes

Snakes excel at moving through cluttered environments, and heterogeneities can be used as propulsive contacts for snakes performing lateral undulation. However, sidewinding, which is often associated with sandy deserts, cuts a broad path through its environment that may increase its vulnerability to obstacles. Our prior work demonstrated that sidewinding can be represented as a pair of orthogonal body waves (vertical and horizontal) that can be independently modulated to achieve high maneuverability and incline ascent, suggesting that sidewinders may also use template modulations to negotiate obstacles. To test this hypothesis, we recorded overhead video of four sidewinder rattlesnakes (Crotalus cerastes) crossing a line of vertical pegs placed in the substrate. Snakes used three methods to traverse the obstacles: a Propagate Through behavior in which the lifted moving portion of the snake was deformed around the peg and dragged through as the snake continued sidewinding (115/160 runs), Reversal turns that reorient the snake entirely (35/160), or switching to Concertina locomotion (10/160). The Propagate Through response was only used if the anterior-most region of static contact would propagate along a path anterior to the peg, or if a new region of static contact could be formed near the head to satisfy this condition; otherwise, snakes could only use Reversal turns or switch to Concertina locomotion. Reversal turns allowed the snake to re-orient and either escape without further peg contact or re-orient into a posture amenable to using the Propagate Through response. We developed an algorithm to reproduce the Propagate Through behavior in a robophysical model using a modulation of the two-wave template. This range of behavioral strategies provides sidewinders with a versatile range of options for effectively negotiating obstacles in their natural habitat, as well as provide insights into the design and control of robotic systems dealing with heterogeneous habitats.

Long Limbless Locomotors Over Land: The Mechanics and Biology of Elongate, Limbless Vertebrate Locomotion

Elongate, limbless body plans are widespread in nature and frequently converged upon (with over two dozen independent convergences in Squamates alone, and many outside of Squamata). Despite their lack of legs, these animals move effectively through a wide range of microhabitats, and have a particular advantage in cluttered or confined environments. This has elicited interest from multiple disciplines in many aspects of their movements, from how and when limbless morphologies evolve to the biomechanics and control of limbless locomotion within and across taxa to its replication in elongate robots. Increasingly powerful tools and technology enable more detailed examinations of limbless locomotor biomechanics, and improved phylogenies have shed increasing light on the origins and evolution of limblessness, as well as the high frequency of convergence. Advances in actuators and control are increasing the capability of "snakebots" to solve real-world problems (e.g., search and rescue), while biological data have proven to be a potent inspiration for improvements in snakebot control. This collection of research brings together prominent researchers on the topic from around the world, including biologists, physicists, and roboticists to offer new perspective on locomotor modes, musculoskeletal mechanisms, locomotor control, and the evolution and diversity of limbless locomotion.

Dietary vitamin D in female rock lizards induces condition-transfer effects in their offspring

One way that maternal effects may benefit the offspring is by informing them about the characteristics of the environment. Through gestation, environmentally induced maternal effects might promote in the offspring-specific behavioral responses like dispersal or residence according to their new habitat characteristics. Females of the Carpetan rock lizard (Iberolacerta cyreni) seem to choose their home ranges using the smell of provitamin D3 in scent marks produced by males. Here, we supplemented gravid females of I. cyreni with dietary provitamin D3 or vitamin D3 to examine whether these food resources, also associated with the scent of males, affect the motivation to disperse and the locomotor performance of their offspring. Our results suggest that the supplementary availability of the resource (vitamin D3) to mothers may provoke condition-transfer maternal effects that motivate the residence or the dispersal of the offspring in their postnatal habitat. Thus, hatchlings of supplemented females had a lower dispersal trend in spite of having a greater climbing ability than hatchlings from nonsupplemented females. This suggests that the levels of provitamin D3 and vitamin D3 inside the body of the mother could act as an informative compound of the habitat quality for the offspring.

Maternal diet affects juvenile Carpetan rock lizard performance and personality

Differences in both stable and labile state variables are known to affect the emergence and maintenance of consistent interindividual behavioral variation (animal personality or behavioral syndrome), especially when experienced early in life. Variation in environmental conditions experienced by gestating mothers (viz. nongenetic maternal effects) is known to have significant impact on offspring condition and behavior; yet, their effect on behavioral consistency is not clear. Here, by applying an orthogonal experimental design, we aimed to study whether increased vitamin D3 content in maternal diet during gestation (vitamin-supplemented vs. vitamin control treatments) combined with corticosterone treatment (corticosterone-treated vs. corticosterone control treatments) applied on freshly hatched juveniles had an effect on individual state and behavioral consistency of juvenile Carpetan rock lizards (Iberolacerta cyreni). We tested the effect of our treatments on (a) climbing speed and the following levels of behavioral variation, (b) strength of animal personality (behavioral repeatability), (c) behavioral type (individual mean behavior), and (d) behavioral predictability (within-individual behavioral variation unrelated to environmental change). We found higher locomotor performance of juveniles from the vitamin-supplemented group (42.4% increase), irrespective of corticosterone treatment. While activity personality was present in all treatments, shelter use personality was present only in the vitamin-supplemented × corticosterone-treated treatment and risk-taking personality was present in corticosterone control treatments. Contrary to our expectations, behavioral type was not affected by our treatments, indicating that individual quality can affect behavioral strategies without affecting group-level mean behavior. Behavioral predictability decreased in individuals with low climbing speed, which could be interpreted as a form of antipredator strategy. Our results clearly demonstrate that maternal diet and corticosterone treatment have the potential to induce or hamper between-individual variation in different components of boldness, often in interactions.

Forelimb position affects facultative bipedal locomotion in lizards

Recent work indicates that bipedal posture in lizards is advantageous during obstacle negotiation. However, little is known about how bipedalism occurs beyond a lizard's acceleratory threshold. Furthermore, no study to date has examined the effects of forelimb position on the body center of mass (BCoM) in the context of bipedalism. This study quantified the frequency of bipedalism when sprinting with versus without an obstacle at 0.8 m from the start of a sprint. Forelimb positions were quantified during bipedal running at the start of a sprint and when crossing an obstacle. Two species with contrasting body forms (and thus different BCoM) were studied (Sceloporus woodi and Aspidoscelis sexlineata) to assess potential variation due to body plan and obstacle-crossing behavior. No significant difference in frequency of bipedalism was observed in S. woodi with or without an obstacle. However, A. sexlineata primarily used a bipedal posture when sprinting. Forelimb positions were variable in S. woodi and stereotyped in A. sexlineata Caudal extension of the forelimbs helped shift the BCoM posteriorly and transition to, or maintain, a bipedal posture in A. sexlineata, but not in S. woodi The posterior shift in BCoM, aided by more caudally placed forelimbs, helps raise the trunk from the ground, regardless of obstacle presence. The body plan, specifically the length of the trunk and tail, and forelimb position work together with acceleration to shift the BCoM posteriorly to transition to a bipedal posture. Thus, species exhibit morphological and behavioral adjustments to transition to and maintain facultative bipedalism while sprinting.

Dynamic traversal of large gaps by insects and legged robots reveals a template

It is well known that animals can use neural and sensory feedback via vision, tactile sensing, and echolocation to negotiate obstacles. Similarly, most robots use deliberate or reactive planning to avoid obstacles, which relies on prior knowledge or high-fidelity sensing of the environment. However, during dynamic locomotion in complex, novel, 3D terrains, such as a forest floor and building rubble, sensing and planning suffer bandwidth limitation and large noise and are sometimes even impossible. Here, we study rapid locomotion over a large gap-a simple, ubiquitous obstacle-to begin to discover the general principles of the dynamic traversal of large 3D obstacles. We challenged the discoid cockroach and an open-loop six-legged robot to traverse a large gap of varying length. Both the animal and the robot could dynamically traverse a gap as large as one body length by bridging the gap with its head, but traversal probability decreased with gap length. Based on these observations, we developed a template that accurately captured body dynamics and quantitatively predicted traversal performance. Our template revealed that a high approach speed, initial body pitch, and initial body pitch angular velocity facilitated dynamic traversal, and successfully predicted a new strategy for using body pitch control that increased the robot's maximal traversal gap length by 50%. Our study established the first template of dynamic locomotion beyond planar surfaces, and is an important step in expanding terradynamics into complex 3D terrains.

It's just sand between the toes: how particle size and shape variation affect running performance and kinematics in a generalist lizard

Animals must cope with and be able to move effectively on a variety of substrates. Substrates composed of granular media, such as sand and gravel, are extremely common in nature, and vary tremendously in particle size and shape. Despite many studies of the properties of granular media and comparisons of locomotion between granular and solid substrates, the effects of systematically manipulating these media on locomotion is poorly understood. We studied granular media ranging over four orders of magnitude in particle size, and differing in the amount of particle shape variation, to determine how these factors affected substrate physical properties and sprinting in the generalist lizard Eremias arguta. We found that media with intermediate particle sizes had high bulk densities, low angles of stability and low load-bearing capacities. Rock substrates with high shape variation had higher values for all three properties than glass bead substrates with low shape variation. We found that E. arguta had the highest maximum velocities and accelerations on intermediate size particles, and higher velocities on rock than glass beads. Lizards had higher stride frequencies and lower duty factors on intermediate particle size substrates, but their stride lengths did not change with substrate. Our findings suggest that sand and gravel may represent different locomotor challenges for animals. Sand substrates provide animals with an even surface for running, but particles shift underfoot. In contrast, gravel particles are heavy, so move far less underfoot, yet provide the animal with an uneven substrate.

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.

Measuring Selection on Physiology in the Wild and Manipulating Phenotypes (in Terrestrial Nonhuman Vertebrates)

To understand why organisms function the way that they do, we must understand how evolution shapes physiology. This requires knowledge of how selection acts on physiological traits in nature. Selection studies in the wild allow us to determine how variation in physiology causes variation in fitness, revealing how evolution molds physiology over evolutionary time. Manipulating phenotypes experimentally in a selection study shifts the distribution of trait variation in a population to better explore potential constraints and the adaptive value of physiological traits. There is a large database of selection studies in the wild on a variety of traits, but very few of those are physiological traits. Nevertheless, data available so far suggest that physiological traits, including metabolic rate, thermal physiology, whole-organism performance, and hormone levels, are commonly subjected to directional selection in nature, with stabilizing and disruptive selection less common than predicted if physiological traits are optimized to an environment. Selection studies on manipulated phenotypes, including circulating testosterone and glucocorticoid levels, reinforce this notion, but reveal that trade-offs between survival and reproduction or correlational selection can constrain the evolution of physiology. More studies of selection on physiological traits in nature that quantify multiple traits are necessary to better determine the manner in which physiological traits evolve and whether different types of traits (dynamic performance vs. regulatory) evolve differently.

Sprint sensitivity and locomotor trade-offs in green anole (Anolis carolinensis) lizards

How well an organism completes an ecologically relevant task – its performance – is often considered a key factor in determining individual fitness. Historically, ecomorphological studies have examined how morphological traits determine individual performance in a static manner, assuming that differential fitness in a population is due indirectly to differences in morphological traits that determine a simple measure of performance. This assumption, however, ignores many ecological factors that can constrain performance in nature, such as substrate variation and individual behavior. We examined some of these complexities in the morphology–performance–fitness paradigm, primarily the impact that substrate variation has on performance. We measured maximal sprint speed of green anole lizards on four substrates that varied in size and complexity and are used by or available to individuals in nature. Performance decreased significantly from a broad substrate to a narrow substrate, and lizards were three times slower on a complex substrate than the broadest substrate. We also detected trade-offs in running on substrates with different diameters and in cluttered versus uncluttered environments. Furthermore, morphological predictors of performance varied among substrates. This indicates that natural selection may act on different morphological traits, depending on which substrates are used by individuals, as well as an individual's ability to cope with changes in substrate rather than maximal capacities.