Speed versus manoeuvrability: association between vertebral number and habitat structure in lacertid lizards

The macroevolutionary singularity of snakes

Snakes and lizards (Squamata) represent a third of terrestrial vertebrates and exhibit spectacular innovations in locomotion, feeding, and sensory processing. However, the evolutionary drivers of this radiation remain poorly known. We infer potential causes and ultimate consequences of squamate macroevolution by combining individual-based natural history observations (>60,000 animals) with a comprehensive time-calibrated phylogeny that we anchored with genomic data (5400 loci) from 1018 species. Due to shifts in the dynamics of speciation and phenotypic evolution, snakes have transformed the trophic structure of animal communities through the recurrent origin and diversification of specialized predatory strategies. Squamate biodiversity reflects a legacy of singular events that occurred during the early history of snakes and reveals the impact of historical contingency on vertebrate biodiversity.

Early amphibians evolved distinct vertebrae for habitat invasions

Living tetrapods owe their existence to a critical moment 360-340 million years ago when their ancestors walked on land. Vertebrae are central to locomotion, yet systematic testing of correlations between vertebral form and terrestriality and subsequent reinvasions of aquatic habitats is lacking, obscuring our understanding of movement capabilities in early tetrapods. Here, we quantified vertebral shape across a diverse group of Paleozoic amphibians (Temnospondyli) encompassing different habitats and nearly the full range of early tetrapod vertebral shapes. We demonstrate that temnospondyls were likely ancestrally terrestrial and had several early reinvasions of aquatic habitats. We find a greater diversity in temnospondyl vertebrae than previously known. We also overturn long-held hypotheses centered on weight-bearing, showing that neural arch features, including muscle attachment, were plastic across the water-land divide and do not provide a clear signal of habitat preferences. In contrast, intercentra traits were critical, with temnospondyls repeatedly converging on distinct forms in terrestrial and aquatic taxa, with little overlap between. Through our geometric morphometric study, we have been able to document associations between vertebral shape and environmental preferences in Paleozoic tetrapods and to reveal morphological constraints imposed by vertebrae to locomotion, independent of ancestry.

Convergent Evolution of Claw Shape in a Transcontinental Lizard Radiation

Species occupying similar selective environments often share similar phenotypes as the result of natural selection. Recent discoveries, however, have led to the understanding that phenotypes may also converge for other reasons than recurring selection. We argue that the vertebrate claw system constitutes a promising but understudied model system for testing the adaptive nature of phenotypic, functional, and genetic convergence. In this study, we combine basic morphometrics and advanced techniques in form analysis to examine claw shape divergence in a transcontinental lizard radiation (Lacertidae). We find substantial interspecific variation in claw morphology and phylogenetic comparative statistics reveal a strong correlation with structural habitat use: ground-dwelling species living in open areas are equipped with long, thick, weakly curved, slender-bodied claws, whereas climbing species carry high, short, strongly curved, full-bodied claws. Species occupying densely vegetated habitats tend to carry intermediately shaped claws. Evolutionary models suggest that claw shape evolves toward multiple adaptive peaks, with structural habitat use pulling species toward a specific selective optimum. Contrary to findings in several other vertebrate taxa, our analyses indicate that environmental pressures, not phylogenetic relatedness, drive convergent evolution of similarly shaped claws in lacertids. Overall, our study suggests that lacertids independently evolved similarly shaped claws as an adaptation to similar structural environments in order to cope with the specific locomotory challenges posed by the habitat. Future biomechanical studies that link form and function in combination with genomic and development research will prove valuable in better understanding the adaptive significance of claw shape divergence.

Anuran forelimb muscle tendinous structures and their relationship with locomotor modes and habitat use

The interaction between organisms and their environment is central in functional morphology. Differences in habitat usage may imply divergent morphology of locomotor systems; thus, detecting which morphological traits are conservative across lineages and which ones vary under environmental pressure is important in evolutionary studies. We studied internal and external morphology in 28 species of Neotropical anurans. Our aim was to determine if internal morphology (muscle and tendons) shows lower phylogenetic signal than external morphology. In addition, we wanted to know if morphology varies in relation to the habitat use and if there are different functional groups. We found differences in the degree of phylogenetic signal on the groups of traits. Interestingly, postaxial regions of the forelimb are evolutionarily more labile than the preaxial regions. Phylomorphospace plots show that arboreal (jumpers and graspers) and swimmer frogs cluster based on length of fingers and the lack of sesamoid, also reflected by the use of habitat. These functional clusters are also related to phylogeny. Sesamoid and flexor plate dimensions together with digit tendons showed to be important to discriminate functional groups as well as use of habitat classification. Our results allow us to identify a "grasping syndrome" in the hand of these frogs, where palmar sesamoid and flexor plate are absent and a third metacarpal with a bony knob are typical. Thus, a lighter skeleton, long fingers and a prensile hand may be key for arboreality.

Habitat use and vestibular system's dimensions in lacertid lizards

The vestibular system is crucial for movement control during locomotion. As the dimensions of the vestibular system determine the fluid dynamics of the endolymph and, as such, the system's function, we investigate the interaction between vestibular system size, head size and microhabitat use in lizards. We grouped 24 lacertid species in three microhabitat types, we acquired three-dimensional models of the bony vestibular systems using micro-computer tomography scanning, and we performed linear and surface measurements. All vestibular measurements scale with a negative allometry with head size, suggesting that smaller heads house disproportionally large ears. As the sensitivity of the vestibular system is positively related to size, a sufficiently large vestibular system in small-headed animals may meet the sensitivity demands during challenged locomotion. We also found that the microhabitat affects the locomotor dynamics: lizards inhabiting open microhabitats run at higher dimensionless speeds. On the other hand, no statistical relationship exists between dimensionless speed and the vestibular system dimensions. Hence, if the vestibular size would differ between microhabitats, this would be a direct effect (i.e. imposed, for instance, by requirements for manoeuvring, balance control, etc.), rather than depending on the lizards' intrinsic running speed. However, we found no effect of the microhabitat on the allometric relationship between head and vestibular system size. The finding that microhabitat is not reflected in the vestibular system size (hence sensitivity) of the lacertids in this study is possibly due to spatial constraints of the skull.

Patterns of growth in the presacral vertebral column of the leopard gecko (Eublepharis macularius)

Postnatal growth patterns within the vertebral column may be informative about body proportions and regionalization. We measured femur length, lengths of all pre-sacral vertebrae, and lengths of intervertebral spaces, from radiographs of a series of 21 Eublepharis macularius, raised under standard conditions and covering most of the ontogenetic body size range. Vertebrae were grouped into cervical, sternal, and dorsal compartments, and lengths of adjacent pairs of vertebrae were summed before analysis. Femur length was included as an index of body size. Principal component analysis of the variance-covariance matrix of these data was used to investigate scaling among them. PC1 explained 94.19% of total variance, interpreted as the variance due to body size. PC1 differed significantly from the hypothetical isometric vector, indicating overall allometry. The atlas and axis vertebrae displayed strong negative allometry; the remainder of the vertebral pairs exhibited weak negative allometry, isometry or positive allometry. PC1 explained a markedly smaller amount of variance for the vertebral pairs of the cervical compartment than for the remainder of the vertebral pairs, with the exception of the final pair. The relative standard deviations of the eigenvalues from the PCAs of the three vertebral compartments indicated that the vertebrae of the cervical compartment were less strongly integrated by scaling than were the sternal or dorsal vertebrae, which did not differ greatly between themselves in their strong integration, suggesting that the growth of the cervical vertebrae is constrained by the mechanical requirements of the head. Regionalization of the remainder of the vertebral column is less clearly defined but may be associated with wave form propagation incident upon locomotion, and by locomotory changes occasioned by tail autotomy and regeneration. Femur length exhibits negative allometry relative to individual vertebral pairs and to vertebral column length, suggesting a change in locomotor requirements over the ontogenetic size range.

How Fast Should an Animal Run When Escaping? An Optimality Model Based on the Trade-Off Between Speed and Accuracy

How fast should animals move when trying to survive? Although many studies have examined how fast animals can move, the fastest speed is not always best. For example, an individual escaping from a predator must run fast enough to escape, but not so fast that it slips and falls. To explore this idea, we developed a simple mathematical model that predicts the optimal speed for an individual running from a predator along a straight beam. A beam was used as a proxy for straight-line running with severe consequences for missteps. We assumed that success, defined as reaching the end of the beam, had two broad requirements: (1) running fast enough to escape a predator, and (2) minimizing the probability of making a mistake that would compromise speed. Our model can be tailored to different systems by revising the predator's maximal speed, the prey's stride length and motor coordination, and the dimensions of the beam. Our model predicts that animals should run slower when the beam is narrower or when coordination is worse.

Vertebral evolution and the diversification of squamate reptiles

Taxonomic, morphological, and functional diversity are often discordant and independent components of diversity. A fundamental and largely unanswered question in evolutionary biology is why some clades diversify primarily in some of these components and not others. Dramatic variation in trunk vertebral numbers (14 to >300) among squamate reptiles coincides with different body shapes, and snake-like body shapes have evolved numerous times. However, whether increased evolutionary rates or numbers of vertebrae underlie body shape and taxonomic diversification is unknown. Using a supertree of squamates including 1375 species, and corresponding vertebral and body shape data, we show that increased rates of evolution in vertebral numbers have coincided with increased rates and disparity in body shape evolution, but not changes in rates of taxonomic diversification. We also show that the evolution of many vertebrae has not spurred or inhibited body shape or taxonomic diversification, suggesting that increased vertebral number is not a key innovation. Our findings demonstrate that lineage attributes such as the relaxation of constraints on vertebral number can facilitate the evolution of novel body shapes, but that different factors are responsible for body shape and taxonomic diversification.

Morphological evolution in Tropidurinae squamates: an integrated view along a continuum of ecological settings

Variation in squamate foot morphology is likely relevant during evolutionary processes of habitat colonization because distinct surfaces differ in energetic and functional demands for locomotion. We combined new foot morphological data with published information of limb and tail lengths to investigate evolutionary changes possibly associated with the differential usage of ecological settings by Tropidurinae species. Several traits exhibited significant phylogenetic signal, and we performed conventional and phylogenetic regressions of PC scores (retained from Principal Components Analyses of morphometric traits) on continuous ecological indices. Tropidurines from sandy habitats exhibit larger foot soles, opposite to the evolution of narrow feet in species that use branches and rocks. Also, species that usually move along trunks present longer femora. This study provides evidence for morphological adaptations associated with substrate usage in Tropidurinae, and suggests that opposite morphological profiles might evolve associated with the use of surfaces energetically and functionally contrasting, possibly leading to trade-offs.

Morphological differentiation and genetic structure in island lizard populations

Only some island populations of Podarcis sicula are hyperchromatic. The study of this phenomenon and its relationship with the lizards of the mainland and other islands, exhibiting a "normal" coloration, provides useful hints in our understanding of evolutionary mechanisms that have created the observed morphological variation. We performed a comparative morphological and genetic analysis of a hyperchromatic lizard population from Licosa Island, and compared the data with that obtained from normal-colored lizard populations both from Ustica and Cirella islands in the Tyrrhenian sea and from nearby mainland Italy. Morphological and microsatellite gene differentiation in the hyperchromatic Licosa population appears to have been much more rapid than the molecular evolution of the mtDNA. We discuss herein that the comparison of hyperchromatism and other types of morphological variation with molecular data in island populations of lizards may provide useful hints as to evolutionary mechanisms.

Triturus newts defy the running-swimming dilemma

Conflicts between structural requirements for carrying out different ecologically relevant functions may result in a compromise phenotype that maximizes neither function. Identifying and evaluating functional trade-offs may therefore aid in understanding the evolution of organismal performance. We examined the possibility of an evolutionary trade-off between aquatic and terrestrial locomotion in females of European species of the newt genus Triturus. Biomechanical models suggest a conflict between the requirements for aquatic and terrestrial locomotion. For instance, having an elongate, slender body, a large tail, and reduced limbs should benefit undulatory swimming, but at the cost of reduced running capacity. To test the prediction of an evolutionary trade-off between swimming and running capacity, we investigated relationships between size-corrected morphology and maximum locomotor performance in females of ten species of newts. Phylogenetic comparative analyses revealed that an evolutionary trend of body elongation (increasing axilla-groin distance) is associated with a reduction in head width and forelimb length. Body elongation resulted in reduced maximum running speed, but, surprisingly, also led to a reduction in swimming speed. The evolution of longer tails was associated with an increase in maximal swimming speed. We found no evidence for an evolutionary trade-off between aquatic and terrestrial locomotor performance, probably because of the unexpected negative effect of body elongation on swimming speed. We conclude that the idea of a design conflict between aquatic and terrestrial locomotion, mediated through antagonistic effects of body elongation, does not apply to our model system.

Convergent evolution of the head retraction escape response in elongate fishes and amphibians

Aquatic escape responses have typically been described as C-starts. However, another aquatic escape response, head retraction, occurs in several elongate species, but has never been studied in detail. The goals of this study are to describe the head retraction escape response in a phylogenetically diverse sample of species, to trace the evolution of head retraction in anamniote vertebrates, and to correlate key morphological traits with escape response behavior. In analyzing the evolution of escape behavior, we found that the head retraction escape response has evolved at least six times in anamniote vertebrates. Using independent contrast analysis, the head retraction escape response was found to be correlated with an increase in the total number of vertebrae and an increase in the elongation of an animal. Results from this study indicate a correlation between head retraction as an escape response, elongation of the axial skeleton, and living in structured habitats.

Effects of loading and size on maximum power output and gait characteristics in geckos

Stride length, stride frequency and power output are all factors influencing locomotor performance. Here, we first test whether mass-specific power output limits climbing performance in two species of geckos(Hemidactylus garnoti and Gekko gecko) by adding external loads to their bodies. We then test whether body size has a negative effect on mass-specific power output. Finally, we test whether loading affects kinematics in both gecko species. Lizards were induced to run vertically on a smooth wooden surface with loads of 0-200% of body mass (BM) in H. garnoti and 0-100% BM in G. gecko. For each stride, we calculated angular and linear kinematics (e.g. trunk angle, stride length),performance (maximum speed) and mean mass-specific power output per stride. The addition of increasingly large loads caused an initial increase in maximum mass-specific power output in both species, but for H. garnoti,mass-specific power output remained constant at higher loads (150% and 200%BM), even though maximum velocity declined. This result, in combination with the fact that stride frequency showed no evidence of leveling off as speed increased in either species, suggests that power limits maximum speed. In addition, the large gecko (G. gecko) produced significantly less power than the smaller H. garnoti, despite the fact that both species ran at similar speeds. This difference disappeared, however, when we recalculated power output based on higher maximum speeds for unloaded G. gecko moving vertically obtained by other researchers. Finally, the addition of external loads did not affect speed modulation in either species:both G. gecko and H. garnoti increase speed primarily by increasing stride frequency, regardless of loading condition. For a given speed, both species take shorter but more strides with heavier loads, but for a given load, G. gecko attains similar speeds to H. garnotiby taking longer but fewer strides.