Evolution of morphology and locomotor performance in anurans: relationships with microhabitat diversification

Comparative muscle anatomy of the anuran pelvis and hindlimb in relation to locomotor mode

Frogs have a highly conserved body plan, yet they employ a diverse array of locomotor modes, making them ideal organisms for investigating the relationships between morphology and locomotor function, in particular whether anatomical complexity is a prerequisite for functional complexity. We use diffusible iodine contrast-enhanced microCT (diceCT) imaging to digitally dissect the gross muscle anatomy of the pelvis and hindlimbs for 30 species of frogs representing five primary locomotor modes, including the first known detailed dissection for some of the world's smallest frogs, forming the largest digital comparative analysis of musculoskeletal structure in any vertebrate clade to date. By linking musculoskeletal dissections and phylogenetic comparative methods, we then quantify and compare relationships between anatomy and function across over 160 million years of anuran evolution. In summary, we have found that bone lengths and pelvic crest sizes are generally not reliable predictors of muscle sizes, which highlights important implications for future palaeontological studies. Our investigation also presents previously unreported differences in muscle anatomy between frogs specialising in different locomotor modes, including several of the smallest frog hindlimb muscles, which are extremely difficult to extract and measure using traditional approaches. Furthermore, we find evidence of many-to-one and one-to-many mapping of form to function across the phylogeny. Additionally, we perform the first quantitative analysis of how the degree of muscle separation can differ between frogs. We find evidence that phylogenetic history is the key contributing factor to muscle separation in the pelvis and thigh, while the separation of shank muscles is influenced more strongly by locomotor mode. Finally, our anatomical 3D reconstructions are published alongside this manuscript to contribute towards future research and serve as educational materials.

Life on both environment in semi-aquatic frogs: Impact of aquatic microplastic (MP) from MP enrichment to growth, immune function and physiological stress

The pervasive distribution of microplastics (MPs) in aquatic ecosystems presents a significant threat to wildlife, with amphibians being particularly vulnerable due to their complex life cycles and ecological roles. This study investigates physiological and ecological impacts of aquatic MP exposure on juvenile black-spotted pond frogs (Pelophylax nigromaculatus), focusing on juvenile frog stage, history of life after metamorphosis. MP examinations in the intestine and body revealed accumulation primarily in the gastrointestinal tracts without evidence of systemic distribution. Experimental exposure to different concentrations of MPs demonstrated adverse effects on growth, physiological stress, and immune function. Notably, higher MP concentrations led to significant reductions in growth and innate immunity, indicative of compromised health. High concentrations of MPs were associated with elevated levels of corticosterone and antioxidant enzymes, indicating physiological stress. However, there was no evidence of extreme hormonal surges or imbalances in antioxidant enzyme activity, suggesting that amphibians were able to effectively cope with the levels of MPs used in the study. Changes in gastrointestinal morphology and fecal microbiota composition were observed, reflecting response of metabolic adaptation to MP exposure. At low concentrations of MPs, adaptive changes in digestive tract morphology and the maintenance of gut microbiota balance were observed, indicating that the frogs were able to manage the exposure below a certain threshold. In contrast, high concentrations of MPs had clear negative effects on amphibians, which could impact biodiversity and ecosystem stability. These findings also suggest that MPs may trigger adaptive responses at lower concentrations, while still posing significant environmental risks at higher levels.

Frog hepatic health and metal pollution: An assemblage-level approach in a hotspot in southeastern Brazil

Chemical pollutants include the harmful effects of various substances on soils, water bodies, and biodiversity. Amphibians are one of the most endangered groups of vertebrates and are impacted by chemical pollutants in various ways due to their complex life cycles. Since trace pollutant concentrations vary across environments, different frog ecomorphs (classified by their microhabitat use) may have different exposures. We aimed to determine the association between frog ecomorphs and the occurrence of histopathological hepatic lesions (HHLs) as an indicator of contaminant exposure. We focused on small forest streams near a large urban region in Brazil, heavily polluted in the 1980s. We examined 104 frog specimens from various families. All specimens exhibited HHLs, with melanomacrophages being the most common (n = 99). Arboreal frogs exhibited more vascular congestion, while terrestrial frogs showed structural hepatic damage. Higher cobalt levels were linked to increased liver necrosis in arboreal frogs and structural issues in both arboreal and terrestrial frogs. Cadmium was associated with hepatitis in terrestrial frogs. Although metal levels had no significant effects on rheophilic frogs, the prevalence of hepatitis and necrosis indicated complex exposure pathways. Iron and aluminum were linked to fewer lesions in rheophilic frogs, suggesting resilience. The high prevalence of HHLs signals an ongoing issue, with variations among ecomorphs suggesting differential exposure to pollutants and posing a complex challenge for community conservation.

Developmental temperature modulates microplastics impact on amphibian life history without affecting ontogenetic microplastic transfer

This study examines how temperature influences the response of Japanese tree frogs (Dryophytes japonicus) to microplastic (MP) pollution, assessing whether temperature can regulate the harmful effects of MPs on their life history and the dispersal of MPs across habitats. This analysis aims to understand the ecological and physiological ramifications of MP pollution. Our results demonstrated an ontogenetic transfer of MP particles across amphibian metamorphosis, possibly allowing and facilitating the translocation of MPs across ecosystems. Temperature did not significantly affect the translocation of aquatic MPs to land. However, high temperatures significantly reduced mortality and hindlimb deformities caused by MPs, thereby mitigating their harmful impact on amphibian life histories. Importantly, our study found that MPs cause hindlimb deformities during amphibian metamorphosis, potentially linked to oxidative stress. Additionally, MP exposure and ingestion induced a plastic response in the morphology of the digestive tract and changes in the fecal microbiome, which were evident at high temperatures but not at low temperatures. The effects of MPs persisted even after the frogs transitioned to the terrestrial stage, suggesting that MPs may have complex, long-term impacts on amphibian population sustainability. Our results enhance the understanding of the intricate environmental challenges posed by MPs and underscore the significant role of temperature in ectotherms regarding ontogenetic impacts and pollutant interactions.

3D Morphological Scanning and Environmental Correlates of Bufo gargarizans in the Yellow River Basin

Morphology plays a crucial role in understanding the intricacies of biological forms. Traditional morphometric methods, focusing on one- or two-dimensional geometric levels, often fall short of accurately capturing the three-dimensional (3D) structure of organisms. The advent of 3D scanning techniques has revolutionized the study of organismal morphology, enabling comprehensive and accurate measurements. This study employs a 3D structured light scanning system to analyze the morphological variations in the Chinese toad (Bufo gargarizans Cantor, 1842) along the Yellow River Basin. The 3D digital model obtained from the scan was used to calculate various morphological parameters including body surface area, volume, fractal dimensions, and limb size. The research explores geographic variability patterns and identifies environmental drivers affecting the 3D phenotypic variation of B. gargarizans. Results reveal a bimodal pattern of variation in the toad population, with higher elevations exhibiting smaller body sizes, greater appendage proportions, and more complex body structures. Linear regression analyses highlight the influence of elevation and annual mean temperature on the morphological variation of B. gargarizans, with elevation playing a significant role. This study underscores the significance of 3D morphometric analysis in unraveling the intricacies of organismal morphology and understanding the adaptive strategies of species in diverse environments.

Does aquatic performance predict terrestrial performance: a case study with an aquatic frog, Xenopus laevis

The physical properties of the environment impose strong selection on organisms and their form-function relationships. In water and on land, selective pressures differ, with water being more viscous and denser than air, and gravity being the most important external force on land for relatively large animals such as vertebrates. These different properties of the environment could drive variation in the design and mechanics of the locomotor system of organisms. Animals that use multiple environments can consequently exhibit locomotion conflicts between the demands imposed by the media, leading to potential trade-offs. Here, we tested for the presence of such locomotor trade-offs depending on the environment (water or land) in a largely aquatic frog, Xenopus laevis. We focused on terrestrial and aquatic exertion capacity (time and distance swum or jumped until exhaustion) and aquatic and terrestrial burst capacity (maximal instantaneous swimming velocity and maximal force jump) given the ecological relevance of these traits. We tested these performance traits for trade-offs, depending on environments (water versus air) and locomotor modes (i.e. exertion and burst performance). Finally, we assessed the contribution of morphological traits to each performance trait. Our data show no trade-offs between the performance traits and between the environments, suggesting that X. laevis is equally good at swimming and jumping thanks to the same underlying morphological specialisations. We did observe, however, that morphological predictors differed depending on the environment, with variation in head shape and forelimb length being good predictors for aquatic locomotion and variation in hindlimb and forelimb segments predicting variation in jumping performance on land.

Locomotor, ecological and phylogenetic drivers of skeletal proportions in frogs

Frogs exhibit complex anatomical features of the pelvis, limbs and spine, long assumed to represent specialisations for jumping. Yet frogs employ a wide range of locomotor modes, with several taxa featuring primary locomotor modes other than jumping. Using a combination of techniques (CT imaging and 3D visualization, morphometrics, phylogenetic mapping), this study aims to determine the link between skeletal anatomy and locomotor style, habitat type and phylogenetic history, shedding new light on how functional demands impact morphology. Body and limb measurements for 164 taxa from all the recognised anuran families are extracted from digitally segmented CT scans of whole frog skeletons and analysed using various statistical techniques. We find that the expansion of the sacral diapophyses is the most important variable for predicting locomotor mode, which was more closely correlated with frog morphology than either habitat type or phylogenetic relationships. Predictive analyses suggest that skeletal morphology is a useful indicator of jumping but less so for other locomotor modes, suggesting that there is a wide range of anatomical solutions to performing locomotor styles such as swimming, burrowing or walking.

Ecology, sexual dimorphism, and jumping evolution in anurans

Sexual dimorphism (SD) is a common feature of animals, and selection for sexually dimorphic traits may affect both functional morphological traits and organismal performance. Trait evolution through natural selection can also vary across environments. However, whether the evolution of organismal performance is distinct between the sexes is rarely tested in a phylogenetic comparative context. Anurans commonly exhibit sexual size dimorphism, which may affect jumping performance given the effects of body size on locomotion. They also live in a wide variety of microhabitats. Yet the relationships among dimorphism, performance, and ecology remain underexamined in anurans. Here, we explore relationships between microhabitat use, body size, and jumping performance in males and females to determine the drivers of dimorphic patterns in jumping performance. Using methods for predicting jumping performance through anatomical measurements, we describe how fecundity selection and natural selection associated with body size and microhabitat have likely shaped female jumping performance. We found that the magnitude of sexual size dimorphism (where females are about 14% larger than males) was much lower than dimorphism in muscle volume, where females had 42% more muscle than males (after accounting for body size). Despite these sometimes-large averages, phylogenetic t-tests failed to show the statistical significance of SD for any variable, indicating sexually dimorphic species tend to be closely related. While SD of jumping performance did not vary among microhabitats, we found female jumping velocity and energy differed across microhabitats. Overall, our findings indicate that differences in sex-specific reproductive roles, size, jumping-related morphology, and performance are all important determinants in how selection has led to the incredible ecophenotypic diversity of anurans.

Morphological Correlates of Locomotion in the Aquatic and the Terrestrial Phases of Pleurodeles waltl Newts from Southwestern Iberia

Animals capable of moving in different environments might face conflicting selection on morphology, thus posing trade-offs on the relationships between morphology and locomotor performance in each of these environments. Moreover, given the distinct ecological roles of the sexes, these relationships can be sexually dimorphic. In this article, I studied the relationships between morphological traits and locomotor performance in male and female semiaquatic Pleurodeles waltl newts in their aquatic and their terrestrial stages. Morphology was sexually dimorphic: males have proportionally longer limbs and tails, as well as a better body condition (only in the aquatic phase), whereas females were larger and had greater body mass in both phases. Nonetheless, these morphological differences did not translate into sexual divergence in locomotor performance in either stage. This finding suggests other functions for the morphological traits measured, among which only SVL showed a positive relationship with locomotor performance in both stages, whereas the effect of SMI was negative only in the terrestrial stage, and that of tail length was positive only in the aquatic stage. In any case, the morphological correlates of terrestrial and aquatic locomotion did not conflict, which suggests no trade-off between both locomotory modes in the newts studied.

Scaling patterns of body plans differ among squirrel ecotypes

Body size is often hypothesized to facilitate or constrain morphological diversity in the cranial, appendicular, and axial skeletons. However, how overall body shape scales with body size (i.e., body shape allometry) and whether these scaling patterns differ between ecological groups remains poorly investigated. Here, we test whether and how the relationships between body shape, body size, and limb lengths differ among species with different locomotor specializations, and describe the underlying morphological components that contribute to body shape evolution among squirrel (Sciuridae) ecotypes. We quantified the body size and shape of 87 squirrel species from osteological specimens held at museum collections. Using phylogenetic comparative methods, we first found that body shape and its underlying morphological components scale allometrically with body size, but these allometric patterns differ among squirrel ecotypes: chipmunks and gliding squirrels exhibited more elongate bodies with increasing body sizes whereas ground squirrels exhibited more robust bodies with increasing body size. Second, we found that only ground squirrels exhibit a relationship between forelimb length and body shape, where more elongate species exhibit relatively shorter forelimbs. Third, we found that the relative length of the ribs and elongation or shortening of the thoracic region contributes the most to body shape evolution across squirrels. Overall, our work contributes to the growing understanding of mammalian body shape evolution and how it is influenced by body size and locomotor ecology, in this case from robust subterranean to gracile gliding squirrels.

Morphological Variation and Its Environmental Correlates in the Taihangshan Swelled-Vented Frog across the Qinling Mountains

Simple Summary

Amphibians have weak dispersal abilities and are sensitive to environmental changes, resulting in their disproportionately high risk of extinction, with many species’ populations rapidly declining. Therefore, it is critical for amphibian conservation to understand their adaptive potential by exploring how amphibians respond to environmental changes based on morphological variations. Our results showed that morphological traits of Feirana taihangnica significantly differed among ages. Along with the increase in annual mean temperature, snout-vent length showed an anti-hump trend, indicating no support for Bergmann’s rule. Mean ultraviolet-B of the highest and lowest months were positively and negatively correlated with head width, thigh length and tibia width, respectively. The present study can help understand the effects of environmental changes on morphological variations of this mountain frog species and its adaptive potential, providing important implications for species conservation.

Abstract

The Taihangshan swelled-vented frog (Feirana taihangnica), an endemic species to the Qinling Mountains, central China, has experienced a dramatic population decline over the last few decades. The aim of this work was to quantify morphological variation in F. taihangnica across the Qinling Mountains and examine environmental correlates of this variation of morphological traits. We implemented a hierarchical partitioning to estimate the independent contribution of each environmental variable on morphological variations. Temperature seasonality was the greatest contributor in variations of snout-vent length (SVL) and head width, and ultraviolet-B (UV-B) radiation of the lowest month was the most influential on both thigh length and tibia width. Then, we used generalized additive models to analyze the relationship between each environmental factor and morphological trait variations. Along the increasing of annual mean temperature, SVL decreased firstly and then increased, indicating no support for Bergmann’s rule. Furthermore, SVL was negatively correlated with annual precipitation, while positively with temperature seasonality. The mean UV-B of the highest and lowest months was positively and negatively correlated with head width, thigh length and tibia width, respectively. The results of this study help us to understand adaptive potential of this mountain frog species via morphological variations in the light of environmental changes.

Reproductive behaviors promote ecological and phenotypic sexual differentiation in the critically endangered Lehmann’s poison frog

Territoriality and parental care are complex reproductive behaviors found in many taxa from insects to mammals. Parental care can be carried out by the female, the male, or both, depending on the species. Territoriality, in contrast, is predominantly displayed by males. Different selective pressures imposed on individuals from the sex performing territorial or parental care behaviors may also lead to sexual differentiation in other life-history traits. Due to their territorial behavior and their diversity of parental care behaviors, Neotropical poison frogs are an excellent study system to investigate whether behavioral traits can influence sexual differentiation in intrinsic or extrinsic traits of individuals. Here, we evaluate whether territorial and parental care behaviors mediate sexual differentiation in ecological (habitat use) and phenotypic (coloration, morphology) traits in the critically endangered Lehmann’s poison frog (Oophaga lehmanni), a species in which males defend territories while females provide parental care. We found sex differences in habitat use and morphological traits, but not in coloration. Males use trunks and green leaves as perches more frequently and are found on higher substrates, than females. We found no sex differences in body size, but females have longer arms than males, which is probably associated with their parental duties (climbing trees to feed the tadpoles). Altogether, our results provide evidence that selection pressures act differently on male and female traits, and that territoriality and parental care may promote the evolution of sexual differentiation in dendrobatids. Long-term wildlife observations are essential to identify important life-history traits and to evaluate hypotheses about the behavioral ecology and conservation of this and other vertebrate species.

Diversity and function of the fused anuran radioulna

In tetrapods, fusion between elements of the appendicular skeleton is thought to facilitate rapid movements during running, flying, and jumping. Although such fusion is widespread, frogs stand out because adults of all living species exhibit fusion of the zeugopod elements (radius and ulna, tibia and fibula), regardless of jumping ability or locomotor mode. To better understand what drives the maintenance of limb bone fusion in frogs, we use finite element modeling methods to assess the functional consequences of fusion in the anuran radioulna, the forearm bone of frogs that is important to both locomotion and mating behavior (amplexus). Using CT scans of museum specimens, measurement tools, and mesh‐editing software, we evaluated how different degrees of fusion between the radius and ulna affect the von Mises stress and bending resistance of the radioulna in three loading scenarios: landing, amplexus, and long‐axis loading conditions. We find that the semi‐fused state observed in the radioulna exhibits less von Mises stress and more resistance to bending than unfused or completely fused models in all three scenarios. Our results suggest that radioulna morphology is optimized to minimize von Mises stress across different loading regimes while also minimizing volume. We contextualize our findings in an evaluation of the diversity of anuran radioulnae, which reveals unique, permanent pronation of the radioulna in frogs and substantial variation in wall thickness. This work provides new insight into the functional consequences of limb bone fusion in anuran evolution.

Why link diverse citizen science surveys? Widespread arboreal habits of a terrestrial amphibian revealed by mammalian tree surveys in Britain

Terrestrial anurans, with their typically short limbs, heavy-set bodies and absent claws or toe pads are incongruous tree climbers, but even occasional arboreal locomotion could offer substantial advantages for evading predators or accessing new shelter or food resources. Despite recent interest, arboreal behaviour remains rarely and unsystematically described for terrestrial amphibians in Europe, likely due to fundamental differences in survey methods and therefore a lack of field data. However, other taxa surveys specifically target trees and tree cavities. We undertook collaborations and large-scale data searches with two major citizen science projects surveying for arboreal mammals in Britain to investigate potential tree climbing by amphibians at a national scale. Remarkably, we found widespread arboreal usage by amphibians in England and Wales, with occupancy of hazel dormouse (Muscardinus avellenarius) nest boxes, tree cavities investigated as potential bat roosts and even a bird nest, by common toads (Bufo bufo), but few additional records of frogs or newts. Of the 277–400 sites surveyed annually for dormice since 2009 at least 18 sites had amphibians recorded in nest boxes while of the 1388 trees surveyed for bats a total 1.4% (19 trees) had toads present. Common toads were found using cavities in seven tree species and especially goat willow (Salix caprea). Toads are potentially attracted to tree cavities and arboreal nests because they provide safe and damp microenvironments which can support an abundance of invertebrate prey but the importance of such tree microhabitats for toad conservation remains unknown and our results should be interpreted cautiously. We encourage expanding and linking of unrelated biodiversity monitoring surveys and citizen science initiatives as valuable tools for investigating ecological traits and interactions.

Individual contributions to group chorus dynamics influence access to mating opportunities in wood frogs

A limitation in bioacoustic studies has been the inability to differentiate individual sonic contributions from group-level dynamics. We present a novel application of acoustic camera technology to investigate how individual wood frogs' calls influence chorus properties, and how variation influences mating opportunities. We recorded mating calls and used playback trials to gauge preference for different chorus types in the laboratory. Males and females preferred chorus playbacks with low variance in dominant frequency. Females preferred choruses with low mean peak frequency. Field studies revealed more egg masses laid in ponds where males chorused with low variance in dominant frequency. We also noted a trend towards more egg masses laid in ponds where males called with low mean frequency. Nearest-neighbour distances influenced call timing (neighbours called in succession) and distances increased with variance in chorus frequency. Results highlight the potential fitness implications of individual-level contributions to a bioacoustic signal produced by groups.

Morphological diversification of Mediterranean anurans: the roles of evolutionary history and climate

Investigation of the ecological and evolutionary mechanisms governing the origin and diversification of species requires integrative approaches that often have to accommodate strong discordance among datasets. A common source of conflict is the combination of morphological and molecular characters with different evolutionary rates. Resolution of these discordances is crucial to assess the relative roles of different processes in generating and maintaining biodiversity. Anuran amphibians provide many examples of morphologically similar, genetically divergent lineages, posing questions about the relative roles of phylogeny and ecological factors in phenotypic evolution. We focused on three circum-Mediterranean anuran genera (Hyla, Alytes and Discoglossus), characterizing morphological and environmental disparity and comparing diversity patterns across biological levels of organization. Using a comparative phylogenetic framework, we tested how shared ancestry and climatic factors come together to shape phenotypic diversity. We found higher morphological differentiation within Hyla and Alytes than in Discoglossus. Body size and limb morphology contributed most to inter- and intraspecific morphological variation in Hyla and Alytes, but there was no strong phylogenetic signal, indicating that shared ancestry does not predict patterns of phenotypic divergence. In contrast, we uncovered a significant association between morphology and climatic descriptors, supporting the hypothesis that morphological disparity between species results from adaptive evolution.

Ecological Drivers of Carnivoran Body Shape Evolution

AbstractMorphological diversity is often attributed as adaptations to distinct ecologies. Although biologists have long hypothesized that distinct ecologies drive the evolution of body shape, these relationships are rarely tested across macroevolutionary scales in mammals. Here, I tested hypotheses that locomotor, hunting, and dietary ecologies influenced body shape evolution in carnivorans, a morphologically and ecologically diverse clade of mammals. I found that adaptive models with ecological trait regimes were poor predictors of carnivoran body shape and the underlying morphological components that contribute to body shape variation. Instead, the best-supported model exhibited clade-based evolutionary shifts, indicating that the complexity and variation of body shape landscape cannot be effectively captured by a priori ecological regimes. However, ecological adaptations of body shapes cannot be ruled out, as aquatic and terrestrial carnivorans exhibited opposite allometric patterns of body shape that may be driven by different gravitational constraints associated with these different environments. Similar to body size, body shape is a prominent feature of vertebrate morphology that may transcend one-to-one mapping relationships between morphology and ecological traits, enabling species with distinct body shapes to exploit similar resources and exhibit similar ecologies. Together, these results demonstrate that the multidimensionality of both body shape morphology and ecology makes it difficult to disentangle the complex relationship among morphological evolution, ecological diversity, and phylogeny across macroevolutionary scales.

Dynamic transcriptome and histomorphology analysis of developmental traits of hindlimb thigh muscle from Odorrana tormota and its adaptability to different life history stages

Background

Systematic studies on the development and adaptation of hindlimb muscles in anura amphibians are rare. Here, we integrated analysis of transcriptome and histomorphological data for the hindlimb thigh muscle of Odorrana tormota (concave-eared torrent frog) at different developmental stages, to uncover the developmental traits of hindlimb thigh muscle from O. tormota and its adaptability to different life history stages.

Results

The development of hindlimb thigh muscle from O. tormota has the following characteristics. Before metamorphosis, myogenous cells proliferate and differentiate into myotubes, and form 11 muscle groups at G41; Primary myofibers and secondary myofibers appeared during metamorphosis; 11 muscle groups differentiated continuously to form myofibers, accompanied by myofibers hypertrophy after metamorphosis; During the growth process of O. tormota from G42 to G46, there were differences between the sexes in the muscle groups that differentiate into muscle fibers, indicating that there was sexual dimorphism in the hindlimb thigh muscles of O. tormota at the metamorphosis stages. Some genes and pathways related to growth, development, and movement ability of O. tormota at different developmental stages were obtained. In addition, some pathways associated with adaptation to metamorphosis and hibernation also were enriched. Furthermore, integrated analysis of the number of myofibers and transcriptome data suggested that myofibers of specific muscle groups in the hindlimbs may be degraded through lysosome and ubiquitin pathways to transform into energy metabolism and other energy-related substances to meet the physiological needs of hibernation.

Conclusions

These results provide further understanding the hindlimb thigh muscle development pattern of frogs and their adaption to life history stages.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-07677-0.

Peculiar relationships among morphology, burrowing performance and sand type in two fossorial microteiid lizards

Associations among ecology, morphology and locomotor performance have been intensively investigated in several vertebrate lineages. Knowledge on how phenotypes evolve in natural environments likely benefits from identification of circumstances that might expand current ecomorphological equations. In this study, we used two species of Calyptommatus lizards from Brazilian Caatingas to evaluate if specific soil properties favor burrowing performance. As a derived prediction, we expected that functional associations would be easily detectable at the sand condition that favors low-resistance burrowing. We collected two endemic lizards and soil samples in their respective localities, obtained morphological data and recorded performance of both species in different sand types. As a result, the two species burrowed faster at the fine and homogeneous sand, the only condition where we detected functional associations between morphology and locomotion. In this sand type, lizards from both Calyptommatus species that have higher trunks and more concave heads were the ones that burrowed faster, and these phenotypic traits did not morphologically discriminate the two Calyptommatus populations studied. We discuss that integrative approaches comprising manipulation of environmental conditions clearly contribute to elucidate processes underlying phenotypic evolution in fossorial lineages.

Ecomorphology of the pectoral girdle in anurans (Amphibia, Anura): Shape diversity and biomechanical considerations

Frogs and toads (Lissamphibia: Anura) show a diversity of locomotor modes that allow them to inhabit a wide range of habitats. The different locomotor modes are likely to be linked to anatomical specializations of the skeleton within the typical frog Bauplan. While such anatomical adaptations of the hind limbs and the pelvic girdle are comparably well understood, the pectoral girdle received much less attention in the past. We tested for locomotor-mode-related shape differences in the pectoral girdle bones of 64 anuran species by means of micro-computed-tomography-based geometric morphometrics. The pectoral girdles of selected species were analyzed with regard to the effects of shape differences on muscle moment arms across the shoulder joint and stress dissipation within the coracoid. Phylogenetic relationships, size, and locomotor behavior have an effect on the shape of the pectoral girdle in anurans, but there are differences in the relative impact of these factors between the bones of this skeletal unit. Remarkable shape diversity has been observed within locomotor groups indicating many-to-one mapping of form onto function. Significant shape differences have mainly been related to the overall pectoral girdle geometry and the shape of the coracoid. Most prominent shape differences have been found between burrowing and nonburrowing species with headfirst and backward burrowing species significantly differing from one another and from the other locomotor groups. The pectoral girdle shapes of burrowing species have generally larger moment arms for (simulated) humerus retractor muscles across the shoulder joint, which might be an adaptation to the burrowing behavior. The mechanisms of how the moment arms were enlarged differed between species and were associated with differences in the reaction of the coracoid to simulated loading by physiologically relevant forces.

Codon usage pattern and evolutionary forces of mitochondrial ND genes among orders of class Amphibia

In this study, we used a bioinformatics approach to analyze the nucleotide composition and pattern of synonymous codon usage in mitochondrial ND genes in three amphibian groups, that is, orders Anura, Caudata, and Gymnophiona to identify the commonality and the differences of codon usage as no research work was reported yet. The high value of the effective number of codons revealed that the codon usage bias (CUB) was low in mitochondrial ND genes among the orders. Nucleotide composition analysis suggested that for each gene, the compositional features differed among Anura, Caudata, and Gymnophiona and the GC content was lower than AT content. Furthermore, a highly significant difference (p < .05) for GC content was found in each gene among the orders. The heat map showed contrasting patterns of codon usage among different ND genes. The regression of GC12 on GC3 suggested a narrow range of GC3 distribution and some points were located in the diagonal, indicating both mutation pressure and natural selection might influence the CUB. Moreover, the slope of the regression line was less than 0.5 in all ND genes among orders, indicating natural selection might have played the dominant role whereas mutation pressure had played a minor role in shaping CUB of ND genes across orders.

Comparative morphology of the humerus in forward-burrowing frogs

Burrowing is one of the many locomotor modes of frogs (order Anura) and is found within many clades. Burrowing is generally categorized into two groups: forward-burrowing and backward-burrowing. While forward-burrowing is more rare than backward-burrowing, we show that it has evolved independently at least eight times across anurans and is correlated with distinct features of the external and internal anatomy. The shape of the humerus is especially important for forward-burrowing, as many species use their forelimbs for digging. Using X-ray computed tomography data, we characterize shape variation in the humerus, including three-dimensional (3D) morphometrics, assess the morphology of muscles related to this variation in the humerus, and discuss the mechanical and evolutionary consequences of our results. We show that the humeri of most forward-burrowing frogs are morphologically distinct from those of non-forward-burrowers, including features such as a curved and thick diaphysis, the presence of a pronounced ventral crest, and relatively large epicondyles and humeral head. Our findings also suggest that pectoral muscle anatomy differs substantially among burrowing modes in frogs. This work provides a framework for predicting locomotor modes in taxa for which the natural history is poorly known as well as extinct taxa.

Unveiling the roles of interspecific competition and local adaptation in phenotypic differentiation of parapatric frogs

Understanding how ecological processes affect phenotypic evolution has been and continues to be an important goal of ecology and evolutionary biology. Interspecific competition for resources can be a selective force driving phenotypic differentiation that reduces competition among sympatric species (character divergence), enabling closely-related species to coexist. However, although patterns of character divergence are well documented in both empirical and theoretical researches, how local adaptation to abiotic environment affects trait evolution in the face of interspecific competition is less known. Here, we investigate how patterns in morphological traits of 2 parapatric frog species, Feirana quadranus and F. taihangnica, vary among allopatric and sympatric regions using range-wide data derived from extensive field surveys. Feirana quadranus was overall larger than F. taihangnica in body size (i.e., snout–vent length [SVL]), and the difference between SVL of both species in sympatry was larger than that in allopatry. From allopatry to sympatry, the 2 species diverged in foot and hand traits, but converged in eye size and interorbital span, even when we controlled for the effects of geographic gradients. Sympatric divergence in SVL, hand and foot traits is likely acting as a case of evolutionary shift caused by interspecific competition. In contrast, sympatric convergence of eye-related traits may derive at least partly from adaptation to local environments. These results imply the relative roles of interspecific competition and local adaptation in shaping phenotypic diversification. Our findings illustrate how traits evolve in parapatric species pair due to sympatric divergent and convergent evolution. It thus provides insights into understanding underlying evolutionary processes of parapatric species, that is, competition and local adaptation.

Evolution of the Unique Anuran Pelvic and Hind limb Skeleton in Relation to Microhabitat, Locomotor Mode, and Jump Performance

Anurans (frogs and toads) have a unique pelvic and hind limb skeleton among tetrapods. Although their distinct body plan is primarily associated with saltation, anuran species vary in their primary locomotor mode (e.g., walkers, hoppers, jumpers, and swimmers) and are found in a wide array of microhabitats (e.g., burrowing, terrestrial, arboreal, and aquatic) with varying functional demands. Given their largely conserved body plan, morphological adaptation to these diverse niches likely results from more fine-scale morphological change. Our study determines how shape differences in Anura's unique pelvic and hind limb skeletal structures vary with microhabitat, locomotor mode, and jumping ability. Using microCT scans of preserved specimens from museum collections, we added 3D landmarks to the pelvic and hind limb skeleton of 230 anuran species. In addition, we compiled microhabitat and locomotor data from the literature for these species that span 52 of the 55 families of frogs and ∼210 million years of anuran evolution. Using this robust dataset, we examine the relationship between pelvic and hind limb morphology and phylogenetic history, allometry, microhabitat, and locomotor mode. We find pelvic and hind limb changes associated with shifts in microhabitat ("ecomorphs") and locomotor mode ("locomorphs") and directly relate those morphological changes to the jumping ability of individual species. We also reveal how individual bones vary in evolutionary rate and their association with phylogeny, body size, microhabitat, and locomotor mode. Our findings uncover previously undocumented morphological variation related to anuran ecological and locomotor diversification and link that variation to differences in jumping ability among species.

Underwater compensation for exaggerated weaponry: The role of morphology and environment on crab locomotor performance

Exaggerated morphologies may increase fitness, but they might be costly to bear; heavy weight, for instance, might hinder locomotion. Evidence supporting these costs are sparse because animals that move on land or swim have traits reducing those costs, called compensatory traits. Animals that walk underwater, however, are under different environmental pressures than land animals. Buoyancy, for instance, reduces the effective weight of any object, which could decrease the locomotion costs of carrying exagerrated traits. Hence, underwater species might maintain performance without compensation. To test this, we compared males of the freshwater anomuran Aegla longirostri that bear an exaggerated claw to females (the natural control). We first tested whether the exaggerated claw decreased male locomotor performance. Next, we tested if sexual dimorphism in performance is associated with differences in leg asymmetry, length, and muscle size. Lastly, we tested if large males have proportionally heavier legs than smaller males. Unexpectedly, females are faster than males while also having relatively longer legs than males. Therefore, females might walk faster because of the longer legs, which might be unrelated to the male exaggerated claw. Furthermore, larger males did not have proportionally heavier legs than smaller males, further suggesting no compensation. Hence, even though aeglid's claw weigh ~25% of their total body weight, we did not find evidence for burden or compensation on males. The environment might thus decrease the costs of exaggerated traits.

Terrestriality constrains salamander limb diversification: Implications for the evolution of pentadactyly

Patterns of phenotypic evolution can abruptly shift as species move between adaptive zones. Extant salamanders display three distinct life cycle strategies that range from aquatic to terrestrial (biphasic), to fully aquatic (paedomorphic) and to fully terrestrial (direct development). Life cycle variation is associated with changes in body form such as loss of digits, limb reduction or body elongation. However, the relationships among these traits and life cycle strategy remain unresolved. Here, we use a Bayesian modelling approach to test whether life cycle transitions by salamanders have influenced rates, optima and integration of primary locomotory structures (limbs and trunk). We show that paedomorphic salamanders have elevated rates of limb evolution with optima shifted towards smaller size and fewer digits compared to all other salamanders. Rate of hindlimb digit evolution is shown to decrease in a gradient as life cycles become more terrestrial. Paedomorphs have a higher correlation between hindlimb digit loss and increases in vertebral number, as well as reduced correlations between limb lengths. Our results support the idea that terrestrial plantigrade locomotion constrains limb evolution and, when lifted, leads to higher rates of trait diversification and shifts in optima and integration. The basic tetrapod body form of most salamanders and the independent losses of terrestrial life stages provide an important framework for understanding the evolutionary and developmental mechanisms behind major shifts in ecological zones as seen among early tetrapods during their transition from water to land.