Complexity and weak integration promote the diversity of reef fish oral jaws

Major trade-offs often manifest as axes of diversity in organismal functional systems. Overarching trade-offs may result in high trait integration and restrict the trajectory of diversification to be along a single axis. Here, we explore the diversification of the feeding mechanism in coral reef fishes to establish the role of trade-offs and complexity in a spectacular ecological radiation. We show that the primary axis of variation in the measured musculo-skeletal traits is aligned with a trade-off between mobility and force transmission, spanning species that capture prey with suction and those that bite attached prey. We found weak or no covariation between about half the traits, reflecting deviations from the trade-off axis. The dramatic trophic range found among reef fishes occurs along the primary trade-off axis, with numerous departures that use a mosaic of trait combinations to adapt the feeding mechanism to diverse challenges. We suggest that morphological evolution both along and independent of a major axis of variation is a widespread mechanism of diversification in complex systems where a global trade-off shapes major patterns of diversity. Significant additional diversity emerges as systems use weak integration and complexity to assemble functional units with many trait combinations that meet varying ecological demands.

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.

Optimal Gearing of Musculoskeletal Systems

Movement is integral to animal life, and most animal movement is actuated by the same engine: striated muscle. Muscle input is typically mediated by skeletal elements, resulting in musculoskeletal systems that are geared: at any instant, the muscle force and velocity are related to the output force and velocity only via a proportionality constant G, the "mechanical advantage". The functional analysis of such "simple machines" has traditionally centered around this instantaneous interpretation, such that a small vs large G is thought to reflect a fast vs forceful system, respectively. But evidence is mounting that a comprehensive analysis ought to also consider the mechanical energy output of a complete contraction. Here, we approach this task systematically, and deploy the theory of physiological similarity to study how gearing affects the flow of mechanical energy in a minimalist model of a musculoskeletal system. Gearing influences the flow of mechanical energy in two key ways: it can curtail muscle work output, because it determines the ratio between the characteristic muscle kinetic energy and work capacity; and it defines how each unit of muscle work is partitioned into different system energies, that is, into kinetic vs "parasitic" energy such as heat. As a consequence of both effects, delivering maximum work in minimum time and with maximum output speed generally requires a mechanical advantage of intermediate magnitude. This optimality condition can be expressed in terms of two dimensionless numbers that reflect the key geometric, physiological, and physical properties of the interrogated musculoskeletal system, and the environment in which the contraction takes place. Illustrative application to exemplar musculoskeletal systems predicts plausible mechanical advantages in disparate biomechanical scenarios, yields a speculative explanation for why gearing is typically used to attenuate the instantaneous force output ($G_{\text{opt}} \lt 1)$, and predicts how G needs to vary systematically with animal size to optimize the delivery of mechanical energy, in superficial agreement with empirical observations. A many-to-one mapping from musculoskeletal geometry to mechanical performance is identified, such that differences in G alone do not provide a reliable indicator for specialization for force vs speed-neither instantaneously, nor in terms of mechanical energy output. The energy framework presented here can be used to estimate an optimal mechanical advantage across variable muscle physiology, anatomy, mechanical environment, and animal size, and so facilitates investigation of the extent to which selection has made efficient use of gearing as a degree of freedom in musculoskeletal "design."

Muscle-Tendon Unit Properties in Mice Bred for High Levels of Voluntary Running: Novel Physiologies, Coadaptation, Trade-Offs, and Multiple Solutions in the Evolution of Endurance Running

AbstractMuscle-tendon unit (MTU) morphology and physiology are likely major determinants of locomotor performance and therefore Darwinian fitness. However, the relationships between underlying traits, performance, and fitness are complicated by phenomena such as coadaptation, multiple solutions, and trade-offs. Here, we leverage a long-running artificial selection experiment in which mice have been bred for high levels of voluntary running to explore MTU adaptation, as well as the role of coadaptation, multiple solutions, and trade-offs, in the evolution of endurance running. We compared the morphological and contractile properties of the triceps surae complex, a major locomotor MTU, in four replicate selected lines to those of the triceps surae complex in four replicate control lines. All selected lines have lighter and shorter muscles, longer tendons, and faster muscle twitch times than all control lines. Absolute and normalized maximum shortening velocities and contractile endurance vary across selected lines. Selected lines have similar or lower absolute velocities and higher endurance than control lines. However, normalized shortening velocities are both higher and lower in selected lines than in control lines. These findings potentially show an interesting coadaptation between muscle and tendon morphology and muscle physiology, highlight multiple solutions for increasing endurance running performance, demonstrate that a trade-off between muscle speed and endurance can arise in response to selection, and suggest that a novel physiology may sometimes allow this trade-off to be circumvented.

Unexpectedly uneven distribution of functional trade-offs explains cranial morphological diversity in carnivores

Functional trade-offs can affect patterns of morphological and ecological evolution as well as the magnitude of morphological changes through evolutionary time. Using morpho-functional landscape modelling on the cranium of 132 carnivore species, we focused on the macroevolutionary effects of the trade-off between bite force and bite velocity. Here, we show that rates of evolution in form (morphology) are decoupled from rates of evolution in function. Further, we found theoretical morphologies optimising for velocity to be more diverse, while a much smaller phenotypic space was occupied by shapes optimising force. This pattern of differential representation of different functions in theoretical morphological space was highly correlated with patterns of actual morphological disparity. We hypothesise that many-to-one mapping of cranium shape on function may prevent the detection of direct relationships between form and function. As comparatively only few morphologies optimise bite force, species optimising this function may be less abundant because they are less likely to evolve. This, in turn, may explain why certain clades are less variable than others. Given the ubiquity of functional trade-offs in biological systems, these patterns may be general and may help to explain the unevenness of morphological and functional diversity across the tree of life.

Many-to-many mapping: A simulation study of how the number of traits and tasks affect the evolution of form and function

Many-to-many mapping of form-to-function posits that multiple morphological and physiological traits affect the performance of multiple tasks in an organism, and that redundancy and multitasking occur simultaneously to shape the evolution of an organism's phenotype. Many-to-many mapping is expected to be ubiquitous in nature, yet little is known about how it influences the evolution of organismal phenotype. The F-matrix is a powerful tool to study these issues because it describes how multiple traits affect multiple tasks. We undertook a simulation study using the F-matrix to test how the number of traits and the number of tasks affect trait integration and evolvability, as well as the relationships among tasks. We found that as the number of traits and/or tasks increases, the relationships between the tasks and the integration between the traits become weaker, and that the evolvability of the traits increases, all resulting in a system that is freer to evolve. We also found that as the number of traits increases, performance tradeoffs tend to become weaker, but only to a point. Our work shows that it is important to consider not only multiple traits, but also the multitude of tasks that those traits carry out when studying form-function relationships. We suggest that evolution of these relationships follows functional lines of least resistance, which are less defined in more complex systems, resulting in a mechanism for diversification.

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.

The radiation continuum and the evolution of frog diversity

Most of life's vast diversity of species and phenotypes is often attributed to adaptive radiation. Yet its contribution to species and phenotypic diversity of a major group has not been examined. Two key questions remain unresolved. First, what proportion of clades show macroevolutionary dynamics similar to adaptive radiations? Second, what proportion of overall species richness and phenotypic diversity do these adaptive-radiation-like clades contain? We address these questions with phylogenetic and morphological data for 1226 frog species across 43 families (which represent >99% of all species). Less than half of frog families resembled adaptive radiations (with rapid diversification and morphological evolution). Yet, these adaptive-radiation-like clades encompassed ~75% of both morphological and species diversity, despite rapid rates in other clades (e.g., non-adaptive radiations). Overall, we support the importance of adaptive-radiation-like evolution for explaining diversity patterns and provide a framework for characterizing macroevolutionary dynamics and diversity patterns in other groups.

Vascularization inside the epidermis of Neotropical anurans (Nobleobatrachia)

Anuran skin is a dynamic organ involved in essential functions that strongly correlate with specific morphological traits. Particularly, gas exchange has been associated with epidermal modifications, such as reduced cell layers and increased vascularization. Here, we describe the epidermal morphology and its association with capillary networks in the dorsal skin of 103 Neotropical anurans (Nobleobatrachia) from different ecomorphs and habitats. Additionally, we examined the lateral and ventral skin for a subset of these species. We report intraepidermal capillaries in (i) dorsal skin of Lepidobatrachus laevis and Lepidobatrachus llanensis (burrowing and semi-aquatic Chacoan species), Hyloscirtus colymba and Hyloscirtus palmeri (arboreal species from humid forests), and Alsodes neuquensis and 15 Telmatobius spp. (aquatic and semi-aquatic species from cold environments); (ii) lateral skin of Boana benitezi and H. colymba (arboreal species from humid forests), and (iii) ventral skin of B. benitezi, H. colymba, Atelognathus patagonicus (aquatic species from cold environments), and four Chacoan species, Chacophrys pierottii, Ceratophrys cranwelli (burrowing/terrestrial species), and Lepidobatrachus asper and L. llanensis (burrowing/semi-aquatic species). Also, verrucae hydrophilicae were observed exclusively in the ventral skin of Leptodactylus fuscus, Leptodactylus laticeps (terrestrial and Chacoan species), and B. benitezi. Regardless of the skin region, the capillaries always penetrate the epidermis from the dermis, while epidermal cell layers are flattened. Our findings support previous hypotheses stating that the environment where species occur influences skin changes related to cutaneous respiration (intraepidermal capillaries in different body regions) and water absorption (intraepidermal capillaries associated with verrucae hydrophilicae within ventral skin). Also, phylogeny might influence the development of these structures, as revealed by the presence of intraepidermal capillaries in almost all analyzed species of Telmatobius. Finally, the co-occurrence of verrucae hydrophilicae in the ventral skin of hylids from humid forests, and leptodactylids from the subhumid Chacoan region suggest an independent origin.

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.

Parallel And Divergent Morphological Adaptations Underlying The Evolution of Jumping Ability in Ants

Jumping is a rapid locomotory mode widespread in terrestrial organisms. However, it is a rare specialization in ants. Forward jumping has been reported within four distantly related ant genera: Gigantiops, Harpegnathos, Myrmecia, and Odontomachus. The temporal engagement of legs/body parts during jump, however, varies across these genera. It is unknown what morphological adaptations underlie such behaviors and whether jumping in ants is solely driven directly by muscle contraction or additionally relies on elastic recoil mechanism. We investigated the morphological adaptations for jumping behavior by comparing differences in the locomotory musculature between jumping and non-jumping relatives using X-ray micro-CT and 3D morphometrics. We found that the size-specific volumes of the trochanter depressor muscle (scm6) of the middle and hind legs are 3-5 times larger in jumping ants, and that one coxal remotor muscle (scm2) is reduced in volume in the middle and/or hind legs. Notably, the enlargement in the volume of other muscle groups is directly linked to the legs or body parts engaged during the jump. Furthermore, a direct comparison of the muscle architecture revealed two significant differences between jumping vs. non-jumping ants: First, the relative Physiological Cross-Sectional Area (PCSA) of the trochanter depressor muscles of all three legs were larger in jumping ants, except in the front legs of Odontomachus rixosus and Myrmecia nigrocincta; second, the relative muscle fiber length was shorter in jumping ants compared to non-jumping counterparts, except in the front legs of O. rixosus and M. nigrocincta. These results suggest that the difference in relative muscle volume in jumping ants is largely invested in the area (PCSA), and not in fiber length. There was no clear difference in the pennation angle between jumping and non-jumping ants. Additionally, we report that the hind leg length relative to body length was longer in jumping ants. Based on direct comparison of the observed vs. possible work and power output during jumps, we surmise that direct muscle contractions suffice to explain jumping performance in three species, except for O. rixosus, where the lack of data on jumping performance prevents us from drawing definitive conclusions for this particular species. We suggest that increased investment in jumping-relevant musculature is a primary morphological adaptation that separates jumping from non-jumping ants. These results elucidate the common and idiosyncratic morphological changes underlying this rare adaptation in ants. まとぅみ (Okinawan language-Uchinaaguchi) (Japanese) РЕЗЮМЕ (Kazakh) ZUSAMMENFASSUNG (German).

Bridging Performance and Adaptive Landscapes to Understand Long-Term Functional Evolution

AbstractUnderstanding functional adaptation demands an integrative framework that captures the complex interactions between form, function, ecology, and evolutionary processes. In this review, we discuss how to integrate the following two distinct approaches to better understand functional evolution: (1) the adaptive landscape approach (ALA), aimed at finding adaptive peaks for different ecologies, and (2) the performance landscape approach (PLA), aimed at finding performance peaks for different ecologies. We focus on the Ornstein-Uhlenbeck process as the evolutionary model for the ALA and on biomechanical modeling to estimate performance for the PLA. Whereas both the ALA and the PLA have each given insight into functional adaptation, separately they cannot address how much performance contributes to fitness or whether evolutionary constraints have played a role in form-function evolution. We show that merging these approaches leads to a deeper understanding of these issues. By comparing the locations of performance and adaptive peaks, we can infer how much performance contributes to fitness in species' current environments. By testing for the relevance of history on phenotypic variation, we can infer the influence of past selection and constraints on functional adaptation. We apply this merged framework in a case study of turtle shell evolution and explain how to interpret different possible outcomes. Even though such outcomes can be quite complex, they represent the multifaceted relations among function, fitness, and constraints.

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.

Using pictographs as traits to explore morphological diversity in sharks

Body shape is a foundational trait on the differences between species. However, morphological measurements can be simplifying and, for many taxa, can be distorted upon preservation or are difficult to collect due to a species' habit or size. Scientific illustrations, or pictographs, provide information on a species' morphology but are rarely used as traits. Here, we demonstrate the use of pictographs using two shark clades: Lamniformes and Carcharhinidae + Sphyrnidae. After collecting 473 pictographs from 67 species across 12 sources, we used landmarking to show that measurements derived from pictographs do not substantially differ from those garnered from specimens. We then used Elliptical Fourier Analysis and principal components analysis to construct a multivariate morphospace. Using global shape measurements, we evaluated whether substantial variability in body shape was introduced by habitat association, endemism, or illustrator. We found that a species' habitat preference strongly influenced the discovery rate of pictographs and the within-species similarity. While illustrations varied within a species, only a limited set of illustrators exhibited significant systematic variability. We also demonstrated the utility of pictographs in two common applications. For ancestral trait reconstruction, we developed a simple extension to estimate body shapes from principal components and, in doing so, observed that the Lamnid body plan diverged from the rest of Lamniformes ~100 MYA. For phylogenetic generalized linear mixed models (PGLMM), we found that the pictographs had greater explanatory power than traditional morphological measurements. We used the PGLMM to show that higher endemism across Carcharhinidae + Sphyrnidae taxa correlates with body shapes that have caudal fins with small heterocercal angles and more pronounced second dorsal/anal fins. We concluded that pictographs are likely an undervalued and easy-to-digitize data source on a species' body shape with numerous established methods for comparing pictographs and assessing variability.

Evolutionary rates and shape variation along the anuran vertebral column with attention to phylogeny, body size, and ecology

The vertebral column is critical to a vertebrate species' flexibility and skeletal support, making vertebrae a clear target for selection. Anurans (frogs and toads) have a unique, truncated vertebral column that appears constrained to provide axial rigidity for efficient jumping. However, no study has examined how presacral vertebrae shape varies among anuran species at the macroevolutionary scale nor how intrinsic (developmental and phylogenetic) and extrinsic (ecological) factors may have influenced vertebrae shape evolution. We used microCT scans and phylogenetic comparative methods to examine the vertebrae of hundreds of anuran species that vary in body size as well as adult and larval ecology. We found variation in shape and evolutionary rates among anuran vertebrae, dispelling any notion that trunk vertebrae evolve uniformly. We discovered the highest evolutionary rates in the cervical vertebrae and in the more caudal trunk vertebrae. We found little evidence for selection pressures related to adult or larval ecology affecting vertebrae evolution, but we did find body size was highly associated with vertebrae shape and microhabitat (mainly burrowing) affected those allometric relationships. Our results provide an interesting comparison to vertebrae evolution in other clades and a jumping-off point for studies of anuran vertebrae evolution and development.

Shifts in morphological covariation and evolutionary rates across multiple acquisitions of the trap-jaw mechanism in Strumigenys

A long-standing question in comparative biology is how the evolution of biomechanical systems influences morphological evolution. The need for functional fidelity implies that the evolution of such systems should be associated with tighter morphological covariation, which may promote or dampen rates of morphological evolution. I examine this question across multiple evolutionary origins of the trap-jaw mechanism in the genus Strumigenys. Trap-jaw ants have latch-mediated, spring-actuated systems that amplify the power output of their mandibles. I use Bayesian estimates of covariation and evolutionary rates to test the hypotheses that the evolution of this high-performance system is associated with tighter morphological covariation in the head and mandibles relative to nontrap-jaw forms and that this leads to shifts in rates of morphological evolution. Contrary to these hypotheses, there is no evidence of a large-scale shift to higher covariation in trap-jaw forms, while different traits show both increased and decreased evolutionary rates between forms. These patterns may be indicative of many-to-one mapping and/or mechanical sensitivity in the trap-jaw LaMSA system. Overall, it appears that the evolution of trap-jaw forms in Strumigenys did not require a correlated increase in morphological covariation, partly explaining the proclivity with which the system has evolved.

Hind limb muscles influence the architectural properties of long bones in frogs

The Mechanostat Theory states that osteocytes sense both the intensity and directionality of the strains induced by mechanical usage and modulate the bone design accordingly. In long bones, this process may adapt anterior-posterior and lateral-medial strength to their mechanical environment showing regional specificity. Anuran species are ideal for analyzing the muscle-bone relationships related to the different mechanical stresses induced by their many locomotor modes and habitat uses. This work aimed to explore the relationships between indicators of the force of the most relevant muscles to locomotion and the mechanical properties of femur and tibia fibula in preserved samples of three anuran species with different habitat use (aquatic, arboreal) and locomotion modes (swimmer, jumper, walker/climber). For that purpose, we measured the anatomical cross-sectional area of each dissected muscle and correlated it with the moments of inertia and bone strength indices. Significant, species-specific covariations between muscle and bone parameters were observed. Pseudis platensis, the aquatic swimmer, showed the largest muscles, followed by Boana faber, the jumper and Phyllomedusa sauvagii, the walker/climber. As we expected, bigger muscles correlate with bone parameters in all the species. Nevertheless, smaller muscles also play an important role in bone design. In aquatic species, muscle interaction enhances mostly lateral bending strength throughout the femur and lateral and antero-posterior bending strength in the tibia fibula. In the jumper species, muscles affected the femur and tibia fibula mostly in anterior-posterior bending. In the walker/climber species, responses involving both antero-posterior and lateral bending strengths were observed in the femur and tibia fibula. These results show that bones will be more or less resistant to lateral and antero-posterior bending according to the different mechanical challenges of locomotion in aquatic vs. arboreal habitats. This study provides new evidence of the muscle-bone relationships in three frog species associated with their different locomotion and habitat uses, highlighting the crucial role of muscle in determining the architectural properties of bones.

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.

Phylogenetic analysis of adaptation in comparative physiology and biomechanics: overview and a case study of thermal physiology in treefrogs

Comparative phylogenetic studies of adaptation are uncommon in biomechanics and physiology. Such studies require data collection from many species, a challenge when this is experimentally intensive. Moreover, researchers struggle to employ the most biologically appropriate phylogenetic tools for identifying adaptive evolution. Here, we detail an established but greatly underutilized phylogenetic comparative framework - the Ornstein-Uhlenbeck process - that explicitly models long-term adaptation. We discuss challenges in implementing and interpreting the model, and we outline potential solutions. We demonstrate use of the model through studying the evolution of thermal physiology in treefrogs. Frogs of the family Hylidae have twice colonized the temperate zone from the tropics, and such colonization likely involved a fundamental change in physiology due to colder and more seasonal temperatures. However, which traits changed to allow colonization is unclear. We measured cold tolerance and characterized thermal performance curves in jumping for 12 species of treefrogs distributed from the Neotropics to temperate North America. We then conducted phylogenetic comparative analyses to examine how tolerances and performance curves evolved and to test whether that evolution was adaptive. We found that tolerance to low temperatures increased with the transition to the temperate zone. In contrast, jumping well at colder temperatures was unrelated to biogeography and thus did not adapt during dispersal. Overall, our study shows how comparative phylogenetic methods can be leveraged in biomechanics and physiology to test the evolutionary drivers of variation among species.

Evolutionary analyses of visual opsin genes in frogs and toads: Diversity, duplication, and positive selection

Among major vertebrate groups, anurans (frogs and toads) are understudied with regard to their visual systems, and little is known about variation among species that differ in ecology. We sampled North American anurans representing diverse evolutionary and life histories that likely possess visual systems adapted to meet different ecological needs. Using standard molecular techniques, visual opsin genes, which encode the protein component of visual pigments, were obtained from anuran retinas. Additionally, we extracted the visual opsins from publicly available genome and transcriptome assemblies, further increasing the phylogenetic and ecological diversity of our dataset to 33 species in total. We found that anurans consistently express four visual opsin genes (RH1, LWS, SWS1, and SWS2, but not RH2) even though reported photoreceptor complements vary widely among species. The proteins encoded by these genes showed considerable sequence variation among species, including at sites known to shift the spectral sensitivity of visual pigments in other vertebrates and had conserved substitutions that may be related to dim-light adaptation. Using molecular evolutionary analyses of selection (dN/dS) we found significant evidence for positive selection at a subset of sites in the dim-light rod opsin gene RH1 and the long wavelength sensitive cone opsin LWS. The function of sites inferred to be under positive selection are largely unknown, but a few are likely to affect spectral sensitivity and other visual pigment functions based on proximity to previously identified sites in other vertebrates. We also found the first evidence of visual opsin duplication in an amphibian with the duplication of the LWS gene in the African bullfrog, which had distinct LWS copies on the sex chromosomes suggesting the possibility of sex-specific visual adaptation. Taken together, our results indicate that ecological factors, such as habitat and life history, as well as behavior, may be driving changes to anuran visual systems.

Hand and foot musculature of Sooglossoidea: synapomorphies, convergences and hind limb digging behaviour in anurans

We describe the hand and foot musculature of the fossorial Indian purple frog, Nasikabatrachus sahyadrensis, and compare it to other members of Sooglossoidea: the Seychellean sooglossid genera Sechellophryne and Sooglossus. Due to the key phylogenetic position of Sooglossoidea, we compare its members with the diversity of Anura and define 52 characters from the hand and foot musculature, among which 26 are novel hypotheses of homology. We found several synapomorphies for Sooglossus, Sooglossidae, Nasikabatrachidae and Sooglossoidea. Additionally, we (1) propose synapomorphies for diverse anuran clades at different taxonomic levels, (2) re-evaluate the identity of some conflicting plantar and palmar muscles in the context of Batrachia and (3) discuss putative adaptations to hind limb digging behaviour resulting from morphological convergences. The lack of a clear pattern of convergences among hind limb digging species suggests the occurrence of a phenomenon of many-to-one mapping from form to function.

The Influence of Fine-Scale Grazing Heterogeneity on Dung Beetle Assemblages: What Trait Analysis Teaches Us

Livestock grazing puts major anthropogenic pressure on biological communities worldwide. Not all species are expected to be affected in the same way, and the impacts will depend on species' traits. Focusing on traits thus helps identify the mechanisms underlying changes in community composition under grazing pressures. We investigated how fine-scale grazing heterogeneity affects the trait composition and diversity of dung beetle assemblages in Western Europe. We sampled dung beetles in habitat patches differing in terms of grazing intensity within rangelands of two distinct biogeographical areas: a Mediterranean lowland steppe and Western alpine meadows. We measured five morphological traits expected to respond to the local-scale filtering pressure exerted by variations in grazing intensity. Using individual-based data, we assessed responses in terms of single-trait mean values in communities and complementary trait diversity indices. We found strong shifts in trait composition and diversity between the habitat patches. In both study areas, variations in habitat conditions are likely to have filtered the local occurrence and abundance of dung beetles by the mean of traits such as body mass (which have several functional implications), as well as traits linked to underground activity. We hypothesize that fine-scale variation in resource availability (i.e., droppings) and disturbance intensity (i.e., trampling) are key drivers of the observed patterns in species assemblages. Trait richness peaks at moderate grazing intensity in both study areas, suggesting that patches with an intermediated level of available resources and soil disturbance enable individuals with a greater range of autecological requirements to coexist.

Morphology of the limb, shell and head explain the variation in performance and ecology across 14 turtle taxa (12 species)

Given that morphology directly influences the ability of an organism to utilize its habitat and dietary resources, it also influences fitness. Comparing the relationship between morphology, performance and ecology is fundamental to understand how organisms evolve to occupy a wide range of habitats and diets. In turtles, studies have documented important relationships between morphology, performance and ecology, but none was field based or considered limb, shell and head morphology simultaneously. We compared the morphology, performance and ecology of 14 turtle taxa (12 species) in Mexico that range in their affinity to water and in their diet. We took linear measurements of limb, shell and head variables. We measured maximum swimming speed, maximum bite force and how often turtles were encountered on land, and we used stable isotopes to assess trophic position. We used these data to test the following three hypotheses: (1) morphology, performance and ecology covary; (2) limb and shell variables, like hand length, are correlated with swimming speed and the percentage of time spent on land; and (3) head variables, such as head width, are correlated with bite force and stable isotopes. We find support for these hypotheses and provide the first evidence that morphology influences performance and ecology in turtles in the field.

Testing for adaptive radiation: A new approach applied to Madagascar frogs

Adaptive radiation is a key topic at the intersection of ecology and evolutionary biology. Yet the definition and identification of adaptive radiation both remain contentious. Here, we introduce a new approach for identifying adaptive radiations that combines key aspects of two widely used definitions. Our approach compares evolutionary rates in morphology, performance, and diversification between the candidate radiation and other clades. We then apply this approach to a putative adaptive radiation of frogs from Madagascar (Mantellidae). We present new data on morphology and performance from mantellid frogs, then compare rates of diversification and multivariate evolution of size, shape, and performance between mantellids and other frogs. We find that mantellids potentially pass our test for accelerated rates of evolution for shape, but not for size, performance, or diversification. Our results demonstrate that clades can have accelerated phenotypic evolution without rapid diversification (dubbed "adaptive non-radiation"). We also highlight general issues in testing for adaptive radiation, including taxon sampling and the problem of including another adaptive radiation among the comparison clades. Finally, we suggest that similar tests should be conducted on other putative adaptive radiations on Madagascar, comparing their evolutionary rates to those of related clades outside Madagascar. Based on our results, we speculate that older Madagascar clades may show evolutionary patterns more similar to those on a continent than an island.

Clinging ability is related to particular aspects of foot morphology in salamanders

The interaction between morphology, performance, and ecology has long been studied in order to explain variation in the natural world. Within arboreal salamanders, diversification in foot morphology and microhabitat use are thought to be linked by the impact of foot size and shape on clinging and climbing performance, resulting in an ability to access new habitats. We examine whether various foot shape metrics correlate with stationary cling performance and microhabitat to explicitly quantify this performance gradient across 14 species of salamander, including both arboreal and nonarboreal species. Clinging performance did not correlate with foot shape, as quantified by landmark-based geometric morphometrics, nor with microhabitat use. Mass-corrected foot centroid size and foot contact area, on the other hand, correlated positively with clinging performance on a smooth substrate. Interestingly, these foot variables correlated negatively with clinging performance on rough substrates, suggesting the use of multiple clinging mechanisms dependent upon the texture of the surface. These findings demonstrate that centroid size and foot contact area are more functionally relevant for clinging in salamanders than foot shape, suggesting that foot shape need not converge in order to achieve convergent performance. More broadly, our results provide an example of how the quantification of the performance gradient can provide the appropriate lens through which to understand the macroevolution of morphology and ecology.

Convergent patterns of adaptive radiation between island and mainland Anolis lizards

Uncovering convergent and divergent patterns of diversification is a major goal of evolutionary biology. On four Greater Antillean islands, Anolis lizards have convergently evolved sets of species with similar ecologies and morphologies (ecomorphs). However, it is unclear whether closely related anoles from Central and South America exhibit similar patterns of diversification. We generated an extensive morphological data set to test whether mainland Draconura-clade anoles are assignable to the Caribbean ecomorphs. Based on a new classification framework that accounts for different degrees of morphological support, we found morphological evidence for mainland representatives of all six Caribbean ecomorphs and evidence that many ecomorphs have also evolved repeatedly on the mainland. We also found strong evidence that ground-dwelling anoles from both the Caribbean and the mainland constitute a new and distinct ecomorph class. Beyond the ecomorph concept, we show that the island and mainland anole faunas exhibit exceptional morphological convergence, suggesting that they are more similar than previously understood. However, the island and mainland radiations are not identical, indicating that regional differences and historical contingencies can lead to replicate yet variable radiations. More broadly, our findings suggest that replicated radiations occur beyond island settings more often than previously recognized.

The Effect of Locomotion Mode on Body Shape Evolution in Teleost Fishes

Teleost fishes vary in their reliance on median and paired fins (MPF) or undulation of the body (BCF) to generate thrust during straight-line, steady swimming. Previous work indicates that swimming mode is associated with different body shapes, though this has never been empirically demonstrated across the diversity of fishes. As the body does not play as active a mechanical role in steady swimming by MPF swimmers, this may relax constraints and spur higher rates of body shape diversification. We test these predictions by measuring the impact of the dominant steady swimming mode on the evolution of body shape across 2295 marine teleost fishes. Aligning with historical expectations, BCF swimmers exhibit a more elongate, slender body shape, while MPF propulsion is associated with deeper and wider body shapes. However, in contrast to expectations, we find that BCF propulsion is associated with higher morphological diversity and greater variance around trait optima. This surprising result is consistent with the interpretation that stronger functional trade-offs stimulate phenotypic evolution, rather than constrain it.

Locomotory mode transitions alter phenotypic evolution and lineage diversification in an ecologically rich clade of mammals

The relationship between organismal function and form is a cornerstone of biology because functional diversity is key to generating and maintaining ecological diversity. Morphological changes often occur in unison with behavioral or ecological transitions, and this process may foster diversification, but alternately could trap a species on an adaptive peak. We estimated the most comprehensive phylogenetic hypothesis of Murinae, a young (∼15 million years) and diverse (∼700 species) clade of mammals. We then tested for correlated evolution among four morphological traits with potential links to locomotor modes (Arboreal, General, Terrestrial, and Amphibious), then investigated the effects of locomotion on morphological and lineage diversification. We found unique combinations of trait values for each locomotor mode, including strong covariance between the tail and hindfoot lengths of specialized Arboreal and ecologically flexible General species. Low diversification rates and long branch lengths suggest that specialized lineages represent stable evolutionary "cul-de-sacs." General species, characterized by the classic "rat-like" body plan and broad locomotor abilities, have narrow optimal trait values and slow phenotypic evolution, but high lineage diversification rates. Our findings suggest that versatile, generalist forms act as seeds of species diversity and morphological specialization, which together build ecologically diverse radiations.

Patterns of correlations and locomotor specialization in anuran limbs: association with phylogeny and ecology

As anuran saltatory locomotion has specific functional requirements achieved through certain intra- and inter-limb proportions, we analyzed pattern and degree of morphological integration in limbs of ten anuran species to reveal the relationship of shared developmental programs of serially homologous structures and locomotor specialization. Our main objectives were (1) to examine if morphological and functional differences in forelimb and hindlimb were associated with reduced covariation between limbs, (2) and to reveal patterns of correlation between species and the roles played by evolutionary history (phylogeny) and ecology (lifestyle and habitat use). Species with different locomotor behaviours (walking, jumping, hopping, running, climbing, swimming and burrowing) were used. Partial correlations showed that species shared similar patterns of functionally based morphological integration, with increased correlations in elements within limbs and reduced correlations between limbs. This was mainly based on strong correlations between proximal elements, humerus-radioulna and femur-tibiofibula. To test the influence of phylogenetic relationships and ecological demands we used different matrices (correlation similarity matrix, ecological similarity matrix, matrices of phylogenetic distance and morphological distance). The changes in correlation patterns are shown to be dissociated from phylogeny. On the other hand, they are to some extent shaped by habitat use and locomotion, as the species with similar locomotor behaviour also tend to have stronger similarity in integration patterns. The results from this study provide insight into the processes underlying the evolutionary change of anuran limbs, highlighting function as the main factor that shaped morphological integration of the examined species.

Hydrodynamic Simulations of the Performance Landscape for Suction-Feeding Fishes Reveal Multiple Peaks for Different Prey Types

The complex interplay between form and function forms the basis for generating and maintaining organismal diversity. Fishes that rely on suction-feeding for prey capture exhibit remarkable phenotypic and trophic diversity. Yet the relationships between fish phenotypes and feeding performance on different prey types are unclear, partly because the morphological, biomechanical, and hydrodynamic mechanisms that underlie suction-feeding are complex. Here we demonstrate a general framework to investigate the mapping of multiple phenotypic traits to performance by mapping kinematic variables to suction-feeding capacity. Using a mechanistic model of suction-feeding that is based on core physical principles, we predict prey capture performance across a broad range of phenotypic trait values, for three general prey types: mollusk-like prey, copepod-like prey, and fish-like prey. Mollusk-like prey attach to surfaces, copepod-like prey attempt to escape upon detecting the hydrodynamic disturbance produced by the predator, and fish-like prey attempt to escape when the predator comes within a threshold distance. This approach allowed us to evaluate suction-feeding performance for any combination of six key kinematic traits, irrespective of whether these trait combinations were observed in an extant species, and to generate a multivariate mapping of phenotype to performance. We used gradient ascent methods to explore the complex topography of the performance landscape for each prey type, and found evidence for multiple peaks. Characterization of phenotypes associated with performance peaks indicates that the optimal kinematic parameter range for suction-feeding on different prey types are narrow and distinct from each other, suggesting different functional constraints for the three prey types. These performance landscapes can be used to generate hypotheses regarding the distribution of extant species in trait space and their evolutionary trajectories toward adaptive peaks on macroevolutionary fitness landscapes.

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.

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.

The Evolutionary Dynamics of Mechanically Complex Systems

Animals use a diverse array of motion to feed, escape predators, and reproduce. Linking morphology, performance, and fitness is a foundational paradigm in organismal biology and evolution. Yet, the influence of mechanical relationships on evolutionary diversity remains unresolved. Here, I focus on the many-to-one mapping of form to function, a widespread, emergent property of many mechanical systems in nature, and discuss how mechanical redundancy influences the tempo and mode of phenotypic evolution. By supplying many possible morphological pathways for functional adaptation, many-to-one mapping can release morphology from selection on performance. Consequently, many-to-one mapping decouples morphological and functional diversification. In fish, for example, parallel morphological evolution is weaker for traits that contribute to mechanically redundant motions, like suction feeding performance, than for systems with one-to-one form-function relationships, like lower jaw lever ratios. As mechanical complexity increases, historical factors play a stronger role in shaping evolutionary trajectories. Many-to-one mapping, however, does not always result in equal freedom of morphological evolution. The kinematics of complex systems can often be reduced to variation in a few traits of high mechanical effect. In various different four-bar linkage systems, for example, mechanical output (kinematic transmission) is highly sensitive to size variation in one or two links, and insensitive to variation in the others. In four-bar linkage systems, faster rates of evolution are biased to traits of high mechanical effect. Mechanical sensitivity also results in stronger parallel evolution-evolutionary transitions in mechanical output are coupled with transition in linkages of high mechanical effect. In other words, the evolutionary dynamics of complex systems can actually approximate that of simpler, one-to-one systems when mechanical sensitivity is strong. When examined in a macroevolutionary framework, the same mechanical system may experience distinct selective pressures in different groups of organisms. For example, performance tradeoffs are stronger for organisms that use the same mechanical structure for more functions. In general, stronger performance tradeoffs result in less phenotypic diversity in the system and, sometimes, a slower rate of evolution. These macroevolutionary trends can contribute to unevenness in functional and lineage diversity across the tree of life. Finally, I discuss how the evolution of mechanical systems informs our understanding of the relative roles of determinism and contingency in evolution.