Invading new climates at what cost? Ontogenetic differences in the thermal dependence of metabolic rate in an invasive amphibian.

Global warming can either promote or constrain the invasive potential of alien species. In ectotherm invaders that exhibit a complex life cycle, success is inherently dependent on the capacity of each developmental stage to cope with environmental change. This is particularly relevant for invasive anurans, which disperse on land while requiring water for reproduction. However, it remains unknown how the different life stages respond in terms of energy expenditure under different climate change scenarios. We here quantified the oxygen uptake of frogs at rest (a proxy of the standard metabolic rate) in the aquatic phase (at the tadpole and climax, i.e. during metamorphosis, stages) and in the terrestrial phase (metamorphosed stage) at three environmental temperatures. To do so, we used marsh frogs (Pelophylax ridibundus), an amphibian with the largest invasive range within the palearctic realm and for which their adaptation to global warming might be key to their invasion success. Beyond an increase of metabolic rate with temperature, our data show variation in thermal adaptation across life stages and a higher metabolic cost during metamorphosis. These results suggest that the cost to shift habitat and face changes in temperature may be a constraint on the invasive potential of species with a complex life cycle which may be particularly vulnerable during metamorphosis.

Cranial muscle architecture in wild boar: Does captivity drive ontogenetic trajectories?

The jaw system in mammals is complex and different muscle morphotypes have been documented. Pigs are an interesting group of animals as they are omnivorous and have a bunodont crushing dentition. Moreover, they have interacted with humans for over 10,000 years and grow nearly two orders of magnitude in size. Despite being a model system for studies on cranial form and function, data on the growth of the jaw adductor muscles are scant. Moreover, whether captivity impacts the growth and architecture of the jaw adductors remains unknown. Based on dissection data of the jaw adductors of 45 animals ranging from less than 1 kg to almost 100 kg, we show that muscle masses, muscle fiber lengths, and cross-sectional areas scale as predicted for geometrically similar systems or with slight negative allometry. Only the fiber length of the lateral pterygoid muscle grew with slight positive allometry. Animals raised in captivity in stalls or in an enclosure were overall very similar to wild animals. However, some muscles were larger in captive animals. Interestingly, variation in bite force in captive animals was well predicted by the variation in the size of the superficial masseter muscle relative to the overall jaw adductor mass.

Comparative analysis of osteoderms across the lizard body.

Osteoderms (ODs) are mineralized tissue embedded within the skin and are particularly common in reptiles. They are generally thought to form a protective layer between the soft tissues of the animal and potential external threats, although other functions have been proposed. The aim of this study was to characterize OD variation across the lizard body. Adults of three lizard species were chosen for this study. After whole body CT scanning of each lizard, single ODs were extracted from 10 different anatomical regions, CT scanned, and characterized using sectioning and nanoindentation. Morphological analysis and material characterization revealed considerable diversity in OD structure across the species investigated. The scincid Tiliqua gigas was the only studied species in which ODs had a similar external morphology across the head and body. Greater osteoderm diversity was found in the gerrhosaurid Broadleysaurus major and the scincid Tribolonotus novaeguineae. Dense capping tissue, like that reported for Heloderma, was found in only one of the three species examined, B. major. Osteoderm structure can be surprisingly complex and variable, both among related taxa, and across the body of individual animals. This raises many questions about OD function but also about the genetic and developmental factors controlling OD shape.

Diving into dual functionality: Swim bladder muscles in lionfish for buoyancy and sonic capabilities.

Although the primary function of the swim bladder is buoyancy, it is also involved in hearing, and it can be associated with sonic muscles for voluntary sound production. The use of the swim bladder and associated muscles in sound production could be an exaptation since this is not its first function. We however lack models showing that the same muscles can be used in both movement and sound production. In this study, we investigate the functions of the muscles associated with the swim bladder in different Pteroinae (lionfish) species. Our results indicate that Pterois volitans, P. radiata and Dendrochirus zebra are able to produce long low-frequency hums when disturbed. The deliberate movements of the fin spines during sound production suggest that these sounds may serve as aposematic signals. In P. volitans and P. radiata, hums can be punctuated by intermittent louder pulses called knocks. Analysis of sonic features, morphology, electromyography and histology strongly suggest that these sounds are most likely produced by muscles closely associated with the swim bladder. These muscles originate from the neurocranium and insert on the posterior part of the swim bladder. Additionally, cineradiography supports the hypothesis that these same muscles are involved in altering the swim bladder's length and angle, thereby influencing the pitch of the fish body and participating in manoeuvring and locomotion movements. Fast contraction of the muscle should be related to sound production whereas sustained contractions allows modifications in swim bladder shape and body pitch.

Plastic and genomic change of a newly established lizard population following a founder event.

Understanding how phenotypic divergence arises among natural populations remains one of the major goals in evolutionary biology. As part of competitive exclusion experiment conducted in 1971, 10 individuals of Italian wall lizard (Podarcis siculus (Rafinesque-Schmaltz, 1810)) were transplanted from Pod Kopiste Island to the nearby island of Pod Mrcaru (Adriatic Sea). Merely 35 years after the introduction, the newly established population on Pod Mrcaru Island had shifted their diet from predominantly insectivorous towards omnivorous and changed significantly in a range of morphological, behavioural, physiological and ecological characteristics. Here, we combine genomic and quantitative genetic approaches to determine the relative roles of genetic adaptation and phenotypic plasticity in driving this rapid phenotypic shift. Our results show genome-wide genetic differentiation between ancestral and transplanted population, with weak genetic erosion on Pod Mrcaru Island. Adaptive processes following the founder event are indicated by highly differentiated genomic loci associating with ecologically relevant phenotypic traits, and/or having a putatively adaptive role across multiple lizard populations. Diverged traits related to head size and shape or bite force showed moderate heritability in a crossing experiment, but between-population differences in these traits did not persist in a common garden environment. Our results confirm the existence of sufficient additive genetic variance for traits to evolve under selection while also demonstrating that phenotypic plasticity and/or genotype by environment interactions are the main drivers of population differentiation at this early evolutionary stage.

What makes a great invader? Anatomical traits as predictors of locomotor performance and metabolic rate in an invasive frog.

Invasive species are characterized by their ability to establish and spread in a new environment. In alien populations of anurans, dispersal and fitness-related traits such as endurance, burst performance and metabolism are key to their success. However, few studies have investigated inter-individual variation in these traits and more specifically have attempted to understand the drivers of variation in these traits. Associations of anatomical features may be excellent predictors of variation in performance and could be targets for selection or subject to trade-offs during invasions. In this study, we used marsh frogs (Pelophylax ridibundus), a species that has been introduced in many places outside its native range and which is now colonizing large areas of Western Europe. We first measured the inter-individual variation in resting metabolism, the time and distance they were able to jump until exhaustion, and their peak jump force, and then measured the mass of specific organs and lengths of body parts suspected to play a role in locomotion and metabolism. Among the 5000 bootstrap replicates on body size-corrected variables, our statistical models most often selected the stomach (75.42%), gonads (71.46%) and the kidneys (67.26%) as predictors of inter-individual variation in metabolism, and the gluteus maximus muscle (97.24%) mass was the most frequently selected predictor of jump force. However, endurance was poorly associated with the anatomical traits (R2distance=0.42, R2time=0.37). These findings suggest that selection on these predictors may lead to physiological changes that may affect the colonization, establishment and dispersal of these frogs.

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 Evolution of Diet and Morphology in Insular Lizards: Insights from a Replicated Island Introduction Experiment.

Resource-limited environments may drive the rapid evolution of phenotypic traits and ecological preferences optimizing the exploitation of resources. Very small islands are often characterized by reduced food availability, seasonal fluctuations in resources and strong unpredictability. These features may drive the evolution of phenotypic traits such as high bite forces, allowing animals to exploit a wider variety of the available resources. They may also lead to more generalist dietary patterns in response to food scarcity. However, the lack of predators and competitors on such small islands often also leads to high densities and the evolution of strong sexual dimorphism, which may also drive the evolution of bite force. Here, we take advantage of a unique replicated introduction experiment to test whether lizards introduced into very small islands alter their feeding ecology and use different resources, resulting in the evolution of a large body size, large head size and large bite forces. Our results show that three years after their introduction, the island lizards were larger and had greater bite forces and more pronounced sexual dimorphism. However, the diets were only marginally different between animals from the source population on a very large nearby island and those on the islets. Moreover, distinct differences in diet between animals on the different islets were observed, suggesting that the local environment is a strong driver of resource use. Overall, lizards with absolutely and relatively (adjusted for body size) large bite forces did eat larger and harder prey. Taken together, our data suggest that intraspecific competition is an important driver of the rapid evolution of bite force, which may allow these lizards to exploit the scarce and fluctuating resources on the islets. Whether or not lizards will evolve to include other types of food such as plants in their diet, facilitated by their large bite forces, remains to be explored in future studies.

Shifts in the Thermal Dependence of Locomotor Performance across an Altitudinal Gradient in Native Populations of Xenopus laevis.

AbstractEctothermic species are dependent on temperature, which drives many aspects of their physiology, including locomotion. The distribution of the native populations of Xenopus laevis is characterized by an exceptional range in latitude and altitude. Along altitudinal gradients, thermal environments change, and populations experience different temperatures. In this study, we compared critical thermal limits and thermal performance curves of populations from the native range across an altitudinal gradient to test whether optimal temperatures for exertion differ depending on altitude. Data on exertion capacity were collected at six different temperatures (8°C, 12°C, 16°C, 19°C, 23°C, and 27°C) for four populations spanning an altitudinal gradient (60, 1,016, 1,948, and 3,197 m asl). Results show that the thermal performance optimum differs among populations. Populations from cold environments at high altitudes exhibit a lower optimal performance temperature than populations from warmer environments at lower altitudes. The ability of this species to change its optimal temperature for locomotor exertion across extremely different climatic environments within the native range may help explain its exceptional invasive potential. These results suggest that ectothermic species capable of adapting to broad altitudinal ranges may be particularly good at invading novel climatic areas, given their ability to cope with a wide range of variation in environmental temperatures.

Measuring the 3D wake of swimming snakes (Natrix tessellata) using volumetric particle image velocimetry.

We describe a method for measuring the 3D vortical structures produced by an anguilliform swimmer using volumetric velocimetry. The wake of freely swimming dice snakes (Natrix tessellata) was quantified, revealing the creation of multiple vortices along the body of the snake due to its undulation. The 3D structure of the vortices generally consisted of paired vortex tubes, some of which were linked together to form a hairpin structure. The observations match predictions from computational fluid dynamic studies of other anguilliform swimmers. Quantitative measurements allowed us to study vortex circulation and size, and global kinetic energy of the flow, which varied with swimming speed, vortex topology and individual characteristics. Our findings provide a baseline for comparing wake structures of snakes with different morphologies and ecologies and investigating the energetic efficiency of anguilliform swimming.

May future climate change promote the invasion of the marsh frog? An integrative thermo-physiological study.

Climate change and invasive species are two major drivers of biodiversity loss and their interaction may lead to unprecedented further loss. Invasive ectotherms can be expected to tolerate temperature variation because of a broad thermal tolerance and may even benefit from warmer temperatures in their new ranges that better match their thermal preference. Multi-trait studies provide a valuable approach to elucidate the influence of temperature on the invasion process and offer insights into how climatic factors may facilitate or hinder the spread of invasive ectotherms. We here used marsh frogs, Pelophylax ridibundus, a species that is invading large areas of Western Europe but whose invasive potential has been underestimated. We measured the maximal and minimal temperatures to sustain physical activity, the preferred temperature, and the thermal dependence of their stamina and jumping performance in relation to the environmental temperatures observed in their invasive range. Our results showed that marsh frogs can withstand body temperatures that cover 100% of the annual temperature variation in the pond they live in and 77% of the observed current annual air temperature variation. Their preferred body temperature and performance optima were higher than the average temperature in their pond and the average air temperature experienced under the shade. These data suggest that invasive marsh frogs may benefit from a warmer climate. Broad thermal tolerances, combined with high thermal preferences and traits maximised at high temperatures, may allow this species to expand their activity period and colonise underexploited shaded habitat, thereby promoting their invasion success.

Show Me Your Teeth And I Will Tell You What You Eat: Differences in Tooth Enamel in Snakes with Different Diets.

Teeth are composed of the hardest tissues in the vertebrate body and have been studied extensively to infer diet in vertebrates. The morphology and structure of enamel is thought to reflect feeding ecology. Snakes have a diversified diet, some species feed on armored lizards, others on soft invertebrates. Yet, little is known about how tooth enamel, and specifically its thickness, is impacted by diet. In this study, we first describe the different patterns of enamel distribution and thickness in snakes. Then, we investigate the link between prey hardness and enamel thickness and morphology by comparing the dentary teeth of 63 species of snakes. We observed that the enamel is deposited asymmetrically at the antero-labial side of the tooth. Both enamel coverage and thickness vary a lot in snakes, from species with thin enamel, only at the tip of the tooth to a full facet covered with enamel. There variations are related with prey hardness: snakes feeding on hard prey have a thicker enamel and a lager enamel coverage while species. Snakes feeding on softer prey have a thin enamel layer confined to the tip of the tooth.

Comparative anatomy and functional implications of variation in the buccal mass in coleoid cephalopods.

In contrast to the well-studied articulated vertebrate jaws, the structure and function of cephalopod jaws remains poorly known. Cephalopod jaws are unique as the two jaw elements do not contact one another, are embedded in a muscular mass and connected through a muscle joint. Previous studies have described the anatomy of the buccal mass muscles in cephalopods and have proposed variation in muscle volume depending on beak shape. However, the general structure of the muscles has been suggested to be similar in octopuses, squids, and cuttlefish. Here we provide a quantitative analysis of the variation in the buccal mass of coleoids using traditional dissections, histological sections and contrast-enhanced computed tomography scans. Our results show that the buccal mass is composed of four main homologous muscles present in both decapodiforms and octopodiforms as suggested previously. However, we also report the presence of a muscle uniquely present in octopodiforms (the postero-lateral mandibular muscle). Our three dimensional reconstructions and quantitative analyses of the buccal mass muscles pave the way for future functional analyses allowing to better model jaw closing in coleoids. Finally, our results suggest differences in beak and muscle function that need to be validated using future in vivo functional analyses.

Pedomorphosis in the ancestry of marsupial mammals.

Within mammals, different reproductive strategies (e.g., egg laying, live birth of extremely underdeveloped young, and live birth of well-developed young) have been linked to divergent evolutionary histories. How and when developmental variation across mammals arose is unclear. While egg laying is unquestionably considered the ancestral state for all mammals, many long-standing biases treat the extreme underdeveloped state of marsupial young as the ancestral state for therian mammals (clade including both marsupials and placentals), with the well-developed young of placentals often considered the derived mode of development. Here, we quantify mammalian cranial morphological development and estimate ancestral patterns of cranial shape development using geometric morphometric analysis of the largest comparative ontogenetic dataset of mammals to date (165 specimens, 22 species). We identify a conserved region of cranial morphospace for fetal specimens, after which cranial morphology diversified through ontogeny in a cone-shaped pattern. This cone-shaped pattern of development distinctively reflected the upper half of the developmental hourglass model. Moreover, cranial morphological variation was found to be significantly associated with the level of development (position on the altricial-precocial spectrum) exhibited at birth. Estimation of ancestral state allometry (size-related shape change) reconstructs marsupials as pedomorphic relative to the ancestral therian mammal. In contrast, the estimated allometries for the ancestral placental and ancestral therian were indistinguishable. Thus, from our results, we hypothesize that placental mammal cranial development most closely reflects that of the ancestral therian mammal, while marsupial cranial development represents a more derived mode of mammalian development, in stark contrast to many interpretations of mammalian evolution.

Armed to the teeth: The underestimated diversity in tooth shape in snakes and its relation to feeding behavior and diet.

The structure, composition, and shape of teeth have been related to dietary specialization in many vertebrate species, but comparative studies on snakes' teeth are lacking. Yet, snakes have diverse dietary habits that may impact the shape of their teeth. We hypothesize that prey properties, such as hardness and shape, as well as feeding behavior, such as aquatic or arboreal predation, or holding vigorous prey, impose constraints on the evolution of tooth shape in snakes. We compared the morphology of the dentary teeth of 63 species that cover the phylogenetic and dietary diversity of snakes, using 3D geometric morphometrics and linear measurements. Our results show that prey hardness, foraging substrate, and the main feeding mechanical challenge are important drivers of tooth shape, size, and curvature. Overall, long, slender, curved teeth with a thin layer of hard tissue are observed in species that need to maintain a grip on their prey. Short, stout, less curved teeth are associated with species that undergo high or repeated loads. Our study demonstrates the diversity of tooth morphology in snakes and the need to investigate its underlying functional implications to better understand the evolution of teeth in vertebrates.

Measurement of voluntary bite forces in large carnivores using a semi-automated reward-driven system.

Bite force is a key performance trait of the feeding system, but maximal in vivo bite force has been measured in few large mammals. The alternative, modelling of bite force from anatomy, cannot be validated without in vivo measurements. To overcome existing limitations of ethics, safety and animal well-being, we propose a semi-automated method to obtain voluntary maximum bite forces from large mammals using bite plates that automatically dispense a food reward if an incrementally increasing threshold force value is reached. We validated our method using two Malayan sun bears, two Andean spectacled bears and a lioness. We show that voluntary bite force measurement using positive reinforcement is a non-invasive and reliable method to record maximum voluntary bite force performance in large mammals. Our results further show that in vivo data are critical as modeling efforts from osteology have greatly underestimated bite forces in Andean spectacled bears.

Evolutionary convergence of muscle architecture in relation to locomotor ecology in snakes.

The epaxial muscles in snakes are responsible for locomotion and as such can be expected to show adaptations in species living in different environments. Here, we tested whether the structural units that comprise the superficial epaxial muscles (semispinalis-spinalis, SSP; longissimus dorsi, LD; iliocostalis, IC) were different in animals occupying similar habitats. To do so, we analyzed and compared the muscle architecture (mass, fiber length, and physiological cross-sectional area) of the superficial epaxial muscle segments in snakes that differ in their habitat use (e.g., arboreal, terrestrial, and aquatic). Our results showed that arboreal species have on average longer muscles and tendons spanning more segments likely important during gap bridging. Moreover, aquatic snakes show relatively heavier semispinalis-spinalis muscles with a greater cross-sectional area. The longissimus dorsi muscles also showed a greater cross-sectional area compared with terrestrial and especially arboreal snakes. Whereas the more strongly developed muscles in aquatic snakes are likely associated with the dense and viscous environment through which they move, the lighter muscles in arboreal snakes may provide an advantage when climbing. Future studies comparing other ecologies (e.g., burrowing snakes) and additional muscle units (e.g., multifidus; hypaxial muscles) are needed to better understand the structural features driving variation in locomotor performance and efficiency in snakes.

The role of cranial osteoderms on the mechanics of the skull in scincid lizards.

Osteoderms (ODs) are calcified organs formed directly within the skin of most major extant tetrapod lineages. Lizards possibly show the greatest diversity in ODs morphology and distribution. ODs are commonly hypothesized to function as a defensive armor. Here we tested the hypothesis that cranial osteoderms also contribute to the mechanics of the skull during biting. A series of in vivo experiments were carried out on three specimens of Tiliqua gigas. Animals were induced to bite a force plate while a single cranial OD was strain gauged. A finite element (FE) model of a related species, Tiliqua scincoides, was developed and used to estimate the level of strain across the same OD as instrumented in the in vivo experiments. FE results were compared to the in vivo data and the FE model was modified to test two hypothetical scenarios in which all ODs were (i) removed from, and (ii) fused to, the skull. In vivo data demonstrated that the ODs were carrying load during biting. The hypothetical FE models showed that when cranial ODs were fused to the skull, the overall strain across the skull arising from biting was reduced. Removing the ODs showed an opposite effect. In summary, our findings suggest that cranial ODs contribute to the mechanics of the skull, even when they are loosely attached.

Evidence for high-performance suction feeding in the Pennsylvanian stem-group holocephalan Iniopera.

The Carboniferous (358.9 to 298.9 Ma) saw the emergence of marine ecosystems dominated by modern vertebrate groups, including abundant stem-group holocephalans (chimaeras and relatives). Compared with the handful of anatomically conservative holocephalan genera alive today-demersal durophages all-these animals were astonishingly morphologically diverse, and bizarre anatomies in groups such as iniopterygians hint at specialized ecological roles foreshadowing those of the later, suction-feeding neopterygians. However, flattened fossils usually obscure these animals' functional morphologies and how they fitted into these important early ecosystems. Here, we use three-dimensional (3D) methods to show that the musculoskeletal anatomy of the uniquely 3D-preserved iniopterygian Iniopera can be best interpreted as being similar to that of living holocephalans rather than elasmobranchs but that it was mechanically unsuited to durophagy. Rather, Iniopera had a small, anteriorly oriented mouth aperture, expandable pharynx, and strong muscular links among the pectoral girdle, neurocranium, and ventral pharynx consistent with high-performance suction feeding, something exhibited by no living holocephalan and never clearly characterized in any of the extinct members of the holocephalan stem-group. Remarkably, in adapting a distinctly holocephalan anatomy to suction feeding, Iniopera is more comparable to modern tetrapod suction feeders than to the more closely related high-performance suction-feeding elasmobranchs. This raises questions about the assumed role of durophagy in the evolution of holocephalans' distinctive anatomy and offers a rare glimpse into the breadth of ecological niches filled by holocephalans in a pre-neopterygian world.

Mammalian forelimb evolution is driven by uneven proximal-to-distal morphological diversity.

Vertebrate limb morphology often reflects the environment due to variation in locomotor requirements. However, proximal and distal limb segments may evolve differently from one another, reflecting an anatomical gradient of functional specialization that has been suggested to be impacted by the timing of development. Here, we explore whether the temporal sequence of bone condensation predicts variation in the capacity of evolution to generate morphological diversity in proximal and distal forelimb segments across more than 600 species of mammals. Distal elements not only exhibit greater shape diversity, but also show stronger within-element integration and, on average, faster evolutionary responses than intermediate and upper limb segments. Results are consistent with the hypothesis that late developing distal bones display greater morphological variation than more proximal limb elements. However, the higher integration observed within the autopod deviates from such developmental predictions, suggesting that functional specialization plays an important role in driving within-element covariation. Proximal and distal limb segments also show different macroevolutionary patterns, albeit not showing a perfect proximo-distal gradient. The high disparity of the mammalian autopod, reported here, is consistent with the higher potential of development to generate variation in more distal limb structures, as well as functional specialization of the distal elements.