Forecasting potential invaders to prevent future biological invasions worldwide.

The ever-increasing and expanding globalisation of trade and transport underpins the escalating global problem of biological invasions. Developing biosecurity infrastructures is crucial to anticipate and prevent the transport and introduction of invasive alien species. Still, robust and defensible forecasts of potential invaders are rare, especially for species without known invasion history. Here, we aim to support decision-making by developing a quantitative invasion risk assessment tool based on invasion syndromes (i.e., generalising typical attributes of invasive alien species). We implemented a workflow based on 'Multiple Imputation with Chain Equation' to estimate invasion syndromes from imputed datasets of species' life-history and ecological traits and macroecological patterns. Importantly, our models disentangle the factors explaining (i) transport and introduction and (ii) establishment. We showcase our tool by modelling the invasion syndromes of 466 amphibians and reptile species with invasion history. Then, we project these models to amphibians and reptiles worldwide (16,236 species [c.76% global coverage]) to identify species with a risk of being unintentionally transported and introduced, and risk of establishing alien populations. Our invasion syndrome models showed high predictive accuracy with a good balance between specificity and generality. Unintentionally transported and introduced species tend to be common and thrive well in human-disturbed habitats. In contrast, those with established alien populations tend to be large-sized, are habitat generalists, thrive well in human-disturbed habitats, and have large native geographic ranges. We forecast that 160 amphibians and reptiles without known invasion history could be unintentionally transported and introduced in the future. Among them, 57 species have a high risk of establishing alien populations. Our reliable, reproducible, transferable, statistically robust and scientifically defensible quantitative invasion risk assessment tool is a significant new addition to the suite of decision-support tools needed for developing a future-proof preventative biosecurity globally.

Using the IUCN Environmental Impact Classification for Alien Taxa to inform decision-making.

The Environmental Impact Classification for Alien Taxa (EICAT) is an important tool for biological invasion policy and management and has been adopted as an International Union for Conservation of Nature (IUCN) standard to measure the severity of environmental impacts caused by organisms living outside their native ranges. EICAT has already been incorporated into some national and local decision-making procedures, making it a particularly relevant resource for addressing the impact of non-native species. Recently, some of the underlying conceptual principles of EICAT, particularly those related to the use of the precautionary approach, have been challenged. Although still relatively new, guidelines for the application and interpretation of EICAT will be periodically revisited by the IUCN community, based on scientific evidence, to improve the process. Some of the criticisms recently raised are based on subjectively selected assumptions that cannot be generalized and may harm global efforts to manage biological invasions. EICAT adopts a precautionary principle by considering a species' impact history elsewhere because some taxa have traits that can make them inherently more harmful. Furthermore, non-native species are often important drivers of biodiversity loss even in the presence of other pressures. Ignoring the precautionary principle when tackling the impacts of non-native species has led to devastating consequences for human well-being, biodiversity, and ecosystems, as well as poor management outcomes, and thus to significant economic costs. EICAT is a relevant tool because it supports prioritization and management of non-native species and meeting and monitoring progress toward the Kunming-Montreal Global Biodiversity Framework (GBF) Target 6.

Uso de la Clasificación de Impacto Ambiental de los Taxones Exóticos de la UICN para la toma de decisiones Resumen La Clasificación de Impacto Ambiental de los Taxones Exóticos (EICAT, en inglés) es una herramienta importante para las políticas y manejo de las invasiones biológicas y ha sido adoptada como un estándar de la Unión Internacional para la Conservación de la Naturaleza (UICN) para medir la seriedad del impacto ambiental causado por los organismos que viven fuera de su extensión nativa. La EICAT ya ha sido incorporada a algunos procedimientos locales y nacionales de toma de decisiones, lo que la vuelve un recurso particularmente relevante para abordar el impacto de las especies no nativas. Algunos principios conceptuales subyacentes de la EICAT han sido cuestionados recientemente, en particular aquellos relacionados con el uso del principio de precaución. Aunque todavía son relativamente nuevas, las directrices para la aplicación e interpretación de la EICAT tendrán una revisión periódica, basada en evidencia científica, por parte de la comunidad de la UICN para mejorar el proceso. Algunas de las críticas recientes están basadas en suposiciones seleccionadas subjetivamente que no pueden generalizarse y podrían perjudicar los esfuerzos globales para manejar las invasiones biológicas. La EICAT adopta un principio de precaución cuando considera el historial de impacto de una especie en cualquier otro lugar ya que algunos taxones tienen características que podrían volverlos más dañinos. Además, las especies no nativas suelen ser factores de pérdida de bidiversidad, incluso bajo otras presiones. Cuando ignoramos el principio de precaución al abordar el impacto de las especies no nativas, hay consecuencias devastadoras para el bienestar humano, la biodiversidad y los ecosistemas, así como resultados pobres de conservación, y por lo tanto con costos económicos importantes. La EICAT es una herramienta relevante porque respalda la priorización y el manejo de las especies no nativas y ayuda con el cumplimiento y monitoreo del progreso para llegar al objetivo 6 del Marco Mundial de Biodiversidad Kunming­Montreal.

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.

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.

The conservation value of freshwater habitats for frog communities of lowland fynbos.

Amphibians are more threatened than any other vertebrate class, yet evidence for many threats is missing. The Cape lowland fynbos (endemic scrub biome) is threatened by habitat loss, and natural temporary freshwater habitats are removed in favour of permanent impoundments. In this study, we determine amphibian assemblages across different freshwater habitat types with special attention to the presence of invasive fish. We find that anuran communities differ primarily by habitat type, with permanent water habitats having more widespread taxa, while temporary water bodies have more range restricted taxa. Invasive fish are found to have a significant impact on frogs with toads most tolerant of their presence. Temporary freshwater habitats are a conservation priority in the area, and their amphibian assemblages represent endemic taxa that are intolerant of invasive fish. Conservation of a biodiverse amphibian assemblage in lowland fynbos areas will rely on the creation of temporary freshwater habitats, rather than a northern hemisphere pond based solution.

Population genomics and subgenome evolution of the allotetraploid frog Xenopus laevis in southern Africa.

Allotetraploid genomes have two distinct genomic components called subgenomes that are derived from separate diploid ancestral species. Many genomic characteristics such as gene function, expression, recombination, and transposable element mobility may differ significantly between subgenomes. To explore the possibility that subgenome population structure and gene flow may differ as well, we examined genetic variation in an allotetraploid frog-the African clawed frog (Xenopus laevis)-over the dynamic and varied habitat of its native range in southern Africa. Using reduced representation genome sequences from 91 samples from 12 localities, we found no strong evidence that population structure and gene flow differed substantially by subgenome. We then compared patterns of population structure in the nuclear genome to the mitochondrial genome using Sanger sequences from 455 samples from 183 localities. Our results provide further resolution to the geographic distribution of mitochondrial and nuclear diversity in this species and illustrate that population structure in both genomes corresponds roughly with variation in seasonal rainfall and with the topography of southern Africa.

The anatomy of the head muscles in caecilians (Amphibia: Gymnophiona): Variation in relation to phylogeny and ecology?

In limbless fossorial vertebrates such as caecilians (Gymnophiona), head-first burrowing imposes severe constraints on the morphology and overall size of the head. As such, caecilians developed a unique jaw-closing system involving the large and well-developed m. interhyoideus posterior, which is positioned in such a way that it does not significantly increase head diameter. Caecilians also possess unique muscles among amphibians. Understanding the diversity in the architecture and size of the cranial muscles may provide insights into how a typical amphibian system was adapted for a head-first burrowing lifestyle. In this study, we use dissection and non-destructive contrast-enhanced micro-computed tomography (µCT) scanning to describe and compare the cranial musculature of 13 species of caecilians. Our results show that the general organization of the head musculature is rather constant across extant caecilians. However, the early-diverging Rhinatrema bivittatum mainly relies on the 'ancestral' amphibian jaw-closing mechanism dominated by the m. adductores mandibulae, whereas other caecilians switched to the use of the derived dual jaw-closing mechanism involving the additional recruitment of the m. interhyoideus posterior. Additionally, the aquatic Typhlonectes show a greater investment in hyoid musculature than terrestrial caecilians, which is likely related to greater demands for ventilating their large lungs, and perhaps also an increased use of suction feeding. In addition to three-dimensional interactive models, our study provides the required quantitative data to permit the generation of accurate biomechanical models allowing the testing of further functional hypotheses.

Island Hopping through Urban Filters: Anthropogenic Habitats and Colonized Landscapes Alter Morphological and Performance Traits of an Invasive Amphibian.

A prominent feature of the modern era is the increasing spread of invasive species, particularly within island and urban ecosystems, and these occurrences provide valuable natural experiments by which evolutionary and invasion hypotheses can be tested. In this study, we used the invasion route of guttural toads (Sclerophrys gutturalis) from natural-native and urban-native populations (Durban, South Africa) to their urban-invasive and natural-invasive populations (Mauritius and Réunion) to determine whether phenotypic changes that arose once the toads became urbanized in their native range have increased their invasive potential before they were transported (i.e., prior adaptation) or whether the observed changes are unique to the invasive populations. This urban/natural by native/invasive gradient allowed us to examine differences in guttural toad morphology (i.e., body size, hindlimb, and hindfoot length) and performance capacity (i.e., escape speed, endurance, and climbing ability) along their invasion route. Our findings indicate that invasive island populations have reduced body sizes, shorter limbs in relation to snout-vent length, decreased escape speeds, and decreased endurance capacities that are distinct from the native mainland populations (i.e., invasion-derived change). Thus, these characteristics did not likely arise directly from a pre-transport anthropogenic "filter" (i.e., urban-derived change). Climbing ability, however, did appear to originate within the urban-native range and was maintained within the invasive populations, thereby suggesting it may have been a prior adaptation that provided this species with an advantage during its establishment in urban areas and spread into natural forests. We discuss how this shift in climbing performance may be ecologically related to the success of urban and invasive guttural toad populations, as well as how it may have impacted other island-derived morphological and performance phenotypes.

Phenotypic variation in Xenopus laevis tadpoles from contrasting climatic regimes is the result of adaptation and plasticity.

Phenotypic variations between populations often correlate with climatic variables. Determining the presence of phenotypic plasticity and local adaptation of a species to different environments over a large spatial scale can provide insight on the persistence of a species across its range. Amphibians, and in particular their larvae, are good models for studies of phenotypic variation as they are especially sensitive to their immediate environment. Few studies have attempted to determine the mechanisms that drive phenotypic variation between populations of a single amphibian species over a large spatial scale especially across contrasting climatic regimes. The African clawed frog, Xenopus laevis, occurs in two regions with contrasting rainfall regimes in southern Africa. We hypothesised that the phenotypic variation of life-history traits of X. laevis tadpoles emerges from a combination of plastic and genetic responses. We predicted that plasticity would allow the development of tadpoles from both regions in each environment. We also predicted that local adaptation of larval traits would drive the differentiation of reaction norms between populations and lower survival in tadpoles reared away from their home environment. We measured growth, time to metamorphosis, and survival in a reciprocal transplant experiment using outdoor mesocosms. Supporting our prediction, we found that the measured variation of all traits was explained by both adaptation and plasticity. However, the reaction norms differed between populations suggesting adaptive and asymmetric plasticity. All tadpoles experienced lower survival when translocated, but only translocated tadpoles from the winter rainfall region matched survival of local tadpoles. This has implications for the dynamics of translocated X. laevis into novel environments, especially from the winter rainfall region. Our discovery of their asymmetric capacity to overcome novel environmental conditions by phenotypic plasticity alone provides insight into their invasion success.

Is vertebral shape variability in caecilians (Amphibia: Gymnophiona) constrained by forces experienced during burrowing?

Caecilians are predominantly burrowing, elongate, limbless amphibians that have been relatively poorly studied. Although it has been suggested that the sturdy and compact skulls of caecilians are an adaptation to their head-first burrowing habits, no clear relationship between skull shape and burrowing performance appears to exist. However, the external forces encountered during burrowing are transmitted by the skull to the vertebral column, and, as such, may impact vertebral shape. Additionally, the muscles that generate the burrowing forces attach onto the vertebral column and consequently may impact vertebral shape that way as well. Here, we explored the relationships between vertebral shape and maximal in vivo push forces in 13 species of caecilian amphibians. Our results show that the shape of the two most anterior vertebrae, as well as the shape of the vertebrae at 90% of the total body length, is not correlated with peak push forces. Conversely, the shape of the third vertebrae, and the vertebrae at 20% and 60% of the total body length, does show a relationship to push forces measured in vivo. Whether these relationships are indirect (external forces constraining shape variation) or direct (muscle forces constraining shape variation) remains unclear and will require quantitative studies of the axial musculature. Importantly, our data suggest that mid-body vertebrae may potentially be used as proxies to infer burrowing capacity in fossil representatives.

Regional differences in vertebral shape along the axial skeleton in caecilians (Amphibia: Gymnophiona).

Caecilians are elongate, limbless and annulated amphibians that, as far as is known, all have an at least partly fossorial lifestyle. It has been suggested that elongate limbless vertebrates show little morphological differentiation throughout the postcranial skeleton. However, relatively few studies have explored the axial skeleton in limbless tetrapods. In this study, we used µCT data and three-dimensional geometric morphometrics to explore regional differences in vertebral shape across a broad range of caecilian species. Our results highlight substantial differences in vertebral shape along the axial skeleton, with anterior vertebrae being short and bulky, whereas posterior vertebrae are more elongated. This study shows that despite being limbless, elongate tetrapods such as caecilians still show regional heterogeneity in the shape of individual vertebrae along the vertebral column. Further studies are needed, however, to understand the possible causes and functional consequences of the observed variation in vertebral shape in caecilians.

No evidence for innate differences in tadpole behavior between natural, urbanized, and invasive populations.

ABSTRACT: Animals are increasingly challenged to respond to novel or rapidly changing habitats due to urbanization and/or displacement outside their native range by humans. Behavioral differences, such as increased boldness (i.e., propensity for risk-taking), are often observed in animals persisting in novel environments; however, in many cases, it is unclear how these differences arise (e.g., through developmental plasticity or evolution) or when they arise (i.e., at what age or developmental stage). In the Guttural Toad (Sclerophrys gutturalis), adult urban toads from both native and invasive ranges are bolder than conspecifics in natural habitats. Here, we reared Guttural Toad tadpoles in a common garden experiment, and tested for innate differences in boldness across their development and between individuals whose parents and lineage came from rural-native, urban-native, and urban-invasive localities (i.e., origin populations). Tadpoles did not differ in their boldness or in how their boldness changed over ontogeny based on their origin populations. In general, tadpoles typically became less bold as they aged, irrespective of origin population. Our findings indicate that differences in boldness in free-living adult Guttural Toads are not innate in the tadpole stage and we discuss three possible mechanisms driving phenotypic divergence in adult boldness for the focus of future research: habitat-dependent developmental effects on tadpole behavior, decoupled evolution between the tadpole and adult stage, and/or behavioral flexibility, learning, or acclimatization during the adult stage. SIGNIFICANCE STATEMENT: To determine if animals can persist in urban areas or become invasive outside their native ranges, it is important to understand how they adapt to life in the city. Our study investigates if differences in boldness that have been found in adult Guttural Toads (Sclerophrys gutturalis) represent heritable differences that can also be found in early life stages by rearing tadpoles from eggs in a common garden experiment. We did not find any differences in boldness among tadpoles from rural-native, urban-native, and urban-invasive origin populations. Our findings suggest that differences in boldness are not innate and/or that boldness is a behavioral trait that is decoupled between the tadpole and the adult stage.

Finding rare species and estimating the probability that all occupied sites have been found.

Detecting occupied sites of rare species, and estimating the probability that all occupied sites are known within a given area, are desired outcomes for many ecological or conservation projects. Examples include managing all occupied sites of a threatened species or eradicating an emerging invader. Occupied sites may remain undetected because (1) sites where the species potentially occurs had not been searched, and (2) the species could have been overlooked in the searched sites. For rare species, available data are typically scant, making it difficult to predict sites where the species probably occurs or to estimate detection probability in the searched sites. Using the critically endangered Rose's mountain toadlet (Capensibufo rosei), known from only two localities, we outline an iterative process aimed at estimating the probability that any unknown occupied sites remain and maximizing the chance of finding them. This includes fitting a species distribution model to guide sampling effort, testing model accuracy and sampling efficacy using the occurrence of more common proxy species, and estimating detection probability using sites of known presence. The final estimate of the probability that all occupied sites were found incorporates the uncertainties of uneven distribution, relative area searched, and detection probability. Our results show that very few occupied sites of C. rosei are likely to remain undetected. We also show that the probability of an undetected occupied site remaining will always be high for large unsearched areas of potential occurrence, but can be low for smaller areas intended for targeted management interventions. Our approach is especially useful for assessing uncertainty in species occurrences, planning the required search effort needed to reduce probability of unknown occurrence to desired levels, and identifying priority areas for further searches or management interventions.

Invasive Amphibian Gut Microbiota and Functions Shift Differentially in an Expanding Population but Remain Conserved Across Established Populations.

Studies of laboratory animals demonstrate extensive variation of host gut microbiomes and their functional capabilities across populations, but how does anthropogenic change impact the microbiomes of non-model species? The anthropogenic movement of species to novel environments can drastically alter animals' microbiomes; however, factors that shape invasive species gut microbiota during introduction remain relatively unexplored. Through 16S amplicon sequencing on guttural toad (Sclerophrys gutturalis) faecal samples, we determine that residence time does not impact microbiome variation between source and introduced populations. The youngest population (~ 20 years in Cape Town) has the most distinct microbiome and associated functional capabilities, whereas longer residence times (~ 100 years in Réunion and Mauritius) produce less divergent microbial compositional, phylogenetic, and predicted functional diversity and differential abundance from source populations (Durban). Additionally, we show extensive variation of microbial and functional diversity, as well as differential abundance patterns in an expanding introduced population (Cape Town) between core and periphery sites. Contrasting previous studies, we suggest that introduction pathways might be an important factor impacting host microbial divergence. These findings also imply that the microbiome can diverge in accordance with host population dynamics.

The relationship between head shape, head musculature and bite force in caecilians (Amphibia: Gymnophiona).

Caecilians are enigmatic limbless amphibians that, with a few exceptions, all have an at least partly burrowing lifestyle. Although it has been suggested that caecilian evolution resulted in sturdy and compact skulls as an adaptation to their head-first burrowing habits, no relationship between skull shape and burrowing performance has been demonstrated to date. However, the unique dual jaw-closing mechanism and the osteological variability of their temporal region suggest a potential relationship between skull shape and feeding mechanics. Here, we explored the relationships between skull shape, head musculature and in vivo bite forces. Although there is a correlation between bite force and external head shape, no relationship between bite force and skull shape could be detected. Whereas our data suggest that muscles are the principal drivers of variation in bite force, the shape of the skull is constrained by factors other than demands for bite force generation. However, a strong covariation between the cranium and mandible exists. Moreover, both cranium and mandible shape covary with jaw muscle architecture. Caecilians show a gradient between species with a long retroarticular process associated with a large and pennate-fibered m. interhyoideus posterior and species with a short process but long and parallel-fibered jaw adductors. Our results demonstrate the complexity of the relationship between form and function of this jaw system. Further studies that focus on factors such as gape distance or jaw velocity will be needed in order to fully understand the evolution of feeding mechanics in caecilians.

Mechanistic reconciliation of community and invasion ecology.

Community and invasion ecology have mostly grown independently. There is substantial overlap in the processes captured by different models in the two fields, and various frameworks have been developed to reduce this redundancy and synthesize information content. Despite broad recognition that community and invasion ecology are interconnected, a process-based framework synthesizing models across these two fields is lacking. Here we review 65 representative community and invasion models and propose a common framework articulated around six processes (dispersal, drift, abiotic interactions, within-guild interactions, cross-guild interactions, and genetic changes). The framework is designed to synthesize the content of the two fields, provide a general perspective on their development, and enable their comparison. The application of this framework and of a novel method based on network theory reveals some lack of coherence between the two fields, despite some historical similarities. Community ecology models are characterized by combinations of multiple processes, likely reflecting the search for an overarching theory to explain community assembly and structure, drawing predominantly on interaction processes, but also accounting largely for the other processes. In contrast, most models in invasion ecology invoke fewer processes and focus more on interactions between introduced species and their novel biotic and abiotic environment. The historical dominance of interaction processes and their independent developments in the two fields is also reflected in the lower level of coherence for models involving interactions, compared to models involving dispersal, drift, and genetic changes. It appears that community ecology, with a longer history than invasion ecology, has transitioned from the search for single explanations for patterns observed in nature to investigate how processes may interact mechanistically, thereby generating and testing hypotheses. Our framework paves the way for a similar transition in invasion ecology, to better capture the dynamics of multiple alien species introduced in complex communities. Reciprocally, applying insights from invasion to community ecology will help us understand and predict the future of ecological communities in the Anthropocene, in which human activities are weakening species' natural boundaries. Ultimately, the successful integration of the two fields could advance a predictive ecology that is urgently required in a rapidly changing world.

Public Awareness and Perceptions of Invasive Alien Species in Small Towns.

Invasive alien species (IAS) are a growing threat globally and cause a variety of ecological, economic, and social impacts. People can introduce IAS and facilitate their spread, and can also implement, support, or oppose their management. Understanding local knowledge, awareness, and perceptions are therefore crucial if management and policy are to be effective. We administered questionnaires to members of the public in eight small towns along the Berg River Catchment in the biodiverse fynbos biome of South Africa. We aimed to assess: (1) awareness of IAS by the general public, (2) local perceptions of the impacts associated with IAS, (3) whether awareness of IAS is correlated with demographic covariates and IAS density, and (4) people's willingness to detect, report, and support IAS management. Overall, 262 respondents participated in the survey. Most respondents (65%) did not know what IAS are, and 10% were unsure. Many respondents also perceived IAS as beneficial. Using a logistic regression, we found that IAS density, educational level, and gender influenced people's knowledge and perceptions about IAS in the region. There were a small number (4%) of respondents currently detecting and reporting IAS, but many respondents were interested to learn more. We concluded that people living in small towns in the Western Cape of South Africa remain largely unaware of IAS and their impacts. It is crucial to increase awareness-raising initiatives, and build support and engagement in management of IAS in small towns.

Under pressure: the relationship between cranial shape and burrowing force in caecilians (Gymnophiona).

Caecilians are elongate, limbless and annulated amphibians that, with the exception of one aquatic family, all have an at least partly fossorial lifestyle. It has been suggested that caecilian evolution resulted in sturdy and compact skulls with fused bones and tight sutures, as an adaptation to their head-first burrowing habits. However, although their cranial osteology is well described, relationships between form and function remain poorly understood. In the present study, we explored the relationship between cranial shape and in vivo burrowing forces. Using micro-computed tomography (µCT) data, we performed 3D geometric morphometrics to explore whether cranial and mandibular shapes reflected patterns that might be associated with maximal push forces. The results highlight important differences in maximal push forces, with the aquatic Typhlonectes producing a lower force for a given size compared with other species. Despite substantial differences in head morphology across species, no relationship between overall skull shape and push force could be detected. Although a strong phylogenetic signal may partly obscure the results, our conclusions confirm previous studies using biomechanical models and suggest that differences in the degree of fossoriality do not appear to be driving the evolution of head shape.

Sex chromosome degeneration, turnover, and sex-biased expression of sex-linked transcripts in African clawed frogs (Xenopus).

The tempo of sex chromosome evolution-how quickly, in what order, why and how their particular characteristics emerge during evolution-remains poorly understood. To understand this further, we studied three closely related species of African clawed frog (genus Xenopus), that each has independently evolved sex chromosomes. We identified population polymorphism in the extent of sex chromosome differentiation in wild-caught Xenopus borealis that corresponds to a large, previously identified region of recombination suppression. This large sex-linked region of X. borealis has an extreme concentration of genes that encode transcripts with sex-biased expression, and we recovered similar findings in the smaller sex-linked regions of Xenopus laevis and Xenopus tropicalis. In two of these species, strong skews in expression (mostly female-biased in X. borealis, mostly male-biased in X. tropicalis) are consistent with expectations associated with recombination suppression, and in X. borealis, we hypothesize that a degenerate ancestral Y-chromosome transitioned into its contemporary Z-chromosome. These findings indicate that Xenopus species are tolerant of differences between the sexes in dosage of the products of multiple genes, and offer insights into how evolutionary transformations of ancestral sex chromosomes carry forward to affect the function of new sex chromosomes. This article is part of the theme issue 'Challenging the paradigm in sex chromosome evolution: empirical and theoretical insights with a focus on vertebrates (Part I)'.

Burrowing in blindsnakes: A preliminary analysis of burrowing forces and consequences for the evolution of morphology.

Burrowing is a common behavior in vertebrates. An underground life-style offers many advantages but also poses important challenges including the high energetic cost of burrowing. Scolecophidians are a group of morphologically derived subterranean snakes that show great diversity in form and function. Although it has been suggested that leptotyphlopids and anomalepidids mostly use existing underground passageways, typhlopids are thought to create their own burrows. However, the mechanisms used to create burrows and the associated forces that animals may be able to generate remain unknown. Here, we provide the first data on push forces in scolecophidians and compare them with those in some burrowing alethinophidian snakes. Our results show that typhlopids are capable of generating higher forces for a given size than other snakes. The observed differences are not due to variation in body diameter or length, suggesting fundamental differences in the mechanics of burrowing or the way in which axial muscles are used. Qualitative observations of skull and vertebral shape suggest that the higher forces exerted by typhlopids may have impacted the evolution of their anatomy. Our results provide the basis for future studies exploring the diversity of form and function in this fascinating group of animals. Quantitative comparisons of the cranial and vertebral shape in addition to collecting functional and ecological data on a wider array of species would be particularly important to test the patterns described here.