Evolutionary Biology

The diversity of body shapes is one of the most prominent features of phenotypic variation in mammals. Yet, mammalian body shapes are poorly quantified and the underlying components contributing to its diversity as well as its relationship to other components of the skeleton are rarely tested. Here, we use lagomorphs (hares, rabbits and pikas) as a model system to (1) investigate which components of the skeleton contributed the most to body shape diversity, (2) examine the relationships between body shape and relative limb lengths, and (3) test how body size, ecotype, burrowing behavior, and locomotor mode influenced variation in lagomorph body shape and appendicular morphology. We quantified the body shape and functional proxies of the appendicular skeleton in 40 lagomorph species from osteological specimens held at museum collections. Using phylogenetic comparative methods, we found the relative length of the ribs and elongation or shortening of the thoracic and lumbar regions contributed the most to body shape evolution across lagomorphs. Second, we found that only leporids (hares and rabbits) exhibited a significant relationship between limb length and body shape, where more elongate species exhibit relatively shorter forelimbs and hindlimbs. Lastly, we found that models incorporating body size were the best predictors of lagomorph body shape and the majority of the appendicular traits, whereas models incorporating burrowing behavior and locomotor mode were largely poor fits. Broadly, these results indicate that larger lagomorphs tend to exhibit more robust body shapes with longer, more gracile forelimbs, whereas smaller lagomorphs tend to exhibit more elongate body shapes with shorter, more robust forelimbs. Overall, this work contributes to the growing understanding of mammalian body shape evolution and demonstrates the importance of not omitting body size in ecomorphological analyses.

Domestic dogs (Canis familiaris) display more morphological variation than any other mammal. Cranial morphology has been extensively studied, as have the relationships with function, development, genetics, veterinary medicine, and breed welfare. Postcrania remain comparatively understudied, despite well-documented breed-specific predispositions to musculoskeletal disease. Here, we apply three-dimensional landmark-free morphometrics to quantify the shape of 743 elements from 213 dogs, including the scapula, humerus, radius, ulna, pelvic girdle, femur, tibia, and fibula. We assess integration among limb elements and investigate drivers of shape variation within and between breeds. Across most breeds, limb bone shape is strikingly similar. Dachshunds, however, exhibit distinct morphology across all elements and one to two orders of magnitude greater variation than any other breed. Despite this disparity, integration remains high between all element pairs. Remarkably, we find no significant relationship between bone shape and body mass, age, or pathology, but comparison with historic specimens reveals marked changes in dachshund long bone shape over the past [~]150 years. These extreme differences are not shared by other sampled chondrodysplastic breeds, underscoring the need to understand morphological diversity beyond simple categorisation. These findings provide a quantitative framework for linking postcranial morphology with function, disease risk, and evidence-based improvements to canine welfare.

Understanding how anatomical structures evolve requires disentangling the roles of integration and modularity in shaping morphological variation. The vertebral column, a serially repeated and regionally differentiated structure, provides a powerful system for investigating these processes. Here, we examine how vertebral morphology evolves in relation to whole-body elongation across the adaptive radiation of Lake Malawi cichlid fishes. We tested for evolutionary integration between the precaudal and caudal domains, as well as assessed the contributions of vertebral count, centrum shape, and intervertebral spacing on body elongation. We find strong evolutionary integration between precaudal and caudal vertebral shape, with both vertebral shapes varying along shared axes of multivariate shape change. Despite this, precaudal and caudal vertebral counts evolve independently, indicating a decoupling between the evolution of identity and morphology. Whole-body elongation is significantly associated with coordinated changes in vertebral and rib morphology, including proportional increases in centrum size, posterior displacement of neural and haemal spines, and increased rib curvature. In contrast, centrum elongation and intervertebral spacing do not independently explain body elongation beyond vertebral counts. These results demonstrate that body elongation in cichlids necessitates integrated, multivariate changes in axial morphology. Our findings highlight the importance of morphological integration in facilitating coordinated evolutionary responses in anatomical systems.

Dental morphology and wear patterns provide insight into the dietary adaptations and ecological niches of living and extinct herbivores. Traditional classification statistics such as Linear Discriminant Analysis (LDA) are limited by assumptions of linearity, normality, and homoscedasticity. This study quantifies mesowear, the shape of molar cusps resulting from occlusal wear, and evaluates the performance of non-linear machine learning models in predicting herbivore diets based on geometric morphometric (GMM) data from adult mandibular second molars (M2) in bovids. We applied Generalized Procrustes Analysis and Principal Component Analysis (PCA) to digitized occlusal shape coordinates from 132 M2 specimens across 64 species. Using the resulting principal component scores, we compared the classification accuracy of LDA with three non-linear models: Random Forest, K-Nearest Neighbors, and Gradient Boosting. While LDA achieved a cross-validated accuracy of just 31%, all non-linear models achieved 99% cross-validation accuracy and 90% test accuracy, demonstrating substantially improved performance. Misclassification analyses revealed that non-linear models more effectively captured complex shape differences, particularly among species with overlapping wear patterns. Our findings support the integration of machine learning with geometric morphometrics to quantify mesowear and improve dietary classification, providing a framework for robust paleoecological inference.

Sarita Lake, British Columbia houses a distinctive population of threespine stickleback (Gastrosteus aculeatus L.) with a phenotype characterized by unusually large individuals relative to nearby conspecifics. We tested the hypothesis that members of this population are not isometrically larger but rather exhibit variation in allometric trajectories that reflect changes in developmental timing impacting the developmental-genetic architecture of the phenotype. We used 3D geometric morphometrics to characterize the size and shape of skulls, pectoral girdles and pelvic girdles from a sample of individuals from nearby freshwater and marine populations and compare them to a sample from Sarita Lake. We showed that individuals from the Sarita Lake population are larger in each body region compared to most other populations examined. Further, these individuals have dorsally expanded skulls and relatively robust pelvic armour. We also showed that the relationship between size and shape is differently structured among body regions and is heavily influenced by non-uniform sexually-mediated variation across populations sampled. Our results reflect complex underlying developmental trajectories, and we suggest that the large phenotype observed may be driven by fecundity selection on female size in combination with a limnetic trophic niche and relatively increased predation pressure in Sarita Lake.

The evolution of body shape reflects the interplay between functional constraints and habitat structure. In fishes, cave environments are well known for promoting regressive traits such as eye and pigment loss, yet their influence on overall body form remains poorly understood. Here, we examine patterns of body shape variation in cave- and surface-dwelling trichomycterid catfishes from northeastern Colombia to assess whether consistent associations exist between habitat type and morphology. Using geometric morphometric analyses, we quantified differences in body shape among species inhabiting subterranean and surface environments. Our results reveal significant habitat-associated differentiation in body shape along the main axes of morphological variation. Cave-dwelling species exhibit more elongated and fusiform body shapes, whereas surface-dwelling species tend to show deeper and more robust morphologies. In a functional context, these contrasting body patterns suggest associations with differing locomotor demands imposed by subterranean versus surface habitats. Although we do not explicitly test convergence or performance, the recurrence of similar body shapes among species from different clades occupying comparable habitats is consistent with repeated morphological responses to shared ecological constraints. Research HighligthsO_LIMultivariate shape analyses reveal significant habitat-associated variation in trichomycterid fishes. Recurrent morphological patterns suggest repeated responses potentially mediated by habitat constraints. C_LIO_LIBody shape differs consistently between cave- and surface-dwelling trichomycterids. Cave species exhibit more elongated and fusiform forms, whereas surface species display deeper body configurations. C_LI

How evolutionary and developmental processes interact to determine axes of neural variation that produce behavioural diversity has been debated for many decades, with alternative hypotheses giving differential emphasis to functional coupling, which favours co-evolution, and developmental constraint, which enforces it. A critical omission is data on the genetic architecture of brain size and structure, which more closely illuminates the shared developmental dependencies between components of an integrated system. Here, we exploit ecological divergence between Astatotilapia calliptera and Aulonocara stuartgranti, two closely related cichlid species from Lake Malawi, to explore the genetic architecture of brain evolution. Using computer vision and machine learning techniques to extract volumetric data from micro-tomographic images, we first demonstrate significant divergence in brain composition between these species. Genomic and micro-tomographic imaging data from a population of hybrids generated between the two species were used to investigate genetic factors shaping this differentiation. We show that the majority of brain components are integrated phenotypically in hybrids, but genetic correlations between them are generally weaker. We further show that variation in multiple brain components is associated with variation in largely structure-specific quantitative trait loci, rather than determined by genetic factors with broad effects across the entire brain. These results suggest a genetic architecture that can facilitate modular changes in brain structure, and imply that individual components are independently evolvable.

Mate preferences are often influenced by the magnitude of sexual signals, which are presumed to indicate underlying aspects of signaller quality. Although the perception of these signals depends on sensory processes, the role of perceptual adaptations and constraints in mate assessment is frequently overlooked. Many sensory systems follow Webers law of proportional processing, where discrimination between signals is based upon their proportional, or relative, difference rather than their absolute difference. Because preference strength varies with relative trait magnitude, Webers law could strongly influence sexual selection, changing the coevolution of traits and preferences. Here, we explore the consequences of Webers law for sexual selection using individual-based models, applying Scalar Utility Theory to mate choice. We investigate the coevolution of male ornaments and female preferences under both Fisherian and good genes scenarios, as well as scrutinizing the sexual selection of multiple ornaments and preferences. Including Webers law in these models either reduced ornament exaggeration, or promoted exaggeration and diversification of ornaments and preferences, depending on the costs of choice and how rapidly female survival decreases when preferences evolve away from the naturally selected optimum. These results highlight the importance of perception and cognitive processing in shaping sexual selection and its evolutionary impacts.

Cities are characterized by elevated temperatures, increased pollution, and high-density human populations which often are accompanied by changes in available resources, like food. These shifts have the potential to drive phenotypic divergence in urban wildlife. Functional morphological traits, like body size, can mediate interactions between wildlife and habitat and are closely tied to life history and fitness. While examples of functional morphological variation associated with urbanization are increasing, variation in such traits as a response to urbanization remains unexplored for most taxa. Here, we investigated morphological divergence between urban and rural populations of house mice (Mus musculus domesticus). House mice are globally distributed in diverse habitats and are a model system with a wealth of phenotypic data, making them useful for the study of the impacts of urbanization on morphology. Using a paired replicate design, we sampled urban and rural populations in three distinct metropolitan regions in the eastern United States. We found that body size was smaller in urban populations. Using 3D geometric morphometrics, we also analyzed variation in cranial shape across habitats. Differences in cranial shape were largely allometric, that is, driven by differences in body size. However, we also uncovered evidence of cranial shape variation between habitats not explained by size. In contrast, we did not find evidence for habitat-driven differences in cranial capacity independent of size. Overall, our results suggest a key role for body size in mediating morphological responses to urbanization and highlight the potential of house mice as a globally-distributed model for urbanization.

The practical utility of many modern phylogenetic comparative methods can depend on how accurately mathematical models capture the evolutionary process of traits. Boucher and Demery (2016) described a new quantitative trait model, Brownian motion with reflective limits, that they anticipated might be of use in testing hypotheses about a particular sort of constraint on phenotypic character evolution. Since their analytic solution for the probability function under this bounded evolutionary scenario was not practical to evaluate for reasonably-sized trees, Boucher and Demery (2016) also identified a creative technique for computing the likelihood of their model. The basis of this methodology derives from the convergence of an equal-rates, symmetric, ordered Markov chain and continuous stochastic diffusion in the limit as the number of steps in our chain goes to {infty} (or, alternatively, as their widths decrease towards zero). We refer to this convergence in the limit as the discretized diffusion approximation or (more compactly) the discrete approximation. We realized that this discrete approximation of Boucher and Demery (2016) unlocked a number of additional models for the phylogenetic comparative analysis of discrete and continuous trait data, and we explore several of these in the present article. Specifically, we examine application of this discretized diffusion approximation to the threshold model from evolutionary quantitative genetics, to a new "semi-threshold" trait evolution model, to a joint model of discrete and continuous traits in which the discrete trait influences the rate of evolution of our continuous character, as well as a model where precisely the converse is true, and to a discrete character dependent multi-trend trended continuous trait evolution model. We conclude with some context for the origins of our article and discussion of other possible applications of this powerful approach.

Social interactions are ubiquitous in nature and have the potential to affect trait evolution, particularly in group-living animals such as cooperative breeders. Interactions among conspecific individuals can affect the amount of additive genetic variation for a trait when the phenotype of an individual is also affected by the genotype of its social partner(s) via indirect genetic effects. Thus, quantifying both direct and indirect genetic effects of social partners is critical for understanding and predicting evolutionary trajectories. While much is known about maternal indirect genetic effects, empirical estimates of indirect genetic effects from other social partners remain limited, particularly in wild populations. Here, we use animal models to assess the contribution of indirect genetic effects from all social partners in a family group (mothers, fathers, and helpers) on juvenile morphometric traits across ontogeny in the cooperatively-breeding Florida scrub-jay (Aphelocoma coerulescens). We found indirect genetic effects of helpers and fathers on nestling weight, but no indirect genetic effect of mothers. Across ontogeny, we found increasing additive genetic variation in both weight and tarsus length. Our study provides a comprehensive assessment of within-group indirect genetic effects in a cooperative breeder and highlights the importance of considering indirect genetic effects beyond maternal effects.

Matching habitat choice provides a mechanism for individuals to maximise their expected fitness by selecting an environment that better fits their phenotype. Many animals choose their local environment by evaluating levels of perceived predation risk against possible resource gain. To test if predation risk is a major driver of habitat choice, we quantify scototaxis, or preference for dark versus light backgrounds, in juvenile guppies. As light backgrounds increase visibility to predators, this aspect of habitat choice captures variation in boldness in small fishes. By rearing and testing 586 fish descended from ten natural populations from Trinidad under common garden conditions, we first quantify (broad sense) heritable variation, i.e. evolutionary potential, within populations. Next, we test for evolutionary divergence among populations in mean preference, and if present, whether ancestral predation regime is a mediator of divergence. Finally, we ask whether families and/or populations differ in the amount of behavioural variation they contain. Habitat choice varied among families (12% of total variance), consistent with heritable variation (0.2). We also found mean preference varies among populations (11% of total variance explained). Evolutionary divergence among-populations is partly explained by ancestral predation regime, with populations from low-predation sites showing a stronger average preference for dark backgrounds than high-predation populations from the same river. Additionally, we find that within-population behavioural variation is greater in high-predation populations. We conclude that guppy populations contain heritable variation that could facilitate adaptive evolution if scototaxis is subject to natural selection. Furthermore, while genetic drift may also contribute to evolutionary divergence among-populations, observed patterns are qualitatively consistent with local adaption to predation regime. Our results suggests that high predation sites favour bolder habitat choice on average, but also that local predation regime shape the evolutionary dynamics of variation, perhaps by maintaining shy-bold variation among-individuals or by favouring individuals with less-predicable behaviour.

Comparative studies in evolutionary biology must account for trait non-independence arising from shared ancestry. While the phylogenetic signal of adult traits has been extensively studied, little is known about how conserved developmental trajectories are across species. Here, I quantify the phylogenetic signal (K) in the ontogeny of 35 traits across 157 primate species, spanning motor, cognitive, life-history, and dental development. Using Blombergs K statistic and a species-level mammalian phylogeny, I test two predictions: (i) that morphological (dental) traits exhibit the strongest phylogenetic signal, and (ii) that earlier-developing traits are more conserved. Results show that life-history traits are the most phylogenetically labile, while dental development is the most conserved (K = 0.7-2.6), with the eruption of the mandibular canine showing the highest signal (K = 2.6). Contrary to expectations, later-developing traits, particularly permanent teeth, display stronger phylogenetic conservation than earlier-developing deciduous teeth. These findings suggest that even within a single developmental system, the strength of phylogenetic constraint varies markedly with timing. The results provide an empirical foundation for identifying reliable temporal anchors in comparative primate ontogeny and have implications for interpreting maturational patterns in human evolution and the fossil record.

The expensive son hypothesis posits that mothers incur higher fitness costs when caring for sons versus daughters in species with male-biased size dimorphism. Evidence for maternal survival costs of sons in humans is limited to shortened overall lifespans; whether having more sons reduces short-term survival during reproductive years is unknown. Here, we utilised life-history data from 5,456 mothers from preindustrial Finland to examine whether mothers with more sons had reduced survival within one year of their last birth. While mothers with few children but more sons showed no differences in survival, at higher family sizes, mothers with more sons had increasingly lower survival. These differences peaked at [~]0.4% lower survival per son among mothers with five children, suggesting accumulated physiological costs of sons. These differences then declined and reversed among mothers with more children, potentially due to selective disappearance of frailer mothers. Our results suggest that studies focusing on post-menopausal mothers may bias estimates of the fitness costs of sons and reproductive costs more broadly. We recommend future research further examines the overlooked short-term fitness costs of sons during reproductive years, which is vital for understanding how life-history trade-offs, sexual dimorphism, and their interaction have shaped human evolution.

We present the first radiographic ageing framework for common dolphins (Delphinus delphis), based on ossification and epiphyseal fusion patterns in the pectoral flipper, demonstrating higher reliability for chronological age estimation than currently available epigenetic approaches for this species. Using individuals of known dental age, we calibrated two modelling approaches to predict dental age from radiographic bone scores: 1) a univariate polynomial regression using a total bone score (sum of 16 scores across all assessed flipper bones), and 2) a multivariate canonical analysis of principal coordinates (CAP) incorporating 16 individual bone-score variables. Both approaches successfully predicted dental age from skeletal ossification patterns. For an age range of 0 to 24 years, polynomial regression demonstrated high predictive accuracy with median absolute errors (MAEs) of 1.25 years in females (Spearmans {rho} = 0.93, R{superscript 2} = 0.90) and 1.08 years in males ({rho} = 0.95, R{superscript 2} = 0.86). The CAP model yielded MAEs of 1.35 years in females ({rho} = 0.90, R{superscript 2} = 0.85) and 1.80 years in males ({rho} = 0.94, R{superscript 2} = 0.84). Notably, both radiographic bone ageing models achieved equal or lower median absolute errors and higher coefficients of determination than a recently developed epigenetic clock for common dolphins derived from the same population (MAE = 1.80, Pearsons correlation (r) = 0.91, R{superscript 2} = 0.82). When applying the bone ageing models to individuals of unknown dental age, both models produced age estimates consistent with expected life-history stages (foetus, neonate, juvenile, subadult, adult), although accuracy declined in dolphins above 20 years, likely as a consequence of subtle age-related variation in skeletal changes in this species. Radiographic ageing provides an accurate non-invasive tool for demographic assessment to support conservation management of common dolphins.

Within species, male testosterone is often linked to mating competition and paternal care, suggesting that sex differences in endogenous testosterone values across mammals may covary with broader reproductive strategies. Using a structured literature search, we compiled 63 studies, spanning 31 non-human species and 9 human populations, reporting endogenous, non-experimentally manipulated testosterone values for both adult males and females within the same population and context. From these studies, we calculated male-to-female testosterone ratios, and analysed these data using Bayesian phylogenetic multilevel models. We tested whether testosterone dimorphism was associated with paternal care and sexual size dimorphism while accounting for sampling matrix, assay method, breeding context, and wild versus captive setting. Across non-human mammals, neither paternal care nor sexual size dimorphism (indexing competition) showed a clear association with testosterone ratios, and the same pattern emerged in the primate-only subset. By contrast, sampling matrix was consistently associated with testosterone dimorphism across all analyses, with lower male-to-female ratios in non-blood than in blood-based measures. In primates, testosterone ratios were also lower in captive than in wild populations, although this pattern was not clearly supported in the broader non-human dataset. In the human-only analysis, testosterone ratios did not clearly differ between industrialized and small-scale societies, whereas the matrix effect remained evident. Overall, our results suggest that sampling matrix is a major source of variation even for ratio-based measures, highlighting the need for caution when inferring between-species endocrine differences from studies using different substrates. More broadly, directly comparable, non-experimentally manipulated testosterone data for both sexes remain rare across mammals, limiting comparative inference.

Alleles with opposing effects on fitness characters are said to exhibit selectional antagonistic pleiotropy (broadly construed so that effects are not necessarily confined to the same individual). A number of theoretical investigations considered the case where a pair of alleles at a locus influences two fitness components and derived the conditions giving rise to stable polymorphism under various assumptions about the mode of trait-interaction. Strikingly, many of these analyses concluded that the potential for maintaining polymorphism is strongly constrained by the joint influence of two factors: (1) the prevalence of weak selection coefficients over coefficients of large magnitude, and (2) the absence of beneficial dominance reversals (where the deleterious effects of each allele are partially or completely masked in the heterozygous genotype). Consequently, the conclusion that selective polymorphism is unlikely to be maintained by intralocus mechanisms of antagonistic pleiotropy has achieved widespread acceptance. Here we argue that such conclusions do not apply to any of the following models of antagonism: (i) additive trait-interaction, (ii) multiplicative trait-interaction, (iii) bivoltine selection, (iv) soft selection, (v) hard selection, and (vi) sexual antagonism. We demonstrate that the parameter space giving rise to stable allelic variation is quite large throughout, and moreover, the plenitude of suitable parameters neither depends on the strength of selection nor requires dominance reversal. Dominance coefficients associated with stringent conditions for stable polymorphism are shown to be atypical as compared to all feasible parameters, and best regarded as an outcome of adherence to a special relation: dominance with a constant magnitude and direction, which includes the case of additive allelic effects at a locus. Properties of single-locus equilibria (heterozygosity, allele frequency differentiation) are investigated, as well as the contribution of dominance schemes to the genetic variance in fitness characters in populations at multilocus linkage equilibrium. Author summaryAllelic variants at a locus with opposing effects on multiple fitness components (antagonistic fitness pleiotropy) have long been appreciated as a possible source of balancing selection. The prevalence of polymorphism owing to this form of natural selection, however, has been doubted on theoretical grounds due to the fact that standard assumptions of genetic models (namely, constant magnitudes for the dominance coefficients) are hardly conducive to the maintenance of polymorphism. The major exception to this conclusion lies with schemes that exhibit dominance reversal (where the direction of dominance for antagonistic alleles flips across fitness components). Here we conduct a geometric analysis of the space of polymorphism-promoting dominance parameters and conclude that the conditions for maintaining balanced alleles is unrestrictive, with non-reversals playing an underappreciated role.

This study explores the proximal and biological mechanisms underlying male same-sex orientation, with a focus on the Fraternal Birth Order Effect (FBOE), a robust phenomenon whereby androphilic men tend to have more older brothers, and its relationship with the Sororal Birth Order Effect (SBOE), whereby older sisters also appear to influence sexual orientation, albeit less consistently. The Maternal Immune Hypothesis (MIH), which posits that maternal immune responses to male-specific antigens accumulate across successive male pregnancies, provides a compelling proximal explanation for the FBOE, but it fails to fully account for the SBOE and other birth order patterns, such as the elevated prevalence of same-sex orientation among only-children compared to firstborns in larger sibships. Through explicit modelling of the MIH, our simulations reveal that the correlation between the number of older brothers and sisters generates a spurious SBOE, which disappears when controlling for older brothers, unless miscarriages are considered, in which case this control becomes insufficient. Additionally, the increased prevalence of same-sex orientation among only-children, relative to firstborns with siblings, only emerges when miscarriages are incorporated into the model. Empirical analyses across eight diverse populations (Indonesia, France, French Polynesia, Greece, Canada, Czech Republic, Samoa, Iran) confirm the presence of an overall significant FBOE and, critically, an overall significant SBOE even after controlling for the number of older brothers. The higher frequency of same-sex orientation men among only-children, compared to firstborns in larger sibships, further supports a possible role of miscarriage. However, the miscarriage rates estimated to explain the observed SBOE (37% - 57%) exceed typical reported rates (10% - 30%), suggesting either that additional mechanisms contribute to a spurious SBOE or that a non-spurious SBOE exists alongside the FBOE. Limitations of this study are discussed, as well as whether the MIH framework can be extended to accommodate these findings, or if alternative explanations are needed to resolve these discrepancies.

Understanding behavioral differences between non-native and closely related endangered species could be important to aid conservation management. In volume 169 of Zoology, Bels et al. (2025) reported on their comparison of display-action-patterns (DAP) between native Iguana delicatissima and non-native iguanas present on islands of the Guadeloupe Archipelago in the Caribbean Lesser Antilles. Here, we address conceptual and methodological concerns about their work and reanalyze their data given our proposed corrections, primarily a literature-informed adjustment of their "species" category. We additionally utilize online videos from South American mainland I. iguana populations, from where the non-native iguanas in the Guadeloupe Archipelago originate, to better understand the different DAPs between native and non-native iguanas in the Guadeloupe Archipelago. Significant differences in DAP characteristics among "species" categories (native I. delicatissima, non-native iguanas, and hybrids) show that Bels et al. (2025) oversimplified their data analyses by merging all non-native populations into one group. This result indicates the presence of behavioral variation among subpopulations within widely hybridizing iguanid populations, which has been poorly studied. Additionally, videos from mainland populations across two major mitochondrial clades of Iguana iguana show that non-native iguanas on Guadeloupe retained DAP characteristics of those populations from which they originate. We discuss these findings in light of the proposed hypotheses put forward by Bels et al. (2025), of which two can be excluded. Overall, our reanalysis shows that studies focusing on characteristics within settings of complex hybridization in diverse species should acknowledge this complexity.

Escape responses are a critical behavioural mechanism influencing survival during predation events. In most species of teleosts and several other lower vertebrates, these responses are triggered by Mauthner cells (M-cells), which generate faster escapes (characterised by higher turning rates and shorter response latencies) than non-M-cell triggered responses. Most adult elasmobranchs lack M-cells and consequently exhibit slower escape response timing than teleosts. Spotted Ratfish (Hydrolagus colliei) are a notable exception in that adults possess M-cells, yet their escape response performance has not been explored. Here, we quantify the kinematics and timing of ratfish escape responses elicited by a mechano-acoustic stimulus. We show that ratfish exhibit higher turning rates and shorter response latencies than other adult chondrichthyans, though their response latencies are also significantly longer than those of teleosts. These findings suggest that retention of M-cells confers enhanced escape performance in ratfish, with important implications for their vulnerability to predator attacks. Summary statementThis study reveals that adult Spotted Ratfish (Hydrolagus colliei) show fast escape response with a performance that is intermediate between teleosts and previously studied elasmobranchs.