Vivien Louppe

Species richness varies widely among taxonomic groups, and understanding the drivers of these disparities remains a key challenge. Using the most recent time-calibrated phylogeny of Caudata (763 species), we investigated how abiotic and biotic factors shape diversification in this amphibian clade, known for its ecological and life-history diversity. We first assessed the respective roles of clade age and diversification rates in explaining species richness using two branch-specific models. We then tested the influence of past temperature, continental fragmentation, and life cycle on diversification using environment- and state-dependent models. Our results show that diversification rates, rather than clade age, primarily explain richness differences. High speciation rates are found in young lineages and in early branches of the three most diverse families, suggesting early radiations followed by slowdowns. Likelihood-based analyses further suggest a positive association between temperature fluctuations and diversification. Weak temperature effects in Plethodontidae and absence of effect in Salamandridae and Hynobiidae point to lineage-specific dynamics. However, Bayesian inference-based models failed to recover reliable diversification dynamics, likely reflecting the inherent difficulty of estimating time-varying speciation and extinction rates from this phylogeny. Consequently, the precise temporal trajectory of diversification and the causal interpretation of environmental associations remain uncertain. State-dependent models show no significant effect of life cycle. Instead, other traits likely drive diversification patterns. Together, our findings highlight the central role of diversification rate heterogeneity in shaping species richness disparity, and the challenges of disentangling the macroevolutionary processes driving salamander evolutionary history.

Synopsis: Facultative paedomorphosis refers to the ability of individuals within a species to reach sexual maturity either by undergoing metamorphosis (metamorph) or by retaining larval traits into adulthood (paedomorph). This developmental flexibility provides a natural framework to investigate how facultative ontogenetic pathways shape morphological variation. Here, we investigate how life cycle variation (metamorphs, paedomorphs, and paedomorphs undergoing metamorphosis) affects the size and morphology of the body and feeding apparatus (cranium, mandible, and hyoid) in the Alpine newt (Ichthyosaura alpestris), using both linear and geometric morphometric approaches. Our results reveal clear differentiation in size, allometric trajectories, and morphology of both the body and the feeding apparatus. Paedomorphs are smaller and exhibit morphologies consistent with aquatic functional demands and the consequences of incomplete metamorphosis, particularly in the feeding apparatus, which normally undergoes extensive remodeling during metamorphosis. Individuals undergoing metamorphosis are morphologically distinct from paedomorphs but overlap with metamorphs in body size, suggesting that metamorphosis involves rapid skeletal reorganization before post-metamorphic growth occurs. Moreover, patterns of morphological integration and modularity do not differ between life cycle categories, suggesting that the developmental interdependence of phenotypic traits does not promote the morphological changes observed in species that experience facultative paedomorphosis. Paedomorphic populations nevertheless show higher morphological disparity in the feeding apparatus, likely reflecting variation in the degree of ossification and remodeling resulting from incomplete metamorphosis. Overall, these results show that facultative life cycles strongly structure body and feeding morphology in Alpine newts, while leaving trait covariation largely unchanged. These findings emphasize the potential of developmental plasticity to generate substantial morphological diversity within a same species, providing a basis for understanding how paedomorphic lineages maintain morphological variation despite persistent developmental constraints.

Synopsis: Caudata (salamanders and newts) exhibit considerable diversity in terms of ecology, life cycle, morphology, and behavior, ranging from species with complete metamorphosis to those with facultative or complete loss of metamorphosis. These developmental differences are often tightly linked to ecological transitions and morphological transformations, influencing how salamanders exploit habitats, access resources, and feed during their lifetime. While skeletal transformations have been widely studied, the impact on cranial musculature remains poorly understood. This study explores how life-cycle variation and associated ecological transitions and morphological transformations affect the architecture of feeding muscles in salamanders. We conducted dissections of the feeding system in 25 salamander species representing different life cycles, ecological transitions, and morphological transformations. We quantified muscle volume and physiological cross-sectional area (PCSA) functional muscle group of the jaw and hyoid muscles. Our results revealed a differentiation in cranial musculature based on different ecological strategies, and that other factors, such as head size, play a prominent role in shaping muscle architecture. We identified consistent patterns associated with whether individuals undergo an ecological transition, experience a morphological transformation, and with their adult habitat use, suggesting that ecological context imposes functional constraints on the muscular organization of the feeding system. These findings suggest that life history, ecological, and developmental strategies impose constraints that influence the muscular organization of the jaw and the hyoid apparatus. Future work should broaden taxonomic sampling, integrate bone and muscular traits together, and examine the evolutionary pathways by which life-cycle variation interacts with functional morphology in Caudata.

In animal behavior, standardizing experimental protocols ensures a rigorous interpretation of the results. However, when working with multisource data, homogeneous standardization is often difficult to attain. This is the case for many studies on feeding kinematics where experimental protocols are carried out using similar but not identical methods. In particular, the impact of the way in which prey is presented remains poorly tested and understood. The aim of this study is to assess whether prey position and the way prey is presented have an impact on the kinematics of prey capture. To do so, we compared the feeding kinematics during the capture of prey presented on the substrate versus prey suspended from tweezers using closely related species of aquatic and terrestrial salamanders. Our results show that changes in prey presentation method directly impact suction feeding kinematics but not terrestrial feeding. In the case of suction feeding, when the prey is suspended by tweezers, mouth opening movements are wider and take more time, the maximum speed and acceleration of mouth opening are higher, and head angle is larger. These changes in kinematics are interpreted as behavioral responses to hydrodynamic changes caused by the different prey presentation methods. This differential sensitivity to prey presentation method between aquatic and terrestrial feeding also highlights differences in the underlying control mechanism: while terrestrial feeding appears to rely on feedback mechanisms, aquatic feeding appears to rely mostly on feedforward mechanisms. As a result, the importance of accounting for prey presentation method is likely context‐dependent, being more relevant when studying feeding systems that rely on feedforward control. Finally, when comparing aquatic and terrestrial feeding, the differences in feeding strategies due to the medium itself outweigh the effects of the prey presentation method.

Size impacts all aspects of life in animals. Not only does it impact metabolism and physiology, it also affects movements, resulting in different selective pressures on animals of diverse sizes. However, beyond size, differences in developmental stage may also impact movements due to the maturation of the nervous and musculoskeletal systems. Here, we tested the influence of size and ontogeny on suction feeding kinematics in adults, juveniles, and larvae of the axolotl (Ambystoma mexicanum) using high-speed video recordings. Our data show that size had an influence on nearly all kinematic variables examined, but kinematics often deviated from the predictions of simple geometric scaling models. Moreover, for both the velocity and acceleration of mouth and hyoid movements, the effect of size differed according to the developmental stage. While overall movements were faster in larger animals, the velocity of movement increased faster with size in adults. This could be explained by the fact that the skull undergoes changes at adulthood due to partial remodeling. Changes in velocity are possibly linked to a shift in diet at adulthood from static to more elusive prey, yet this remains to be tested.

Metamorphosis, a common and spectacular developmental process, involves major phenotypic changes often linked to environmental transitions. Understanding how these changes affect morphological evolution is crucial for examining metamorphosis's evolutionary impacts. In this study, we analysed the role of metamorphosis and life cycle variation on the morphological evolution of the mandible in Caudata (salamanders and newts), a clade that exhibits a great diversity of life cycles and ecologies. We focused on the mandible, a key component of the masticatory apparatus, to investigate how life cycle variation may influence the diversification of feeding modes. Using 3D geometric morphometrics on 317 extant species of Caudata, we found that both life cycle and size influence mandible shape, with complex interactions between these factors. The morphology of the mandibular bones evolved more independently in species undergoing complete metamorphosis compared to those with partial metamorphosis, likely facilitating feeding diversification in terrestrial adults. On the other hand, high morphological integration in species undergoing partial metamorphosis could promote the rapid evolution of traits, resulting in the differentiation of more disparate morphologies, potentially aiding paedomorphic species in adapting to challenging environments. This study highlights the interplay between metamorphosis and the environment in shaping morphological evolution among species and provides new insights into the role of heterochrony in the morphological diversification of organisms. Read the free Plain Language Summary for this article on the Journal blog.

Species displaced outside their native range may face new pressures resulting from both environmental gradients and important differences in ecosystem structure. We investigated how this may impact the morphological variation of a globally introduced carnivoran, the small Indian mongoose. Previous research showed size variations in several introduced populations, suggesting that these differences resulted from character release in response to the absence of competition from native species. Here, we contrasted the morphological evolution of indigenous and introduced populations using geometric morphometric approaches on the head system of a large sample of specimens belonging to several regions. Our analyses revealed that differences in size and shape between introduced and native populations do not support hypothesis of character release. These results suggest that morphological variations in introduced populations may reflect the introduction history of the species, rather than evolutionary processes resulting from changes in ecosystem composition.

The introduction of exotic species is one of the major causes of the decline of global biodiversity. Tropical insular ecosystems, including many biodiversity hotspots, are particularly threatened by biological invasions. Two wild carnivores have been introduced in the Caribbean, the northern raccoon Procyon lotor and the small Indian mongoose Urva auropunctata. Understanding the spatial distribution and activity patterns of both species is crucial for conservation purposes. Here we used camera trap data to model single‐season occupancy and detection of these two species on two Caribbean islands, Guadeloupe and Martinique. Our survey highlighted the broad distribution of both species on these islands, with the exception of the northern raccoon population in Martinique which appears very limited. Moreover, spatio‐temporal co‐occurrence with other bird and mammal species revealed that the northern raccoon and the small Indian mongoose face few or no competitors. Finally, our models show that the occupancy of both species was not influenced by any variable tested (i.e. elevation, precipitation, temperature and land cover) and that the probability to detect small Indian mongooses was influenced by land cover and camera model. These results highlight the potential of both the northern raccoon and the small Indian mongoose to have a significant impact on the native ecosystems in these hotspots of biodiversity and demonstrate the necessity to develop conservation actions towards control and limitation of these invasive carnivores.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.