In primates, food sharing from mothers to infants may serve two main functions: supporting offspring nutritional needs (nutritional hypothesis) and facilitating the acquisition of dietary knowledge, such as what and how to eat (informational hypothesis). We investigated these hypotheses in a wild population of bearded capuchin monkeys Sapajus libidinosus, at Fazenda Boa Vista, Brazil. We analyzed 194 h of footage of 10 infants filmed over their first 18 months of life using focal animal sampling. Mothers shared food exclusively in response to infant solicitation, with solicitation events observed across 48 mother feeding episodes. Infants were successful in obtaining food in 29 bouts but failed in 19. In 15 of the 29 successful bouts, mothers only shared food after repeated solicitations, suggesting that food sharing involves a negotiated process. We found partial support for the nutritional hypothesis: food sharing increased as the infants aged. Infants also solicited more high-quality and hard-to-process food items, but food sharing did not differ according to these food categories. Additionally, mothers were less likely to share lizards. Our findings suggest that food sharing in capuchins is driven primarily by infant begging, rather than active maternal investment, highlighting the dynamic interplay between mother and infant in promoting infant development.

Helodermatid lizard envenomations are rarely reported in the literature, and symptoms are usually mild to moderate, primarily causing pain and localized effects. However, severe toxicity can occur with systemic effects, such as angioedema, hemodynamic instability, and myocardial infarction. Although there are reports of Heloderma bites that are reputedly fatal, the data are inconclusive at best. We report a fatal case following a Gila monster (Heloderma suspectum) bite likely resulting from cascading events initiated at the time of envenomation. This case reinforces the importance of recognizing that envenomated patients may have complex and occasionally idiosyncratic responses to venom and that treatment should be tailored accordingly.

Beaded lizards (Heloderma spp) are venomous lizards capable of inflicting local and systemic envenoming. An exotic pet collector presented with a reportedly prolonged bite by a presumed Guatemalan beaded lizard (Heloderma charlesbogerti). He reported immediate symptoms of envenoming, including severe pain, paresthesia, local swelling, dizziness, and nausea. Follow-up assessments conducted over a period of up to 4 y revealed a persistent boutonnière deformity of the right middle finger. This case report describes envenomation caused by the uncommonly documented bite of a beaded lizard (Heloderma spp) that was presumed by the patient to be the rare Guatemalan beaded lizard (H charlesbogerti). It illustrates the risks of long-term sequelae that may result from noncompliance with treatment after a helodermatid bite and also reemphasizes concerns about the underground sale of rare, endangered wildlife. Beaded lizard bites can inflict severe local envenomation and potentially cause permanent hand deformities. Proper referral and follow-up are essential to prevent long-term sequelae.

Linking morphology to performance is essential for understanding how skeletal growth shapes functional capabilities. In sexually dimorphic species, males and females often experience distinct selection pressures, leading to differences in growth rate, allometric patterns and adult performance. Using longitudinal X-ray imaging, we quantified skeletal growth in the agamid lizard, Psammophilus dorsalis, from juvenile to adult stages, to examine sex differences in growth trajectories, allometric patterns, whole-body performance and morphology-performance relationships. We found that body length increased non-linearly in both sexes and growth trajectories were parallel. Key morphological traits scaled isometrically relative to body length. As adults, sexes did not differ in absolute sprint speed, but males had higher absolute bite force. Hindlimb length and body shape poorly predicted sprint speed, but head length predicted bite force in females. Overall, our results suggest broadly similar growth patterns and performance outcomes in both sexes, indicating possible constraints on morphology and performance.

Understanding how form-function relationships scale across levels of biological organization is essential for uncovering the mechanisms driving morphological and performance diversity. We examined the association between head shape and bite force in lacertid lizards across three hierarchical levels: individuals within species, species within the genus Podarcis, and species across the family Lacertidae. Using geometric morphometrics of dorsal and lateral head shape combined with bite force measurements, we tested whether the strength and direction of the form-function relationship is conserved across scales and whether body size mediates these patterns. Our analyses revealed significant associations between head shape and bite force at all levels, with body size exerting a strong but not exclusive influence. Importantly, while the form-function link persisted after removing allometric effects, the evolutionary trajectories of this relationship were not aligned across scales: regression vectors differed randomly rather than following consistent directions. These results indicate that performance consistently constrains head morphology, yet the evolutionary pathways linking form and function vary across scales, reflecting a flexible interplay between selective pressures, developmental constraints, and phylogenetic history.

Sexual dimorphism in lizards arises from the dynamic interplay between natural and sexual selection, manifesting in divergent phenotypic traits across taxa. A key unresolved question is whether the relationship between such sexually dimorphic traits and functional performance also differs between the sexes. This study investigated this question in the mountain dragon (Diploderma vela), a protected species endemic to the upper Lancang River basin in southwestern China, by quantifying its sexual dimorphism in morphology and coloration and assessing their sex-specific correlations with bite force. A total of 94 individuals were assessed for nine morphological traits, maximum bite force capacity, and body coloration across 15 anatomical regions. After controlling for body size, significant male-biased dimorphism was detected in most morphological traits, whereas abdomen length was female-biased. Coloration also differed between sexes across all measured regions except the abdomen. Crucially, the relationship between morphology and bite force was sex-specific; different suites of traits predicted bite force in males versus females. In contrast, no correlation was found between coloration and bite force in either sex. These divergences reflect the species' flexible phenotypic responses to varying reproductive and ecological pressures. These findings demonstrate that sexual dimorphism extends beyond trait means to encompass sex-specific phenotype-performance relationships, highlighting differential adaptive responses. This work provides a functional framework for understanding trait evolution in D. vela and underscores the need for sex-specific considerations in its conservation.

Desert vipers (Eristicophis and Pseudocerastes) employ diverse procoagulant strategies despite their close phylogenetic relationships, yet the biochemical mechanisms underlying the variations in their coagulotoxic activities remain poorly understood. We conducted comprehensive coagulation factor activation studies across all four species in the clade: Eristicophis macmahoni (Ema), Pseudocerastes fieldi (Pfi), P. persicus (Ppe), and P. urarachnoides (Pur). Plasma clotting assays revealed extreme variation in coagulation speed (17.3-376.1 s), with E. macmahoni and P. urarachnoides emerging as rapid coagulators. Fluorometric zymogen activation studies demonstrated that all species require Factor Va as a cofactor for optimal activity, with species-specific preferences for either the endogenous form of Factor Va produced by circulating thrombin or the FVa produced by the venom directly cleaving Factor V. This suggests that the venoms cleave FV at sites distinct from the cleavage site utilised by endogenous thrombin. The species had dramatically distinct patterns of factor activation: Factor XII was most strongly activated factor, and had a venom rank order of Pur ≫ Ema > Pfi = Ppe; Factor X was the next most potently activated factor, with a venom rank order of Ema = Pfi > Pur > Ppe; followed by Factor VII Pfi > Ppe > Ema = Pur; and prothrombin (Factor II) Ema > Pur, with activation by Pfi or Ppe not detectable. None of the venoms activated Factor XI. Notably, despite being suggested as a bird-specialist, P. urarachnoides exhibiting the highest Factor XII activation capacity is significant as avians have secondarily lost FXII as a component in their coagulation cascade. As such, the potent activation of FXII suggest non-avian prey species such as lizards or small rodents make up a significant proportion of this species diet. Antivenom efficacy studies revealed species-specific patterns, with the VINS Jorven antivenom showing superior neutralization against P. urarachnoides and Eristicophis macmahoni relative to the Inosan Inoserp-MENA antivenom and National Antivenom and Vaccine Production Centre of Saudi Arabia antivenom. These toxicology findings reveal that despite morphological similarity and shared ecological niches, desert vipers have evolved fundamentally distinct coagulotoxicity approaches to prey immobilization. These species-specific pathophysiological profiles have important clinical implications for antivenom development and bite treatment protocols in endemic regions.

A major problem in current biomechanical literature is the extent to which in silico data can be validated by in vivo data across taxonomic scales. Despite frequent incongruence between in silico and in vivo data gained from precisely the same individual, biologists and palaeontologists continue to publish in silico data of single bones intended to represent entire species. Here, we aim to bridge this gap by investigating whether jaw morphology alone can be used to validate biomechanical models on the intraspecific level in a phenotypically diverse lizard, Podarcis pityusensis. We tested this by investigating how effectively in vivo bite force measurements from eight populations of this species are predicted by biomechanical models. We used alcohol-preserved specimens from each location to generate population-average and male-average morphologies of mandibles and dentaries, from which we calculated mechanical advantage as well as strength estimates from finite element analysis. Overall, we found a general lack of population-level correlation between in vivo and in silico data; however, strength estimates from finite element analysis did follow the same bite∼size relationship as in vivo bite, suggesting that biomechanical analysis of even a single bone can produce useful bite force estimates. We encourage researchers to create in silico models with maximally complex shape data and caution that intraspecific variation is a crucial aspect of in vivo and in silico biomechanics.

The veiled chameleon, Chamaeleo calyptratus, is a model organism in the study of reptile development. The extreme size of its cranial casque relative to other chameleons has been linked to high bite forces and conspecific signaling, yet its unique head anatomy has not been described. In general, detailed accounts of chameleon cranial musculature are scarce and not in formats that are easily accessible for use by researchers and educators. In this study, we provide the first complete description of both the cranial osteology and soft tissues in two adult specimens, exploring differences between sexes. Utilizing contrast-enhancing staining techniques, microCT scanning and deep learning models for segmentation, we have generated the first digital anatomical atlases of the chameleon skull, nervous system, cranial muscles and, in the female, hyolingual muscles. This includes the first maps of chameleon cranial sutures and muscle attachment sites. Our results show that the sexes differ most in the structure of the facial skeleton and the enlarged cranial casque. Many osteological differences are reflected in muscle topology and size, where the male presents a considerably larger jaw adductor. These atlases represent novel insights and potential applications for future research and education in reptile evolution and development. As an example, we have published a series of lesson plans alongside our atlases to help educators bridge the gap between theoretical knowledge and hands-on analysis in comparative anatomy.

Despite extensive research, it is still poorly understood how microgeographic phenotypic variation translates to the macroevolutionary level. Here, we use the Ibiza wall lizard, Podarcis pityusensis, an endemic species of the Balearic Islands, to study microgeographic variation across different scales of evolutionary isolation. We quantify bite force and morphology alongside biotic and abiotic environment in 11 populations, which have been variably isolated from one another over the Quaternary period. While we generally find increasing divergence in form and function as populations become more isolated from each other, this is not true when isolation is the highest; phenotypic differences between the 2 major clades of P. pityusensis are negligible, despite populations being isolated for over 100,000 years. Our results show that how environmental selective pressures drive form-function evolution differ by sex. Natural selection appears the most important driver of female evolution, while male phenotypes are apparently driven by both natural and sexual selection, but precise drivers of form-function evolution vary according to the scale of isolation investigated. Our study demonstrates incongruence in form-function-environment relationships within a constrained geographical area, highlighting how convergence at greater evolutionary scales can obscure microevolutionary diversity.

The jaw muscles of the southern alligator lizard, Elgaria multicarinata, are used in prolonged mate-holding behavior, and also to catch fast prey. In both males and females, these muscles exhibit an unusual type of high endurance known as sustained force in which contractile force does not return to baseline between subsequent contractions. This phenomenon is assumed to facilitate the prolonged mate-holding observed in this species. Skeletal muscle is often subject to a speed-endurance trade-off. Here, we determined the isometric twitch, tetanic and isotonic force-velocity properties of the jaw muscles at ∼24°C as metrics of contractile speed and compared these properties with a more typical thigh locomotory muscle to determine whether endurance by sustained force allows for circumvention of the speed-endurance trade-off. The specialized jaw muscle was generally slower than the more typical thigh muscle: time to peak twitch force, twitch 90% relaxation time (P<0.01), and tetanic 90% and 50% relaxation times (P<0.001) were significantly longer, and force-velocity properties were significantly slower (P<0.001) in the jaw than the thigh muscle. However, there seemed to be greater effects on relaxation rates and shortening velocity than on force rise times: there was no effect of muscle on time to peak, or 50% of tetanic force. Hence, the jaw muscle of the southern alligator lizard does not seem to circumvent the speed-endurance trade-off. However, the maintenance of force rise times despite slow relaxation, potentially enabled by the presence of hybrid fibers, may allow this muscle to meet the functional demand of prey capture.

Rensch's rule describes a pattern of interspecific allometry in which sexual size dimorphism (SSD) increases with size among closely related species (i.e., among a group of related species, the largest ones tend to show more male-biased SSD). Sexual selection is often invoked to explain Rensch's rule, as larger male body size is assumed to be favoured by sexual selection for increased fighting performance in contests for mating opportunities. Often, however, the correlation between size and performance is not well described. We studied a sexually selected performance trait, bite force in Anolis lizards, to determine whether patterns of SSD are linked to size-associated patterns of performance dimorphism at the macroevolutionary level, as expected under the sexual selection hypothesis for Rensch's rule. Additionally, we tested whether allometric patterns of performance dimorphism differ between mainland and island species, as the latter have likely evolved under a stronger sexual selection regime. We found that SSD overwhelmingly explains the relationship between performance dimorphism and size in anoles, as expected under a sexual selection model for Rensch's rule. However, residual performance dimorphism was higher in island than in mainland species, suggesting that these groups differ in performance dimorphism for reasons unrelated to size. Head size dimorphism was associated with residual performance dimorphism, but did not fully explain the difference in performance dimorphism between island and mainland species. Together, these findings highlight the need to interpret Rensch's rule patterns of body size evolution cautiously, as allometric patterns of performance dimorphism and size dimorphism might not be equivalent.

Animal coloration offers a unique opportunity to explore the evolutionary mechanisms underlying phenotypic diversity. Conspicuous coloration caused by pigments plays a crucial role in social signaling across multiple species by conveying information about individual quality, social ranks, or reproductive condition. Nevertheless, most previous studies have focused predominantly on colors produced by the exogenous pigments-carotenoids. Pterins are another prevalent group of conspicuous pigments, which can be produced endogenously and have received comparatively little attention. Whether pterin-based colors represent reliable signals remains elusive. The remarkable red ventrolateral coloration exhibited by males of the Guinan toad-headed lizard (Phrynocephalus guinanensis) in the Mugetan Desert presents an ideal model for investigating pterin-based coloration. Through electron microscopy and metabolomic identification, we discovered three types of pterin pigments within xanthophores. Integrating a series of morphological measurements and behavioral experiments, we found that this red coloration was not correlated with body size, bite force, and testosterone level, nor did females show a preference bias toward it. However, the red intensity predicted male-male competition outcomes, with deeper red males being more likely to emerge as winners. Our results indicated that the pterin-based coloration could convey information about male quality, suggesting its potential role in honest signaling, given the vital importance of pterin metabolism in physiological processes. This study provides a novel case into the understanding of pterin-based colors in animals.

The Mabuyinae subfamily exhibits remarkable diversity, encompassing 26 genera and 236 currently recognized species. Traditionally, the entire range of the group was attributed to the single genus Mabuya, which had a wide distribution along tropical regions of the Planet. In recent studies, phylogenetic hypotheses based on molecular data have identified four major groups, which have been further divided into geographically distinct clades. At least two phylogenetically distinct lineages of Mabuyinae are distributed in the Neotropical Region: Trachylepis atlantica and the remaining 16 genera within the Mabuyinae clade from the mainland and the Caribbean islands. Our understanding of Mabuyinae osteology is still quite limited, particularly concerning interspecific variation. This lack of information hinders our ability to make strong contributions to the phylogenetic relationships within this group or even to confirm the existence of certain new taxa considering their relatively conserved external morphology. This work provides a comprehensive anatomical reference for the adult skull of Neotropical Mabuyinae lizards, highlighting osteological features that might be useful for delimiting each genus. This descriptive guide includes illustrations and employs multiple techniques, such as dry preparation, clearing and staining, and high-resolution computerized microtomography. Our results provide additional diagnostic characteristics that include specific cranial bone arrangements, dental patterns, and cranial adaptations, such as dorsoventral head flattening, and their functional implications for bite force and cranial biomechanics. This study reinforces the importance of cranial morphology in understanding the phylogenetic relationships and evolutionary trajectories of New World Mabuyinae lizards, advocating for broader morphological sampling to enrich our understanding of these diverse reptiles.

Being aggressive and by extension, dominant, is an important mechanism for determining access to resources such as mates or territories. While predictors of contest outcome and dominance are increasingly studied, we have a poor understanding of how they vary across populations. Here, I use the widely distributed Australian agamid lizard, the Jacky dragon (Amphibolurus muricatus), to quantify variation in features predicting contest outcome among males of different populations. I measured physical attributes, maximal physiological performance capacity (sprint speed, endurance, bite force) and visual displays during staged encounters. I found that morphology, performance capacity and the type and frequency of visual displays used during agonistic interactions varied significantly across populations. Contest winners from the Cann River State Forest population favored tail-flicks and push-up/body-rocks, while those from Royal National Park were more likely to chase and individuals from Yarratt State Forest performed more bite-lunges than other populations. The losers of contests also differed in their displays. Individuals from the Cann River population were dominant over the others based on behavioral attributes (i.e., aggressive visual displays, chases and bite-lunges). I suggest that population differences in signal form and function could have implications for range dynamics as populations come into contact in an era of rapid environmental change.