What doesn't kill you might make you stronger: functional basis for variation in body armour

Reinterpretation of tuberculate cervical vertebrae of Eocene birds as an exceptional anti-predator adaptation against the mammalian craniocervical killing bite

We report avian cervical vertebrae from the Quercy fissure fillings in France, which are densely covered with villi-like tubercles. Two of these vertebrae stem from a late Eocene site, another lacks exact stratigraphic data. Similar cervical vertebrae occur in avian species from Eocene fossils sites in Germany and the United Kingdom, but the new fossils are the only three-dimensionally preserved vertebrae with pronounced surface sculpturing. So far, the evolutionary significance of this highly bizarre morphology, which is unknown from extant birds, remained elusive, and even a pathological origin was considered. We note the occurrence of similar structures on the skull of the extant African rodent Lophiomys and detail that the tubercles represent true osteological features and characterize a distinctive clade of Eocene birds (Perplexicervicidae). Micro-computed tomography (μCT) shows the tubercles to be associated with osteosclerosis of the cervical vertebrae, which have a very thick cortex and much fewer trabecles and pneumatic spaces than the cervicals of most extant birds aside from some specialized divers. This unusual morphology is likely to have served for strengthening the vertebral spine in the neck region, and we hypothesize that it represents an anti-predator adaptation against the craniocervical killing bite ("neck bite") that evolved in some groups of mammalian predators. Tuberculate vertebrae are only known from the Eocene of Central Europe, which featured a low predation pressure on birds during that geological epoch, as is evidenced by high numbers of flightless avian species. Strengthening of the cranialmost neck vertebrae would have mitigated attacks by smaller predators with weak bite forces, and we interpret these vertebral specializations as the first evidence of "internal bony armor" in birds.

Foraging mode constrains the evolution of cephalic horns in lizards and snakes

A phylogenetically diverse minority of snake and lizard species exhibit rostral and ocular appendages that substantially modify the shape of their heads. These cephalic horns have evolved multiple times in diverse squamate lineages, enabling comparative tests of hypotheses on the benefits and costs of these distinctive traits. Here, we demonstrate correlated evolution between the occurrence of horns and foraging mode. We argue that although horns may be beneficial for various functions (e.g. camouflage, defence) in animals that move infrequently, they make active foragers more conspicuous to prey and predators, and hence are maladaptive. We therefore expected horns to be more common in species that ambush prey (entailing low movement rates) rather than in actively searching (frequently moving) species. Consistent with that hypothesis, our phylogenetic comparative analysis of published data on 1939 species reveals that cephalic horns occur almost exclusively in sit-and-wait predators. This finding underlines how foraging mode constrains the morphology of squamates and provides a compelling starting point for similar studies in other animal groups.

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

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

Osteoderms as calcium reservoirs: Insights from the lizard Ouroborus cataphractus

The functional significance of osteoderms-bony elements embedded in the dermis-remains a topic of much debate. Although many hypotheses have been put forward in the past, the idea that osteoderms can serve as calcium reservoirs has received little experimental attention thus far. In this study, we use micro-computed tomography to investigate inter- and intrasexual variation in osteoderm density in the viviparous lizard Ouroborus cataphractus and conduct histochemical analyses to unravel the potential mechanism involved in mineral resorption from the osteoderms. Our results show that females have denser, more compact osteoderms than males of similar body sizes, regardless of the season during which they were collected and their reproductive state. Furthermore, a histochemical study demonstrates the presence of mononucleated TRAP-positive cells in the vascular canals of the osteoderms. Based on the findings of this study, we suggest that the mineral storage hypothesis merits further attention as a candidate explanation for osteoderm evolution.

Unravelling the structural variation of lizard osteoderms

Vertebrate skin is a remarkable organ that supports and protects the body. It consists of two layers, the epidermis and the underlying dermis. In some tetrapods, the dermis includes mineralised organs known as osteoderms (OD). Lizards, with over 7,000 species, show the greatest diversity in OD morphology and distribution, yet we barely understand what drives this diversity. This multiscale analysis of five species of lizards, whose lineages diverged ∼100-150 million years ago, compared the micro- and macrostructure, material properties, and bending rigidity of their ODs, and examined the underlying bones of the skull roof and jaw (including teeth when possible). Unsurprisingly, OD shape, taken alone, impacts bending rigidity, with the ODs of Corucia zebrata being most flexible and those of Timon lepidus being most rigid. Macroscopic variation is also reflected in microstructural diversity, with differences in tissue composition and arrangement. However, the properties of the core bony tissues, in both ODs and cranial bones, were found to be similar across taxa, although the hard, capping tissue on the ODs of Heloderma and Pseudopus had material properties similar to those of tooth enamel. The results offer evidence on the functional adaptations of cranial ODs, but questions remain regarding the factors driving their diversity. STATEMENT OF SIGNIFICANCE: Understanding nature has always been a significant source of inspiration for various areas of the physical and biological sciences. Here we unravelled a novel biomineralization, i.e. calcified tissue, OD, forming within the skin of lizards which show significant diversity across the group. A range of techniques were used to provide an insight into these exceptionally diverse natural structures, in an integrated, whole system fashion. Our results offer some suggestions into the functional and biomechanical adaptations of OD and their hierarchical structure. This knowledge can provide a potential source of inspiration for biomimetic and bioinspired designs, applicable to the manufacturing of light-weight, damage-tolerant and multifunctional materials for areas such as tissue engineering.

Lizard osteoderms - Morphological characterisation, biomimetic design and manufacturing based on three species

Osteoderms (OD) are mineralised dermal structures consisting mainly of calcium phosphate and collagen. The sheer diversity of OD morphologies and their distribution within the skin of lizards makes these reptiles an ideal group in which to study ODs. Nonetheless, our understanding of the structure, development, and function of lizard ODs remains limited. The specific aims of this study were: (1) to carry out a detailed morphological characterisation of ODs in three lizard species; (2) to design and manufacture biomimetic sheets of ODs corresponding to the OD arrangement in each species; and (3) to evaluate the impact resistance of the manufactured biomimetic sheets under a drop weight test. Skin samples of the anguimorphsH. suspectumandO. ventralis, and the skinkC. zebratawere obtained from frozen lab specimens. Following a series of imaging and image characterisations, 3D biomimetic models of the ODs were developed. 3D models were then printed using additive manufacturing techniques and subjected to drop weight impact tests. The results suggest that a 3D printed compound of overlapping ODs as observed inCoruciacan potentially offers a higher energy absorption by comparison with the overlapping ODs ofOphisaurusand the non-overlapping ODs ofHeloderma.Compound overlapping ODs need to be further tested and explored as a biomimetic concept to increase the shock absorption capabilities of devices and structures.

Armed stem to stinger: a review of the ecological roles of scorpion weapons

Scorpions possess two systems of weapons: the pincers (chelae) and the stinger (telson). These are placed on anatomically and developmentally well separated parts of the body, that is, the oral appendages and at the end of the body axis. The otherwise conserved body plan of scorpions varies most in the shape and relative dimensions of these two weapon systems, both across species and in some cases between the sexes. We review the literature on the ecological function of these two weapon systems in each of three contexts of usage: (i) predation, (ii) defense and (iii) sexual contests. In the latter context, we will also discuss their usage in mating. We first provide a comparative background for each of these contexts of usage by giving examples of other weapon systems from across the animal kingdom. Then, we discuss the pertinent aspects of the anatomy of the weapon systems, particularly those aspects relevant to their functioning in their ecological roles. The literature on the functioning and ecological role of both the chelae and the telson is discussed in detail, again organized by context of usage. Particular emphasis is given on the differences in morphology or usage between species or higher taxonomic groups, or between genders, as such cases are most insightful to understand the roles of each of the two distinct weapon systems of the scorpions and their evolutionary interactions. We aimed to synthesize the literature while minimizing conjecture, but also to point out gaps in the literature and potential future research opportunities.

A review of the osteoderms of lizards (Reptilia: Squamata)

Osteoderms are mineralised structures consisting mainly of calcium phosphate and collagen. They form directly within the skin, with or without physical contact with the skeleton. Osteoderms, in some form, may be primitive for tetrapods as a whole, and are found in representatives of most major living lineages including turtles, crocodilians, lizards, armadillos, and some frogs, as well as extinct taxa ranging from early tetrapods to dinosaurs. However, their distribution in time and space raises questions about their evolution and homology in individual groups. Among lizards and their relatives, osteoderms may be completely absent; present only on the head or dorsum; or present all over the body in one of several arrangements, including non-overlapping mineralised clusters, a continuous covering of overlapping plates, or as spicular mineralisations that thicken with age. This diversity makes lizards an excellent focal group in which to study osteoderm structure, function, development and evolution. In the past, the focus of researchers was primarily on the histological structure and/or the gross anatomy of individual osteoderms in a limited sample of taxa. Those studies demonstrated that lizard osteoderms are sometimes two-layered structures, with a vitreous, avascular layer just below the epidermis and a deeper internal layer with abundant collagen within the deep dermis. However, there is considerable variation on this model, in terms of the arrangement of collagen fibres, presence of extra tissues, and/or a cancellous bone core bordered by cortices. Moreover, there is a lack of consensus on the contribution, if any, of osteoblasts in osteoderm development, despite research describing patterns of resorption and replacement that would suggest both osteoclast and osteoblast involvement. Key to this is information on development, but our understanding of the genetic and skeletogenic processes involved in osteoderm development and patterning remains minimal. The most common proposition for the presence of osteoderms is that they provide a protective armour. However, the large morphological and distributional diversity in lizard osteoderms raises the possibility that they may have other roles such as biomechanical reinforcement in response to ecological or functional constraints. If lizard osteoderms are primarily for defence, whether against predators or conspecifics, then this 'bony armour' might be predicted to have different structural and/or mechanical properties compared to other hard tissues (generally intended for support and locomotion). The cellular and biomineralisation mechanisms by which osteoderms are formed could also be different from those of other hard tissues, as reflected in their material composition and nanostructure. Material properties, especially the combination of malleability and resistance to impact, are of interest to the biomimetics and bioinspired material communities in the development of protective clothing and body armour. Currently, the literature on osteoderms is patchy and is distributed across a wide range of journals. Herein we present a synthesis of current knowledge on lizard osteoderm evolution and distribution, micro- and macrostructure, development, and function, with a view to stimulating further work.

The evolution of dermal shield vascularization in Testudinata and Pseudosuchia: phylogenetic constraints versus ecophysiological adaptations

Studies on living turtles have demonstrated that shells are involved in the resistance to hypoxia during apnea via bone acidosis buffering; a process which is complemented with cutaneous respiration, transpharyngeal and cloacal gas exchanges in the soft-shell turtles. Bone acidosis buffering during apnea has also been identified in crocodylian osteoderms, which are also known to employ heat transfer when basking. Although diverse, many of these functions rely on one common trait: the vascularization of the dermal shield. Here, we test whether the above ecophysiological functions played an adaptive role in the evolutionary transitions between land and aquatic environments in both Pseudosuchia and Testudinata. To do so, we measured the bone porosity as a proxy for vascular density in a set of dermal plates before performing phylogenetic comparative analyses. For both lineages, the dermal plate porosity obviously varies depending on the animal lifestyle, but these variations prove to be highly driven by phylogenetic relationships. We argue that the complexity of multi-functional roles of the post-cranial dermal skeleton in both Pseudosuchia and Testudinata probably is the reason for a lack of obvious physiological signal, and we discuss the role of the dermal shield vascularization in the evolution of these groups. This article is part of the theme issue 'Vertebrate palaeophysiology'.

Convergent evolution of tail spines in squamate reptiles driven by microhabitat use

The repeated evolution of convergent or analogous traits is often used as evidence for adaptive evolution. Squamate reptiles show a high degree of convergence in a variety of morphological traits; however, the evolutionary mechanisms driving these patterns are not fully understood. Here, we investigate the evolution of tail spines, a trait that evolved multiple times in evolutionarily independent clades of lizards. Taking a comparative phylogenetic approach, we use 2877 squamate species to demonstrate that the evolution of spiny tails is correlated with microhabitat use, with species that live in rocky habitats significantly more likely to have evolved spiny tails. In the light of previous behavioural observations, our results suggest that spiny-tailed lizards have an advantage in rocky habitats through predation avoidance, where tail spines are used to prevent extraction from rocky crevices. In concordance with previous research on lizard body armour, our results suggest that the evolution of tail spines is coupled to both a rock-dwelling lifestyle and predator avoidance strategies, and highlight a complex interplay between different selective pressures on the evolution of defensive morphologies in reptiles.

Ontogenetic scaling patterns of lizard skin surface structure as revealed by gel-based stereo-profilometry

The skin surface structure of squamate reptiles varies greatly among species, likely because it plays a key role in a range of tasks, such as camouflage, locomotion, self-cleaning, mitigation of water loss and protection from physical damage. Although we have foundational knowledge about squamate skin morphology, we still know remarkably little about how intraspecific variation in skin surface structure translates to functional variation. This gap in our understanding can be in part traced back to: (i) our lack of knowledge on how body size determines skin surface structure; and (ii) the lack of means to perform high-throughput and detailed analysis of the three-dimensional (3D) anatomy of reptilian skin surfaces in a non-destructive manner. To fill this gap, we explored the possibilities of a new imaging technique, termed gel-based stereo-profilometry, to visualize and quantify the 3D topography of reptilian skin surface structure. Using this novel approach, we investigated intra-specific and intra-individual variation in the skin surface morphology of a focal lizard species, Anolis cristatellus. We assessed how various characteristics of surface topography (roughness, skew and kurtosis) and scale morphology (area, height, width and shape) scale with body size across different body regions. Based on an ontogenetic series of A. cristatellus males, we show that skin roughness increases with body size. Skin patches on the ventral body region of lizards were rougher than on the dorsum, but this was a consequence of ventral scales being larger than dorsal scales. Dorsal surface skew and kurtosis varied with body size, but surfaces on the ventral skin showed no such relationship. Scale size scaled isometrically with body size, and while ventral scales differed in shape from dorsal scales, scale shape did not change with ontogeny. Overall, this study demonstrates that gel-based stereo-profilometry is a promising method to rapidly assess the 3D surface structure of reptilian skin at the microscopic level. Additionally, our findings of the explanatory power of body size on skin surface diversity provide a foundation for future studies to disentangle the relationships among morphological, functional and ecological diversity in squamate reptile skin surfaces.

On dangerous ground: the evolution of body armour in cordyline lizards

Animal body armour is often considered an adaptation that protects prey against predatory attacks, yet comparative studies that link the diversification of these allegedly protective coverings to differential predation risk or pressure are scarce. Here, we examine the evolution of body armour, including spines and osteoderms, in Cordylinae, a radiation of southern African lizards. Using phylogenetic comparative methods, we attempt to identify the ecological and environmental correlates of body armour that may hint at the selective pressures responsible for defensive trait diversification. Our results show that species inhabiting arid environments are more likely to possess elaborated body armour, specifically osteoderms. We did not find any effect of estimated predation pressure or risk on the degree of body armour. These findings suggest that body armour might not necessarily evolve in response to direct interactions with predators, but rather as a result of increased habitat-mediated predation risk. Furthermore, we discuss the possibility that osteoderms might have been shaped by factors unrelated to predation.

Ecological opportunity and predator-prey interactions: linking eco-evolutionary processes and diversification in adaptive radiations

Much of life's diversity has arisen through ecological opportunity and adaptive radiations, but the mechanistic underpinning of such diversification is not fully understood. Competition and predation can affect adaptive radiations, but contrasting theoretical and empirical results show that they can both promote and interrupt diversification. A mechanistic understanding of the link between microevolutionary processes and macroevolutionary patterns is thus needed, especially in trophic communities. Here, we use a trait-based eco-evolutionary model to investigate the mechanisms linking competition, predation and adaptive radiations. By combining available micro-evolutionary theory and simulations of adaptive radiations we show that intraspecific competition is crucial for diversification as it induces disruptive selection, in particular in early phases of radiation. The diversification rate is however decreased in later phases owing to interspecific competition as niche availability, and population sizes are decreased. We provide new insight into how predation tends to have a negative effect on prey diversification through decreased population sizes, decreased disruptive selection and through the exclusion of prey from parts of niche space. The seemingly disparate effects of competition and predation on adaptive radiations, listed in the literature, may thus be acting and interacting in the same adaptive radiation at different relative strength as the radiation progresses.

The evolution of tail weaponization in amniotes

Weaponry, for the purpose of intraspecific combat or predator defence, is one of the most widespread animal adaptations, yet the selective pressures and constraints governing its phenotypic diversity and skeletal regionalization are not well understood. Here, we investigate the evolution of tail weaponry in amniotes, a rare form of weaponry that nonetheless evolved independently among a broad spectrum of life including mammals, turtles and dinosaurs. Using phylogenetic comparative methods, we test for links between morphology, ecology and behaviour in extant amniotes known to use the tail as a weapon, and in extinct taxa bearing osseous tail armaments. We find robust ecological and morphological correlates of both tail lashing behaviour and bony tail weaponry, including large body size, body armour and herbivory, suggesting these life-history parameters factor into the evolution of antipredator behaviours and tail armaments. We suggest that the evolution of tail weaponry is rare because large, armoured herbivores are uncommon in extant terrestrial faunas, as they have been throughout evolutionary history.

Functional trade-off between strength and thermal capacity of dermal armor: Insights from girdled lizards

The presence of dermal armor is often unambiguously considered the result of an evolutionary predator-prey arms-race. Recent studies focusing predominantly on osteoderms - mineralized elements embedded in the dermis layer of various extant and extinct vertebrates - have instead proposed that dermal armor might exhibit additional functionalities besides protection. Multiple divergent functionalities could impose conflicting demands on a phenotype, yet, functional trade-offs in dermal armor have rarely been investigated. Here, we use high-resolution micro-computed tomography and voxel-based simulations to test for a trade-off between the strength and thermal capacity of osteoderms using two armored cordylid lizards as model organisms. We demonstrate that high vascularization, associated with improved thermal capacity might limit the strength of osteoderms. These results call for a holistic, cautionary future approach to studies investigating dermal armor, especially those aiming to inspire artificial protective materials.

Some Like It Hot: Camera Traps Unravel the Effects of Weather Conditions and Predator Presence on the Activity Levels of Two Lizards

It is generally assumed that favourable weather conditions determine the activity levels of lizards, because of their temperature-dependent behavioural performance. Inactivity, however, might have a selective advantage over activity, as it could increase survival by reducing exposure to predators. Consequently, the effects of weather conditions on the activity patterns of lizards should be strongly influenced by the presence of predators. Using remote camera traps, we test the hypothesis that predator presence and weather conditions interact to modulate daily activity levels in two sedentary cordylid lizards, Karusasaurus polyzonus and Ouroborus cataphractus. While both species are closely related and have a fully overlapping distribution, the former is a fast-moving lightly armoured lizard, whereas the latter is a slow-moving heavily armoured lizard. The significant interspecific difference in antipredator morphology and consequently differential vulnerability to aerial and terrestrial predators, allowed us to unravel the effects of predation risk and weather conditions on activity levels. Our results demonstrate that K. polyzonus is predominantly active during summer, when ambient temperatures are favourable enough to permit activity. In contrast, a peak in activity during spring was observed in O. cataphractus, with individuals being inactive during most of summer. While favourable weather conditions had a strong effect on the activity levels of K. polyzonus, no such relationship was present in O. cataphractus. Contrary to our hypothesis, the presence of terrestrial predators does not seem to affect daily activity levels or alter the influence of weather conditions on activity levels. We conclude that inactivity in O. cataphractus appears to be related to seasonal differences in vulnerability to predators, rather than the presence of predators, and highlight the importance of additional selective pressures, such as food abundance, in determining the species' activity levels.