Get to the point: Claw morphology impacts frictional interactions on rough substrates.

Claws are a common anatomical feature among limbed amniotes and contribute to a variety of functions including prey capture, locomotion, and attachment. Previous studies of both avian and non-avian reptiles have found correlations between habitat use and claw morphology, suggesting that variation in claw shape permits effective functioning in different microhabitats. How, or if, claw morphology influences attachment performance, particularly in isolation from the rest of the digit, has received little attention. To examine the effects of claw shape on frictional interactions, we isolated the claws of preserved specimens of Cuban knight anoles (Anolis equestris), quantified variation in claw morphology via geometric morphometrics, and measured friction on four different substrates that varied in surface roughness. We found that multiple aspects of claw shape influence frictional interactions, but only on substrates for which asperities are large enough to permit mechanical interlocking with the claw. On such substrates, the diameter of the claw's tip is the most important predictor of friction, with narrower claw tips inducing greater frictional interactions than wider ones. We also found that claw curvature, length, and depth influence friction, but that these relationships depend on the substrate's surface roughness. Our findings suggest that although claw shape plays a critical role in the effective clinging ability of lizards, its relative importance is dependent upon the substrate. Description of mechanical function, as well as ecological function, is critical for a holistic understanding of claw shape variation.

Ontogeny of the paraphalanges and derived phalanges of Hemidactylus turcicus (Squamata: Gekkonidae).

Gekkotan lizards of the genus Hemidactylus exhibit derived digital morphologies. These include heavily reduced antepenultimate phalanges of digits III and IV of the manus and digits III-V of the pes, as well as enigmatic cartilaginous structures called paraphalanges. Despite this well-known morphological derivation, no studies have investigated the development of these structures. We aimed to determine if heterochrony underlies the derived antepenultimate phalanges of Hemidactylus. Furthermore, we aimed to determine if convergently evolved paraphalanges exhibit similar or divergent developmental patterns. Herein we describe embryonic skeletal development in the hands and feet of four gekkonid species, exhibiting a range of digital morphologies. We determined that the derived antepenultimate phalanges of Hemidactylus are the products of paedomorphosis. Furthermore, we found divergent developmental patterns between convergently evolved paraphalanges.

Ankle structure of the Tokay gecko (Gekko gecko) and its role in the deployment of the subdigital adhesive system.

The remarkable ability of geckos to adhere to smooth surfaces is often thought of in terms of external structures, including the branching setae that make contact with the surface producing van der Waals forces. Some geckos also exhibit unique movements of the distal segments of the limbs during locomotion and static clinging, including active digital hyperextension and considerable pedal rotation. During static clinging, geckos can exhibit considerable adduction/abduction of the pes while the crus and thigh remain firmly adpressed to the substratum. This decoupling of pedal adduction/abduction from ankle flexion/extension and pedal long-axis rotation is a significant departure from pedal displacements of a typical lizard lacking adhesive ability. The structure of the ankle is likely key to this decoupling, although no detailed comparison of this complex joint between pad-bearing geckos and other lizards is available. Here we compare the configuration of the mesotarsal joint of nongekkotan lizards (Iguana and Pristidactylus) with that of the Tokay gecko (Gekko gecko) using prepared skeletons, scanning electron microscopy, and micro-computed tomographic (µCT) scans. We focus on the structure of the astragalocalcaneum and the fourth distal tarsal. The mesotarsal joint exhibits a suite of modifications that are likely associated with the secondarily symmetrical pes of pad-bearing geckos. For example, the lateral process of the astragalocalcaneum is much more extensive in G. gecko compared with other lizards. The mesotarsal joint exhibits several other differences permitting dissociation of long-axis rotation of the pes from flexion-extension movement, including a reduced ventral peg on the fourth distal tarsal, an articulatory pattern dominated by a well-defined, expansive distomesial notch of the astragalocalcaneum, and an associated broad proximodorsal articulatory facet of the fourth distal tarsal. Pad-bearing geckos are capable of effectively deploying their intricate adhesive system across a broad array of body angles because of this highly modified ankle. Future research should determine whether the differences encountered in G. gecko (and their extent) apply to the Gekkota as a whole and should examine how the elements of the ankle move dynamically during locomotion across a range of taxa.

Revisiting the classification of squamate adhesive setae: historical, morphological and functional perspectives.

Research on gecko-based adhesion has become a truly interdisciplinary endeavour, encompassing many disciplines within the natural and physical sciences. Gecko adhesion occurs by the induction of van der Waals intermolecular (and possibly other) forces between substrata and integumentary filaments (setae) terminating in at least one spatulate tip. Gecko setae have increasingly been idealized as structures with uniform dimensions and a particular branching pattern. Approaches to developing synthetic simulacra have largely adopted such an idealized form as a foundational template. Observations of entire setal fields of geckos and anoles have, however, revealed extensive, predictable variation in setal form. Some filaments of these fields do not fulfil the morphological criteria that characterize setae and, problematically, recent authors have applied the term 'seta' to structurally simpler and likely non-adhesively competent fibrils. Herein we briefly review the history of the definition of squamate setae and propose a standardized classificatory scheme for epidermal outgrowths based on a combination of whole animal performance and morphology. Our review is by no means comprehensive of the literature regarding the form, function, and development of the adhesive setae of squamates and we do not address significant advances that have been made in many areas (e.g. cell biology of setae) that are largely tangential to their classification and identification. We contend that those who aspire to simulate the form and function of squamate setae will benefit from a fuller appreciation of the diversity of these structures, thereby assisting in the identification of features most relevant to their objectives.

The same but different: setal arrays of anoles and geckos indicate alternative approaches to achieving similar adhesive effectiveness.

The functional morphology of squamate fibrillar adhesive systems has been extensively investigated and has indirectly and directly influenced the design of synthetic counterparts. Not surprisingly, the structure and geometry of exemplar fibrils (setae) have been the subject of the bulk of the attention in such research, although variation in setal morphology along the length of subdigital adhesive pads has been implicated to be important in the effective functioning of these systems. Adhesive setal field configuration has been described for several geckos, but that of the convergent Anolis lizards, comprised of morphologically simpler fibrils, remains largely unexplored. Here, we examine setal morphology along the proximodistal axis of the digits of Anolis equestris and compare our findings to those for a model gecko, Gekko gecko. Consistent with previous work, we found that the setae of A. equestris are generally thinner, shorter, and present at higher densities than those of G. gecko and terminate in a single spatulate tip. Contrastingly, the setae of G. gecko are hierarchically branched in structure and carry hundreds of spatulate tips. Although the splitting of contacts into multiple smaller tips is predicted to increase the adhesive performance of a fiber compared to an unbranched one, we posited that the adhesive performance of G. gecko and A. equestris would be relatively similar when the configuration of the setal fields of each was accounted for. We found that, as in geckos, setal morphology of A. equestris follows a predictable pattern along the proximodistal axis of the pad, although there are several critical differences in the configuration of the setal fields of these two groups. Most notably, the pattern of variation in setal length of A. equestris is effectively opposite to that exhibited by G. gecko. This difference in clinal variation mirrors the difference in the direction in which the setal fields of anoles and geckos are peeled from the substrate, consistent with the hypothesis that biomechanical factors are the chief determinants of these patterns of variation. Future empirical work, however, is needed to validate this. Our findings set the stage for future comparative studies investigating the functional morphology of these convergent adhesive apparatuses. Such investigations will lead to an enhanced understanding of the interactions between form, function, and environment of fibril-based biological adhesive systems.

Vocalization by extant nonavian reptiles: A synthetic overview of phonation and the vocal apparatus.

Among amniote vertebrates, nonavian reptiles (chelonians, crocodilians, and lepidosaurs) are regarded as using vocal signals rarely (compared to birds and mammals). In all three reptilian clades, however, certain taxa emit distress calls and advertisement calls using modifications of regions of the upper respiratory tract. There is no central tendency in either acoustic mechanisms or the structure of the vocal apparatus, and many taxa that vocalize emit only relatively simple sounds. Available evidence indicates multiple origins of true vocal abilities within these lineages. Reptiles thus provide opportunities for studying the early evolutionary stages of vocalization. The early literature on the diversity of form of the laryngotracheal apparatus of reptiles boded well for the study of form-function relationships, but this potential was not extensively explored. Emphasis shifted away from anatomy, however, and centered instead on acoustic analysis of the sounds that are produced. New investigative techniques have provided novel ways of studying the form-function aspects of the structures involved in phonation and have brought anatomical investigation to the forefront again. In this review we summarize what is known about hearing in reptiles in order to contextualize the vocal signals they generate and the sound-producing mechanisms responsible for them. The diversity of form of the sound producing apparatus and the increasing evidence that reptiles are more dependent upon vocalization as a communication medium than previously thought indicates that they have a significant role to play in the understanding of the evolution of vocalization in amniotes.

Ecomorphological associations of scapulocoracoid form in Greater Antillean Anolis lizards.

External morphological metrics have featured prominently in comparative studies examining the morphological convergence that characterizes anoline ecomorphs. To what degree the appendicular-skeletal morphology of Greater Antillean island Anolis lizards tracks their diversity and ecological adaptation, however, remains relatively unexplored. Here we employ computed tomographic scanning techniques to visualize in situ the scapulocoracoid of ecomorph representatives (trunk-ground, trunk-crown, crown-giant, twig) from three islands (Jamaica, Hispaniola, and Puerto Rico), and compare its three-dimensional geometry using qualitative-descriptive and quantitative-morphometric techniques. In contrast to our previous, similarly-conducted study of the pelvic girdle of these same species, the form of the scapulocoracoid varies markedly both within and between species, with much of the variation relating to phylogenetic relationship, specimen size, and assigned ecomorph category. Morphometric variation that correlates with size and/or phylogenetic signal varies between species and cannot be eliminated from the data set without markedly reducing its overall variability. The discovered patterns of skeletal variation are consistent with the demands of locomotor mechanics imposed by the structural configuration of the microhabitat of these ecomorphs. Most pertinently the ecomorphs differ in the anteroposterior length of the coracoid, the dorsoventral height of the scapulocoracoid, the dorsoventral height of the scapula in relation to the height of the suprascapula, and the relative positioning of the borders of the scapulocoracoid fenestra. In the examined ecomorph categories these skeletal differences likely relate to microhabitat usage by permitting different degrees of tilting and displacement of the scapulocoracoid in the parasagittal plane and influencing the sizes of muscle origins and the vectors of their actions. These differences relate to the amount of humeral adduction applied during its protraction, and to the structural stability of the shoulder girdle during acrobatic maneuvers, thus influencing the perch diameter that can be effectively negotiated, a critical factor in the microhabitat structure of Anolis ecomorphs.

Evolution of pedal digit orientation and morphology in relation to acquisition and secondary loss of the adhesive system in geckos.

Among geckos, the acquisition of the adhesive system is associated with several morphological changes of the feet that are involved in the operation of the adhesive apparatus. However, analyses using a comparative framework are lacking. We applied traditional morphometrics and geometric morphometric analysis with phylogenetic comparative methods to morphological data, collected from X-ray scans, to examine patterns of morphological evolution of the pes in association with the gain and loss of adhesive capabilities, and with habitat occupancy among 102 species of gecko. Padbearing gecko lineages tend to have shorter digits and greater inter-digital angles than padless ones. Arboreal and saxicolous species have shorter digits than terrestrial species. Our results suggest repeated shifts that converge upon a similar padbearing morphology, with some modifications being associated with the habitat occupied. We demonstrate that functional innovation and habitat can operate on, and influence, different components of foot morphology.

The Integrative Biology of Gecko Adhesion: Historical Review, Current Understanding, and Grand Challenges.

Geckos are remarkable in their ability to reversibly adhere to smooth vertical, and even inverted surfaces. However, unraveling the precise mechanisms by which geckos do this has been a long process, involving various approaches over the last two centuries. Our understanding of the principles by which gecko adhesion operates has advanced rapidly over the past 20 years and, with this knowledge, material scientists have attempted to mimic the system to create artificial adhesives. From a biological perspective, recent studies have examined the diversity in morphology, performance, and real-world use of the adhesive apparatus. However, the lack of multidisciplinarity is likely a key roadblock to gaining new insights. Our goals in this paper are to 1) present a historical review of gecko adhesion research, 2) discuss the mechanisms and morphology of the adhesive apparatus, 3) discuss the origin and performance of the system in real-world contexts, 4) discuss advancement in bio-inspired design, and 5) present grand challenges in gecko adhesion research. To continue to improve our understanding, and to more effectively employ the principles of gecko adhesion for human applications, greater intensity and scope of interdisciplinary research are necessary.

Going Out on a Limb: How Investigation of the Anoline Adhesive System Can Enhance Our Understanding of Fibrillar Adhesion.

The remarkable ability of geckos to adhere to a wide-variety of surfaces has served as an inspiration for hundreds of studies spanning the disciplines of biomechanics, functional morphology, ecology, evolution, materials science, chemistry, and physics. The multifunctional properties (e.g., self-cleaning, controlled releasability, reversibility) and adhesive performance of the gekkotan adhesive system have motivated researchers to design and fabricate gecko-inspired synthetic adhesives of various materials and properties. However, many challenges remain in our attempts to replicate the properties and performance of this complex, hierarchical fibrillar adhesive system, stemming from fundamental, but unanswered, questions about how fibrillar adhesion operates. Such questions involve the role of fibril morphology in adhesive performance and how the gekkotan adhesive apparatus is utilized in nature. Similar fibrillar adhesive systems have, however, evolved independently in two other lineages of lizards (anoles and skinks) and potentially provide alternate avenues for addressing these fundamental questions. Anoles are the most promising group because they have been the subject of intensive ecological and evolutionary study for several decades, are highly speciose, and indeed are advocated as squamate model organisms. Surprisingly, however, comparatively little is known about the morphology, performance, and properties of their convergently-evolved adhesive arrays. Although many researchers consider the performance of the adhesive system of Anolis lizards to be less accomplished than its gekkotan counterpart, we argue here that Anolis lizards are prime candidates for exploring the fundamentals of fibrillar adhesion. Studying the less complex morphology of the anoline adhesive system has the potential to enhance our understanding of fibril morphology and its relationship to the multifunctional performance of fibrillar adhesive systems. Furthermore, the abundance of existing data on the ecology and evolution of anoles provides an excellent framework for testing hypotheses about the influence of habitat microstructure on the performance, behavior, and evolution of lizards with subdigital adhesive pads.

Evolution of the Gekkotan Adhesive System: Does Digit Anatomy Point to One or More Origins?

Recently-developed, molecularly-based phylogenies of geckos have provided the basis for reassessing the number of times adhesive toe-pads have arisen within the Gekkota. At present both a single origin and multiple origin hypotheses prevail, each of which has consequences that relate to explanations about digit form and evolutionary transitions underlying the enormous variation in adhesive toe pad structure among extant, limbed geckos (pygopods lack pertinent features). These competing hypotheses result from mapping the distribution of toe pads onto a phylogenetic framework employing the simple binary expedient of whether such toe pads are present or absent. It is evident, however, that adhesive toe pads are functional complexes that consist of a suite of integrated structural components that interact to bring about adhesive contact with the substratum and release from it. We evaluated the competing hypotheses about toe pad origins using 34 features associated with digit structure (drawn from the overall form of the digits; the presence and form of adhesive scansors; the proportions and structure of the phalanges; aspects of digital muscular and tendon morphology; presence and form of paraphalangeal elements; and the presence and form of substrate compliance-enhancing structures). We mapped these onto a well-supported phylogeny to reconstruct their evolution. Nineteen of these characters proved to be informative for all extant, limbed geckos, allowing us to assess which of them exhibit co-occurrence and/or clade-specificity. We found the absence of adhesive toe pads to be the ancestral state for the extant Gekkota as a whole, and our data to be consistent with independent origins of adhesive toe pads in the Diplodactylidae, Sphaerodactylidae, Phyllodactylidae, and Gekkonidae, with a strong likelihood of multiple origins in the latter three families. These findings are consistent with recently-published evidence of the presence of adhesively-competent digits in geckos generally regarded as lacking toe pads. Based upon morphology we identify other taxa at various locations within the gekkotan tree that are promising candidates for the expression of the early phases of adhesively-assisted locomotion. Investigation of functionally transitional forms will be valuable for enhancing our understanding of what is necessary and sufficient for the transition to adhesively-assisted locomotion, and for those whose objectives are to develop simulacra of the gekkotan adhesive system for biotechnological applications.

The Ecomechanics of Gecko Adhesion: Natural Surface Topography, Evolution, and Biomimetics.

The study of gecko adhesion is necessarily interdisciplinary due to the hierarchical nature of the adhesive system and the complexity of interactions between the animals and their habitats. In nature, geckos move on a wide range of surfaces including soft sand dunes, trees, and rocks, but much of the research over the past two decades has focused on their adhesive performance on artificial surfaces. Exploring the complex interactions between geckos and their natural habitats will reveal aspects of the adhesive system that can be applied to biomimetic research, such as the factors that facilitate movement on dirty and rough surfaces with varying microtopography. Additionally, contrasting suites of constraints and topographies are found on rocks and plants, likely driving differences in locomotion and morphology. Our overarching goals are to bring to light several aspects of ecology that are important for gecko-habitat interactions, and to propose a framework for how they can inspire material scientists and functional ecologists. We also present new data on surface roughness and topography of a variety of surfaces, and adhesive performance of Phelsuma geckos on surfaces of varying roughness. We address the following key questions: (1) why and how should ecology be incorporated into the study of gecko adhesion? (2) What topographical features of rocks and plants likely drive adhesive performance? (3) How can ecological studies inform material science research? Recent advances in surface replication techniques that eliminate confounding factors among surface types facilitate the ability to address some of these questions. We pinpoint gaps in our understanding and identify key initiatives that should be adopted as we move forward. Most importantly, fine details of locomotor microhabitat use of both diurnal and nocturnal geckos are needed.

Lower rotational inertia and larger leg muscles indicate more rapid turns in tyrannosaurids than in other large theropods.

SYNOPSIS: Tyrannosaurid dinosaurs had large preserved leg muscle attachments and low rotational inertia relative to their body mass, indicating that they could turn more quickly than other large theropods. METHODS: To compare turning capability in theropods, we regressed agility estimates against body mass, incorporating superellipse-based modeled mass, centers of mass, and rotational inertia (mass moment of inertia). Muscle force relative to body mass is a direct correlate of agility in humans, and torque gives potential angular acceleration. Agility scores therefore include rotational inertia values divided by proxies for (1) muscle force (ilium area and estimates of m. caudofemoralis longus cross-section), and (2) musculoskeletal torque. Phylogenetic ANCOVA (phylANCOVA) allow assessment of differences in agility between tyrannosaurids and non-tyrannosaurid theropods (accounting for both ontogeny and phylogeny). We applied conditional error probabilities a(p) to stringently test the null hypothesis of equal agility. RESULTS: Tyrannosaurids consistently have agility index magnitudes twice those of allosauroids and some other theropods of equivalent mass, turning the body with both legs planted or pivoting over a stance leg. PhylANCOVA demonstrates definitively greater agilities in tyrannosaurids, and phylogeny explains nearly all covariance. Mass property results are consistent with those of other studies based on skeletal mounts, and between different figure-based methods (our main mathematical slicing procedures, lofted 3D computer models, and simplified graphical double integration). IMPLICATIONS: The capacity for relatively rapid turns in tyrannosaurids is ecologically intriguing in light of their monopolization of large (>400 kg), toothed dinosaurian predator niches in their habitats.

Population genetic structure and species delimitation of a widespread, Neotropical dwarf gecko.

Amazonia harbors the greatest biological diversity on Earth. One trend that spans Amazonian taxa is that most taxonomic groups either exhibit broad geographic ranges or small restricted ranges. This is likely because many traits that determine a species range size, such as dispersal ability or body size, are autocorrelated. As such, it is rare to find groups that exhibit both large and small ranges. Once identified, however, these groups provide a powerful system for isolating specific traits that influence species distributions. One group of terrestrial vertebrates, gecko lizards, tends to exhibit small geographic ranges. Despite one exception, this applies to the Neotropical dwarf geckos of the genus Gonatodes. This exception, Gonatodes humeralis, has a geographic distribution almost 1,000,000 km2 larger than the combined ranges of its 30 congeners. As the smallest member of its genus and a gecko lizard more generally, G. humeralis is an unlikely candidate to be a wide-ranged Amazonian taxon. To test whether or not G. humeralis is one or more species, we generated molecular genetic data using restriction-site associated sequencing (RADseq) and traditional Sanger methods for samples from across its range and conducted a phylogeographic study. We conclude that G. humeralis is, in fact, a single species across its contiguous range in South America. Thus, Gonatodes is a unique clade among Neotropical taxa, containing both wide-ranged and range-restricted taxa, which provides empiricists with a powerful model system to correlate complex species traits and distributions. Additionally, we provide evidence to support species-level divergence of the allopatric population from Trinidad and we resurrect the name Gonatodes ferrugineus from synonymy for this population.

What is bred in the bone: Ecomorphological associations of pelvic girdle form in greater Antillean Anolis lizards.

Ecological niche partitioning of Anolis lizards of the Greater Antillean islands has been the focus of many comparative studies, and much is known about external morphological convergence that characterizes anole ecomorphs. Their internal anatomy, however, has rarely been explored in an ecomorphological context, and it remains unknown to what degree skeletal morphology tracks the diversity and ecological adaptation of these lizards. Herein, we employ CT scanning techniques to visualise the skeleton of the pelvic girdle in situ, and 3D geometric morphometrics to compare the form of the ilium, ischium, and pubis within and between ecomorphs. We examine 26 species of anoles representing four ecomorphs (trunk-ground, trunk-crown, crown-giant, twig) from three islands (Jamaica, Hispaniola, and Puerto Rico). The subtle variations in pelvic girdle morphology discovered are directly associable with all three parameters that we set out to focus on: phylogenetic relationship, specimen size, and assigned ecomorph category. Morphometric variation that correlates with size and/or phylogenetic signal varies between species and cannot be eliminated from the data set without markedly reducing its overall variability. The discovered patterns of skeletal variation are consistent with the demands of locomotor mechanics pertinent to the structural configuration of the microhabitat of three of the four ecomorphs, with the fourth having no discernible distinctive features. This manifests itself chiefly in the relative anteroposterior extent and anteroventral inclination of the ilium and pubis, which differ between ecomorphs and are postulated to reflect optimization of the direction of muscle vectors of the femoral protractors and retractors. Our investigation of the form of the pelvic girdle of anoles allows us to generalize our findings to entire ecomorph categories within a broad phylogenetic and biogeographic context. Differences in the form and configuration of the postcranial skeleton are directly related to ecological patterns.

Patterns of growth in the presacral vertebral column of the leopard gecko (Eublepharis macularius).

Postnatal growth patterns within the vertebral column may be informative about body proportions and regionalization. We measured femur length, lengths of all pre-sacral vertebrae, and lengths of intervertebral spaces, from radiographs of a series of 21 Eublepharis macularius, raised under standard conditions and covering most of the ontogenetic body size range. Vertebrae were grouped into cervical, sternal, and dorsal compartments, and lengths of adjacent pairs of vertebrae were summed before analysis. Femur length was included as an index of body size. Principal component analysis of the variance-covariance matrix of these data was used to investigate scaling among them. PC1 explained 94.19% of total variance, interpreted as the variance due to body size. PC1 differed significantly from the hypothetical isometric vector, indicating overall allometry. The atlas and axis vertebrae displayed strong negative allometry; the remainder of the vertebral pairs exhibited weak negative allometry, isometry or positive allometry. PC1 explained a markedly smaller amount of variance for the vertebral pairs of the cervical compartment than for the remainder of the vertebral pairs, with the exception of the final pair. The relative standard deviations of the eigenvalues from the PCAs of the three vertebral compartments indicated that the vertebrae of the cervical compartment were less strongly integrated by scaling than were the sternal or dorsal vertebrae, which did not differ greatly between themselves in their strong integration, suggesting that the growth of the cervical vertebrae is constrained by the mechanical requirements of the head. Regionalization of the remainder of the vertebral column is less clearly defined but may be associated with wave form propagation incident upon locomotion, and by locomotory changes occasioned by tail autotomy and regeneration. Femur length exhibits negative allometry relative to individual vertebral pairs and to vertebral column length, suggesting a change in locomotor requirements over the ontogenetic size range.

Leaping lizards landing on leaves: escape-induced jumps in the rainforest canopy challenge the adhesive limits of geckos.

The remarkable adhesive capabilities of geckos have garnered attention from scientists and the public for centuries. Geckos are known to have an adhesive load-bearing capacity far in excess (by 100-fold or more) of that required to support their body mass or accommodate the loading imparted during maximal locomotor acceleration. Few studies, however, have investigated the ecological contexts in which geckos use their adhesive system and how this may influence its properties. Here we develop a modelling framework to assess whether their prodigious adhesive capacity ever comes under selective challenge. Our investigation is based upon observations of escape-induced aerial descents of canopy-dwelling arboreal geckos that are rapidly arrested by clinging to leaf surfaces in mid-fall. We integrate ecological observations, adhesive force measurements, and body size and shape measurements of museum specimens to conduct simulations. Using predicted bending mechanics of petioles and leaf midribs, we find that the drag coefficient of the gecko, the size of the gecko and the size of the leaf determine impact forces. Regardless of the landing surface, safety factors for geckos range from a maximum of just over 10 to a minimum of well under one, which would be the point at which the adhesive system fails. In contrast to previous research that intimates that gecko frictional adhesive capacity is excessive relative to body mass, we demonstrate that realistic conditions in nature may result in frictional capacity being pushed to its limit. The rapid arrest of the lizard from its falling velocity likely results in the maximal loading to which the adhesive system is exposed during normal activities. We suggest that such activities might be primary determinants in driving their high frictional adhesive capacity.

A whole lamella perspective on the origin of the epidermal free margin of Anolis (Reptilia: Dactyloidae) toe pads.

Anoline lamellae terminate in an epidermal free margin carrying the majority of its setae. How the free margin is extruded from the body of the scale is not well understood. Two hypotheses have been advanced to account for it, one advocating distal migration of the outer epidermal layers relative to the body of the lamella, and the other proposing regression of the dermal core. Available evidence provides partial support for both. We assembled a series of specimens of Anolis grahami representing all shedding cycle stages, and prepared histological sections of the toe pads to allow measurement of appropriate lamellar components through the shedding cycle. Through its proliferative phases the lamellae increase markedly in length, with the distance between the distal tip of the dermal core and that of the lamella accounting for most of this, indicating that epidermal extrusion is responsible for production of the new free margin. The dermal core showed no evidence of regression. Concomitant with epidermal extrusion, the lacunar cells on the inner lamellar face hypertrophy and keep pace with the increasing thickness of the outer lamellar face resulting from the lengthening of the replacement setae. The integrated changes observed are consistent with continuity of functioning and alignment of the exposed setal carpet of the outer epidermal generation while ensuring that the new setal carpet is fully aligned and functional immediately after shedding. At shedding the original proportions of the lamellae are restored. Development of the new free margin results from a combination of distal displacement of Oberhäutchen cells along with arrested maturation of the epidermis in this region. Changes in length of the lamellae during the proliferative stages may impact the overall size of the adhesive toe pad, which may have consequences for assessments of the relationship between whole animal clinging ability and adhesive pad area. J. Morphol. 278:360-368, 2017. © 2017 Wiley Periodicals, Inc.

The phylogenetic distribution, anatomy and histology of the post-cloacal bones and adnexa of geckos.

Post-cloacal bones of gekkotans may be present as a single (medial) pair, two pairs (medial and lateral), or may be lacking. We, herein, demonstrate that the presence of a single medial pair is the ancestral condition for the Gekkota, that the lateral pair is of sporadic occurrence within and between families, except for the Eublepharidae where it is universal, and that absence is also of sporadic occurrence except for the Sphaerodactylidae where it is the ancestral condition. Adult male Tokay geckos (Gekko gecko) possess only the medial pair of bones, and these exhibit a regionally-specific expression of woven, fibrolamellar, and lamellar bone, and an enclosed medullary cavity. Females and small juvenile males lack bony elements but exhibit a conspicuous band of dense connective tissue located about the anterior and lateral margins of the cloacal sacs. As males grow and attain sexual maturity, the medial post-cloacal bones condense in this band of dense connective tissue, and are thus shown to be dermal ossifications, similar to osteoderms but with muscular associations (although this is also known for crocodylians). Based upon ontogenetic data we set forth a scenario to explain the loss of the medial post-cloacal bones in various lineages. Differential staining of the cloacal sacs failed to reveal any specialized glandular structures. Investigation of the post-cloacal spurs shows them to be associated with cellular connective tissue of a type similar to that found in the vicinity of the medial post-cloacal bones. This suggests that the lateral post-cloacal bones may also be dermal bones, but histological evidence is needed to corroborate this.

The evolution of digit form in Gonatodes (Gekkota: Sphaerodactylidae) and its bearing on the transition from frictional to adhesive contact in gekkotans.

Although the phenomenon of adhesion in geckos has been intensively studied for over 200 years, our understanding of how the morphological apparatus associated with this arose is less clear. Indeed, whether or not all of the intricate morphological hierarchy that is implicated in the attachment and removal of the adhesive setae originated at the same time is unknown. To explore whether setae may have arisen prior to the other parts of this structural hierarchy, we undertook morphological observations of Gonatodes, an ancestrally padless, sphaerodatyline genus known to exhibit the expression of incipient subdigital pads in some species. Focusing on this geographically and morphologically well-circumscribed genus, for which intraspecific relationships are adequately known and ecology is quite well documented, allowed us to deduce trends in digit proportions, shape, scalation, and skeletal structure, and associate these with the micro-ornamentation of the subdigital surfaces. Our findings indicate that in Gonatodes, setae capable of inducing adhesion are present without the modifications of the digital musculotendinous, circulatory and skeletal systems that are generally considered to be necessary for the operation of a functional adhesive apparatus. The acquisition of these latter characteristics (independently in many lineages of gekkotans, and incipiently so in Anolis) may have been preceded by a suite of modifications of the digits that enhanced static clinging in relation to sit-and-wait predation and the ability to take refuge on surfaces unavailable to other taxa. These possibilities await further testing.