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

Epidermal microstructures on the paired fins of marine sculpins suggest new functional hypotheses supporting benthic station-holding

Harsh environments, such as those with breaking waves and turbulent flows, present extreme challenges to organismal survival. Many animals exploiting these habitats possess adaptations to maintain position under dynamic flow conditions, such as reversible or permanent attachment systems. However, some station-holding fishes (e.g. sculpins) instead rely on morphological and behavioural modifications of their pectoral fins to increase friction with the substrate and combat drag. Despite epidermal microstructures on the fins of other benthic fishes, little exploration of pectoral fin surfaces at the microscopic scale has been undertaken in sculpins. Using scanning electron microscopy, we discovered microscopic, fibrillar projections contained within single cells on the ventral surfaces of the paired fin rays of two intertidal and two subtidal species of marine sculpins. In contrast to subtidal species, the intertidal species possessed epidermal cells with discrete channels separating groups of fibrillar projections. These features bear a striking resemblance to epidermal microstructures described in other fishes but have distinct morphological differences. We suggest the hypothesis that these previously overlooked features contribute to sculpin station-holding performance via enhanced mechanical interactions with the substrate, suggesting new taxa within which to explore potential mechanisms of underwater friction enhancement and adhesion.

Frictional adhesion of geckos predicts maximum running performance in nature

Despite the myriad studies examining the diversity and mechanisms of gecko adhesion in the lab, we have a poor understanding of how this translates to locomotion in nature. It has long been assumed that greater adhesive strength should translate to superior performance in nature. Using 13 individuals of Bradfield's Namib day gecko (Rhoptropus bradfieldi) in Namibia, I tested the hypothesis that maximum running performance in nature (speed and acceleration) is driven by maximum frictional adhesive strength. Specifically, those individuals with greater frictional adhesion should escape with faster speed and acceleration because of increased contact with the surface from which to apply propulsive forces. I tested this prediction by quantifying laboratory adhesive performance and then releasing the geckos into the field while simultaneously recording the escape using high-speed videography. Additional measurements included how this species modulates maximum running speed (stride length and/or stride frequency) and how temperature influences field performance. I found that maximum acceleration was significantly correlated with maximum frictional adhesive strength, whereas maximum sprinting speed was only correlated with increases in stride frequency (not stride length) and temperature. Thus, different measures of performance (acceleration and speed) are limited by very different variables. Acceleration is key for rapidly escaping predation and, given their correlation, maximum frictional adhesion likely plays a key role in fitness.

Toe pad morphology and adhesion in the miniaturized gecko, Chatogekko amazonicus (Gekkota: Sphaerodactylidae)

Chatogekko amazonicus is a miniaturized gecko from northern South America and is among the smallest of toe pad bearing lizards. The toe pads of C. amazonicus are miniscule, between 18% and 27% of the plantar surface area. We aimed to investigate the relationship between adhesive toe pad morphology, body size, and adhesive capabilities. Using scanning electron microscopy, we determine that the adhesive pads of C. amazonicus exhibit branched setae similar to those of other geckos, but that are generally much smaller. When compared with other gecko taxa, we show that C. amazonicus setae occupy a similar range of seta length: snout-vent length ratio and aspect ratio as other gekkonoid species (i.e. Gekkonidae, Phyllodactylidae, and Sphaerodactylidae). We demonstrate that C. amazonicus-even with its relatively small toe pads-is capable of climbing a smooth glass surface at a nearly vertical angle. We suggest that sphaerodactylids like C. amazonicus offer an excellent system for studying toe pad morphology and function in relation to miniaturization.

Actin-templated Structures: Nature's Way to Hierarchical Surface Patterns (Gecko's Setae as Case Study)

The hierarchical design of the toe pad surface in geckos and its reversible adhesiveness have inspired material scientists for many years. Micro- and nano-patterned surfaces with impressive adhesive performance have been developed to mimic gecko's properties. While the adhesive performance achieved in some examples has surpassed living counterparts, the durability of the fabricated surfaces is limited and the capability to self-renew and restore function-inherent to biological systems-is unimaginable. Here the morphogenesis of gecko setae using skin samples from the Bibron´s gecko (Chondrodactylus bibronii) is studied. Gecko setae develop as specialized apical differentiation structures at a distinct cell-cell layer interface within the skin epidermis. A primary role for F-actin and microtubules as templating structural elements is necessary for the development of setae's hierarchical morphology, and a stabilization role of keratins and corneus beta proteins is identified. Setae grow from single cells in a bottom layer protruding into four neighboring cells in the upper layer. The resulting multicellular junction can play a role during shedding by facilitating fracture of the cell-cell interface and release of the high aspect ratio setae. The results contribute to the understanding of setae regeneration and may inspire future concepts to bioengineer self-renewable patterned adhesive surfaces.

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.

Climbing and Clinging of Urban Lizards are Differentially Affected by Morphology, Temperature, and Substrate

Urbanization alters the environment along many dimensions, including changes to structural habitat and thermal regimes. These can present challenges, but may also provide suitable habitat for certain species. Importantly, the functional implications of these habitat shifts can be assessed through the morphology-performance-fitness paradigm, though these relationships are complicated by interactions among habitat choice, other abiotic factors, and morphology across scales (i.e., micromorphology and gross anatomy). The common wall lizard (Podarcis muralis) is one example of a cosmopolitan and successful urban colonizer. Quantifying both shifts in morphology over time and morphology-performance relationships under various ecological contexts can provide insight into the success of species in a novel environment. To examine how morphological variation influences performance, we measured seven gross morphological characteristics and utilized scanning electron microscopy to obtain high-resolution images of a claw from individuals living in established populations in Cincinnati, Ohio, USA. We used a geometric morphometric approach to describe variation in claw shape and then compared the claws of contemporary lizards to those of museum specimens collected approximately 40 years ago, finding that claw morphology has not shifted over this time. We then performed laboratory experiments to measure the clinging and climbing performance of lizards on materials that mimic ecologically relevant substrates. Each individual was tested for climbing performance on two substrates (cork and turf) and clinging performance on three substrates (cork, turf, and sandpaper) and at two temperatures (24ºC and 34ºC). Clinging performance was temperature insensitive, but determined by substrate-specific interactions between body dimensions and claw morphology. Conversely, the main determinant of climbing performance was temperature, though lizards with more elongate claws, as described by the primary axis of variation in claw morphology, climbed faster. Additionally, we found strong evidence for within-individual trade-offs between performance measures such that individuals who are better at clinging are worse at climbing and vice versa. These results elucidate the complex interactions shaping organismal performance in different contexts and may provide insight into how certain species are able to colonize novel urban environments.

The Lesser-Known Transitions: Organismal Form and Function Across Abiotic Gradients

From minute-to-minute changes, or across daily, seasonal, or geological timescales, animals are forced to navigate dynamic surroundings. Their abiotic environment is continually changing. These changes could include alterations to the substrates animals locomote on, flow dynamics of the microhabitats they feed in, or even altitudinal shifts over migration routes. The only constancy in any organism's day-to-day existence is the heterogeneity of the habitats they move through and the gradients in the physical media (e.g., air, water) they live in. We explored a broad range of organismal transitions across abiotic gradients and investigated how these organisms modify their form, function, and behavior to accommodate their surrounding media. We asked the following questions: (1) What are some challenges common to animals in changing media or moving between media? (2) What are common solutions to these recurring problems? (3) How often are these common solutions instances of either convergence or parallelism? Our symposium speakers explored these questions through critical analysis of numerous datasets spanning multiple taxa, timescales, and levels of analysis. After discussions with our speakers, we suggest that the role of physical principles (e.g., drag, gravity, buoyancy, viscosity) in constraining morphology and shaping the realized niche has been underappreciated. We recommend that investigations of these transitions and corresponding adaptations should include comparisons at multiple levels of biological organization and timescale. Relatedly, studies of organisms that undergo habitat and substrate changes over ontogeny would be worthwhile to include in comparisons. Future researchers should ideally complement lab-based morphological and kinematic studies with observational and experimental approaches in the field. Synthesis of the findings of our speakers across multiple study systems, timescales, and transitional habitats suggests that behavioral modification and exaptation of morphology play key roles in modulating novel transitions between substrates.

Parameters of the adhesive setae and setal fields of the Jamaican radiation of anoles (Dactyloidae: Anolis): potential for ecomorphology at the microscopic scale

The subdigital adhesive pads of Caribbean Anolis lizards are considered to be a key innovation that permits occupation of novel ecological niches. Although previous work has demonstrated that subdigital pad morphology and performance vary with habitat use, such investigations have only considered the macroscale aspects of these structures (e.g. pad area). The morphological agents of attachment, however, are arrays of hair-like fibres (setae) that terminate in an expanded tip (spatula) and have not been examined in a similar manner. Here we examine the setal morphology and setal field configuration of ecologically distinct species of the monophyletic Jamaican Anolis radiation from a functional and ecological perspective. We find that anoles occupying the highest perches possess greater setal densities and smaller spatulae than those exploiting lower perches. This finding is consistent with the concept of contact splitting, whereby subdivision of an adhesive area into smaller and more densely packed fibres results in an increase in adhesive performance. Micromorphological evidence also suggests that the biomechanics of adhesive locomotion may vary between Anolis ecomorphs. Our findings indicate that, in a similar fashion to macroscale features of the subdigital pad, its microstructure may vary in relation to performance and habitat use in Caribbean Anolis.

The Comparative approach to bio-inspired design: integrating biodiversity and biologists into the design process

Biodiversity provides a massive library of ideas for bio-inspired design, but the sheer number of species to consider can be daunting. Current approaches for sifting through biodiversity to identify relevant biological models include searching for champion adapters that are particularly adept at solving a particular design challenge. While the champion adapter approach has benefits, it tends to focus on a narrow set of popular models while neglecting the majority of species. An alternative approach to bio-inspired design is the comparative method, which leverages biodiversity by drawing inspiration across a broad range of species. This approach uses methods in phylogenetics to map traits across evolutionary trees and compare trait variation to infer structure-function relationships. Although comparative methods have not been widely used in bio-inspired design, they have led to breakthroughs in studies on gecko-inspired adhesives and multifunctionality of butterfly wing scales. Here we outline how comparative methods can be used to complement existing approaches to bioinspired design, and we provide an example focused on bio-inspired lattices, including honeycomb and glass sponges. We demonstrate how comparative methods can lead to breakthroughs in bio-inspired applications as well as answer major questions in biology, which can strengthen collaborations with biologists and produce deeper insights into biological function.

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.