Synopsis 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 phylo- genetic 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 theirmore »
Convergent developmental patterns underlie the repeated evolution of adhesive toe pads among lizards
Abstract How developmental modifications produce key innovations, which subsequently allow for rapid diversification of a clade into new adaptive zones, has received much attention. However, few studies have used a robust comparative framework to investigate the influence of evolutionary and developmental constraints on the origin of key innovations, such as the adhesive toe pad of lizards. Adhesive toe pads evolved independently at least 16 times in lizards, allowing us to examine whether the patterns observed are general evolutionary phenomena or unique, lineage-specific events. We performed a high-resolution comparison of plantar scale development in 14 lizard species in Anolis and geckos, encompassing five independent origins of toe pads (one in Anolis, four in geckos). Despite substantial evolutionary divergence between Anolis and geckos, we find that these clades have undergone similar developmental modifications to generate their adhesive toe pads. Relative to the ancestral plantar scale development, in which scale ridges form synchronously along the digit, both padded geckos and Anolis exhibit scansor formation in a distal-to-proximal direction. Both clades have undergone developmental repatterning and, following their origin, modifications in toe pad morphology occurred through relatively minor developmental modifications, suggesting that developmental constraints governed the diversification of the adhesive toe pad in lizards.
- Award ID(s):
- 1657662
- Publication Date:
- NSF-PAR ID:
- 10328007
- Journal Name:
- Biological Journal of the Linnean Society
- Volume:
- 135
- Issue:
- 3
- Page Range or eLocation-ID:
- 518 to 532
- ISSN:
- 0024-4066
- Sponsoring Org:
- National Science Foundation
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Among the most specialized integumentary outgrowths in amniotes are the adhesive, scale-like scansors and lamellae on the digits of anoles and geckos. Less well-known are adhesive tail pads exhibited by 21 gecko genera. While described over 120 years ago, no studies have quantified their possible adhesive function or described their embryonic development. Here, we characterize adult and embryonic morphology and adhesive performance of crested gecko ( Correlophus ciliatus ) tail pads. Additionally, we use embryonic data to test whether tail pads are serial homologues to toe pads. External morphology and histology of C . ciliatus tail pads are largely similar to tail pads of closely related geckos. Functionally, C . ciliatus tail pads exhibit impressive adhesive ability, hypothetically capable of holding up to five times their own mass. Tail pads develop at approximately the same time during embryogenesis as toe pads. Further, tail pads exhibit similar developmental patterns to toe pads, which are markedly different from non-adhesive gecko toes and tails. Our data provide support for the serial homology of adhesive tail pads with toe pads.
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Abstract 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 themore »
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Abstract 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.
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Synopsis Urbanization, despite its destructive effects on natural habitats, offers species an opportunity to colonize novel niches. Previous research found that urban Anolis lizards in Puerto Rico had increased adhesive toepad area and more ventral toepad scales, traits that are likely adaptive and genetically based. We further investigated these phenotypic changes using geometric morphometrics to measure differences in toe shape, toepad shape, and lamellar morphology. Our results indicate that the increased toepad area of urban Anolis cristatellus lizards in Puerto Rico is not simply an isometric increase in toe size. Toes of urban populations exhibit multiple disproportional changes compared to forest lizards, with a larger proportion of the toe length covered in adhesive toepad. In addition, the toepads of urban lizards increase more in length than width. Lastly, lizards in urban populations exhibit both increased number of lamellae as well as increased spacing between individual lamellae. We also observed regional variation, with urban specimens having significantly more disparity, suggesting similar processes of urban adaptation are likely happening in parallel across the island, yet with region-specific idiosyncrasies, possibly generating more variation in toepad morphology across urban specimens as compared to forest specimens. Considering the use of geometric morphometrics, we found thatmore »
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Accepted Manuscript1.0
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- https://doi.org/10.1093/biolinnean/blab164
