Geckos running with dynamic adhesion: towards integration of ecology, energetics and biomechanics

Morphological specializations often enable animals to deal with challenges in nature, a prime example being the adhesive system of geckos. With this, geckos can access smooth and vertical (and even inverted) areas of the habitat that most other animals cannot. However, what is known about how geckos cling stems primarily from laboratory studies of static adhesion, with an emphasis on the integumentary component of the adhesive apparatus. In reality, the system is hierarchical, with complex musculotendinous, vascular and sensory systems that are crucial for achieving attachment, modulation of attachment strength and ultimately, detachment. Experiments examining these additional components are virtually non-existent. Additionally, there is a paucity of information about the surfaces on which geckos move, how geckos move in their natural habitat and how the adhesive system is controlled during running over complex surfaces. It is unclear whether having an adhesive system reduces the energetic costs of running compared with lizards that lack the system. We propose a complimentary set of laboratory and field studies to fill major gaps in our understanding of gecko adhesion and locomotion. Key outstanding questions are: (1) How does surface structure influence locomotion? (2) How might geckos modulate adhesion through physiological mechanisms? (3) How do geckos locomote in complex natural habitats that vary in structural properties? (4) What are the underlying energetic costs of moving dynamically in nature with an adhesive system? We address these questions and generate a roadmap for future work, including the framing of testable hypotheses. The results of such studies will help us to understand the evolution of fast locomotion in small ectothermic vertebrates and the energetic costs of moving in complex habitats. In addition, they may inform the development of small adhesive robots.

Variation in density, but not morphology, of cutaneous sensilla among body regions in nine species of Australian geckos

Skin sense organs, i.e., cutaneous sensilla, are a well-known feature of the integument of squamate reptiles, and particularly geckos. They vary widely in morphology among species, and are thought to be mechanosensitive, associated with prey capture and handling, tail autotomy, and placement of the adhesive toepads in pad-bearing species. Some authors suggest that they may also sense abiotic environmental features, such as temperature, or humidity. Here, we describe the morphology and distribution of cutaneous sensilla among body regions of nine Australian gecko species, in four genera. We hypothesised that if sensilla morphology was distinct, or sensilla density high, around the mouth, on the tail, and on extremities, sensilla were likely used for these direct tactile functions. We found that sensilla morphology was uniform among body regions within species, but varied among species, while sensilla densities varied among species and body regions. In gecko species studied, sensilla density was highest on the labials and the dorsal tail scales, and low on the feet, head and body, providing strong support for the hypothesis that sensilla serve tactile mechanoreceptive functions for prey capture and handling and for predator avoidance, but not for toepad placement. We suggest sensilla density may be explained by mechanoreception, whereas structure may be influenced by other factors.

Spinal ganglia and peripheral nerves innervating the regenerating tail and muscles of lizards

The present review summarizes available information on the contribution of regenerating nerves to the process of regeneration in the tail of lizards. From the last three segments of the spinal cord and ganglia proximal to the regenerating tail, motor, sensory somatic and autonomous nerves regenerate and richly innervate the growing blastema. However, experimental studies have indicated that peripheral nerves are not essential for stimulating the regeneration of the tail that instead is mainly sustained by the interaction of the apical ependyma with the wound epidermis. Ganglion neurons innervating the regenerating blastema increase their size and some satellite cells multiply but no ganglion neurons are regenerated. Numerous Schwann cells proliferate to keep pace with nerve regeneration, and they form myelin starting from 3 to 4 weeks of tail regeneration. The hypertrophic ganglion neurons synthesize growth factors and signaling proteins such as FGFs and Wnts that are transported into the regenerating blastema through the regenerating nerves. Nerves form synaptic-like contacts with mesenchymal cells or fibroblasts at the tip of the regenerating blastema but not synaptic boutons. These terminals may discharge stimulating factors that favor cell proliferation but this is not experimentally demonstrated. Most of the innervation is directed to differentiating muscles where nerve endings form cholinergic motor-plates. Transcriptome data on the regenerating blastema-cone detect up-regulation of various genes coding for ionic channels, neurotransmitter receptors and signaling proteins. The latter suggests that the neurotrophic stimulation may control cell proliferation but is most directed to the functionality of regenerating muscles.

The dendrite arbor of Purkinje cells is altered following to tail regeneration in the leopard gecko

Purkinje cells of the cerebellum have a complex arborized arrangement of dendrites and are amongst the most distinctive cell types of the nervous system. Although the neuromorphology of Purkinje cells has been well described for some mammals and teleost fish, for most vertebrates less is known. Here we used a modified Golgi-Cox method to investigate the neuromorphology of Purkinje cells from the lizard Eublepharis macularius, the leopard gecko. Using Sholl and Branch Structure Analyses, we sought to investigate whether the neuromorphology of gecko Purkinje cells was altered is response to tail loss and regeneration. Tail loss is an evolved mechanism commonly used by geckos to escape predation. Loss of the tail represents a significant and sudden change in body length and mass, which is only partially recovered as the tail is regenerated. We predicted that tail loss and regeneration would induce a quantifiable change in Purkinje cell dendrite arborization. Post hoc comparisons of Sholl analyses data showed that geckos with regenerated tails have significant changes in dendrite diameter and the number of dendrite intersections in regions corresponding to the position of parallel fiber synapses. We propose that the neuromorphological alterations observed in gecko Purkinje cells represent a compensatory response to tail regrowth, and perhaps a role in motor learning.