Comparative and functional analysis of the digital mucus glands and secretions of tree frogs

Abstract

Background Mucus and mucus glands are important features of the amphibian cutis. In tree frogs, the mucus glands and their secretions are crucial components of the adhesive digital pads of these animals. Despite a variety of hypothesised functions of these components in tree frog attachment, the functional morphology of the digital mucus glands and the chemistry of the digital mucus are barely known. Here, we use an interdisciplinary comparative approach to analyse these components, and discuss their roles in tree frog attachment. Results Using synchrotron micro-computer-tomography, we discovered in the arboreal frog Hyla cinerea that the ventral digital mucus glands differ in their morphology from regular anuran mucus glands and form a subdermal gland cluster. We show the presence of this gland cluster also in several other—not exclusively arboreal—anuran families. Using cryo-histochemistry as well as infrared and sum frequency generation spectroscopy on the mucus of two arboreal ( H. cinerea and Osteopilus septentrionalis ) and of two terrestrial, non-climbing frog species ( Pyxicephalus adspersus and Ceratophrys cranwelli ), we find neutral and acidic polysaccharides, and indications for proteinaceous and lipid-like mucus components. The mucus chemistry varies only little between dorsal and ventral digital mucus in H. cinerea , ventral digital and abdominal mucus in H. cinerea and O. septentrionalis , and between the ventral abdominal mucus of all four studied species. Conclusions The presence of a digital mucus gland cluster in various anuran families, as well as the absence of differences in the mucus chemistry between arboreal and non-arboreal frog species indicate an adaptation towards generic functional requirements as well as to attachment-related requirements. Overall, this study contributes to the understanding of the role of glands and their secretions in tree frog attachment and in bioadhesion in general, as well as the evolution of anurans. Electronic supplementary material The online version of this article (10.1186/s12983-019-0315-z) contains supplementary material, which is available to authorized users.

Design of Tree-Frog-Inspired Adhesives

“…A similar effect may be caused by the low surface tension of the mucus ( Drotlef et al. 2013 ; Langowski et al. 2019b ), which presumably leads to strong wetting, the distribution of a given mucus volume over a large surface area, and thus to a reduced height of the skin-inherent mucus layer.…”

“…Liquid drainage during pad sliding has been shown to strengthen attachment in insects ( Federle and Labonte 2019 ). Conversely, tree frogs may actively ‘flood’ the pad-substrate gap with mucus to reduce attachment strength, or add surfactants to the mucus to control attachment strength ( Langowski et al. 2019b ).…”

How Can Interfacial Phenomena in Nature Inspire Smaller Robots

“…In the case of tree frog adhesion depends on the pad morphology, distribution and chemistry of the mucus. [ 101 ] The feet have gland clusters, with tubular ventral glands that secrete mucus through long thin channels onto the toe pad surface. The feet are carved by micro‐ and nanochannels which act as mucus storage and liquid drainage.…”

“…Estimations suggest an amount of 2.3 nL (1.1 mm 2 pad area) for safe adhesion using only capillary interactions. [ 101 ]…”

Influence of Toad Parotid Gland Secretion from Indian Toad (Bufo melanostictus) in Diabetic Rats: An Experimental Evidence of P-Glycoprotein Inhibition

“…Toads contain alveolar mucus and acinar granular glands with different types of functions [1,2]. Frogs and toads skin, glandular secretions contain bioactive host-defensive molecules from Australian anurans and, there are several types of glands available for the secretion of these compounds [3][4][5]. These natural compounds with biological activities like anti-bacterial, antiinflammatory and anti-cancer activities [6][7][8][9][10][11] are reported surprisingly there are no studies on Indian Toad secretions.…”