Abstract
BackgroundMucus 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.ResultsUsing 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.ConclusionsThe 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.
Highlights
Mucus and mucus glands are important features of the amphibian cutis
This study offers a comparative analysis of the mucus glands and their secretory products in the digital pads of tree frogs, in order to evaluate the function(s) of gland morphology and mucus chemistry related to the attachment of these animals
We present here the first 3D, quantitative assessment of anuran mucus glands in the adhesive digital pads of the tree frog H. cinerea
Summary
Mucus and mucus glands are important features of the amphibian cutis. 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. Climbing is an important aspect of terrestrial locomotion, as it allows animals to avoid ground-dwelling predators and to access elevated habitats, but poses significant challenges [1], especially on wet and slippery surfaces. Amphibians have a moist skin [14] that contains mucus, granular ( serous, poisonous, or venomous), mixed ( seromucous), and lipid glands, which secrete various muco- and other substances [14,15,16,17]; see Additional file 1 for the associated mucus nomenclature. The epidermal mucus is involved in various functions including thermoregulation, cutaneous gas exchange, reproduction, and defense against predators and pathogens [14, 16, 18, 19]
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