Dynamic mucus secretion in ventral surfaces of toe pads of the tree frog (Dryophytes japonica).

The tree frog is a prominent amphibian among terrestrial vertebrates known for its ability to adhere to various surfaces through the capillary forces of water in the microchannels between micropillars on its disc-shaped toe pads, a phenomenon known as wet adhesion. However, the secretion pattern of mucus on the attachment surface of living tree frog toe pads and the distribution of active mucus pores (AMPs) have not yet been fully elucidated. In this study, we utilized synchrotron X-ray micro-computed tomography and interference reflection microscopy to obtain the spatial distribution of the entire population of ventral mucus glands on the toe pads of living tree frogs and the real-time mucus secretion patterns from the ventral mucus pores on the contact surface under different environmental conditions. We observed that the number and secretion frequency of AMPs on the toe pad are regulated according to environmental conditions. Such dynamic mucus secretion on the tree frog's toe pad could contribute to the understanding of capillary force regulation for wet adhesion and the development of adhesive surfaces by mimicking the mucus-secreting toe pad.