Tree frogs use a special mucus that acts like glue when they stay still and like oil when they move.

Tree frogs, such as the Hyla arborea, possess specialized toe pads covered in a hexagonal pattern of epithelial cells. These cells secrete a thin layer of mucus that acts as a non-Newtonian fluid. Specifically, the mucus exhibits 'shear-thinning' properties, meaning its viscosity decreases under pressure or movement.When the frog is stationary, the mucus remains viscous and creates high surface tension and capillary forces. This allows the frog to adhere to smooth, vertical, or even overhanging surfaces like wet leaves or glass. Research published in the Journal of Experimental Biology has shown that these forces are strong enough to support many times the frog's body weight.As soon as the frog begins to move, the mechanical shear stress applied to the mucus causes it to thin out instantly. This transition allows the toe pad to slide across the surface with minimal resistance, functioning much like a lubricant or oil. This rapid change in state prevents the frog from being permanently bonded to its environment.Scientists at the University of Glasgow and the Max Planck Institute have studied these biomimetic properties for potential industrial applications. Understanding this mechanism could lead to the development of reversible adhesives that work in wet conditions. This unique evolutionary adaptation ensures the frog can transition from a rock-solid grip to agile movement in milliseconds.