The mystery of how frogs cling to surfaces - even if their feet are wet - may have been solved by scientists.
Tree frogs have a remarkable ability to stick
A study of tree frogs has revealed their toe pads are covered in tiny bumps that can directly touch a surface to create friction.
The scientists found this direct contact occurs even though the pads are covered with a film of watery mucus.
The findings, published in the journal Interface, may aid the development of anti-slip devices.
Researchers, based in Germany and the UK, investigated how the frogs could cling onto a smooth surface without either slipping, as their wet feet would suggest, or sticking too hard, making it difficult for them to jump.
Previous research has suggested that the "wet adhesion" could be linked to the viscosity of the mucus, but the new study suggests other forces may be at work.
"The toe pads are patterned with a fine structure of hexagonal cells with channels running between them," explained Dr Jon Barnes, an author on the paper and a zoologist from Glasgow University.
"One imagines if you are sticking to a leaf, that each cell, even if it is separate from the other cells, can form its own closest orientation."
If you looked closer still, he said, each cell was covered in bumps at the nanometre scale (One nanometre is a billionth of a metre).
"It really does look as if there is direct contact with the surface, i.e. you are getting real friction."
To see how this could take place even though the liquid film was present, the team measured the thickness of the mucus.
They found it was just one to 100nm thick; much thinner than they had expected.
At this thickness, said Dr Barnes, direct contact could still occur.
The mucus also proved much less viscous than previously thought.
"We found that the fluid is just 1.6 times more viscous than water. If you have syrup, it is thousands of times more viscous than water. So, this mucus is really very watery," said Dr Barnes.
This added weight, he said, to the idea that it was friction caused by direct contact rather than viscosity that was enabling these frogs to cling onto surfaces.
Very viscous mucus would also make it difficult for the frogs to jump, he added.
The work on the tree frogs may help engineers to develop novel anti-slip devices, such as wet-weather tyres. But Dr Barnes also added a note of caution: "There is a real problem, which is that of scaling.
"Motor cars are a good deal bigger than tree frogs, and what you find at a micro-level - you need to ask if it still works when you can scale up 100-fold.
"You might be able to do this, but there is no guarantee. Although I suspect there may be useful devices - especially smaller ones."