Geckos are perfectly adapted for clinging to dry surfaces in rainforests and urban environments.

The secret to their success are weak intermolecular forces called van der Waals forces, which are associated with simple attraction between two molecules. In the case of the geckos, the attraction comes from the tiny adhesive hairs on the bottoms of their toes, which flatten out when the gecko’s feet make contact with a surface, and the surface itself.

“By having hairs, instead of just skin like our fingers, the weak van der Waals forces are multiplied due to the increase in surface area,” says researcher Alyssa Stark from the University of Akron. But there are some situations where these lizards can lose their grip, usually sliding down a wet surface after a few steps.

To find out how geckos cope with in the wet, Stark and her colleagues conducted a study, published in the Journal of Experimental Biology, to see how well they could stick to the surface when it was dry. They fitted a harness around the gecko’s pelvis and attached the other end to a force sensor, which could move backwards and forwards at a controlled speed once the lizards had had a chance to get a good grip.

The geckos would cling or stick to the surface up to some force, but eventually their feet would begin to slip across the glass plate, Stark explains. “Our maximum force was determined to be the point right as all four of their feet began to slip along the surface.”

They then tested geckos with wet feet, who used the same mechanism to stick to the glass. But this time the water disrupted the close contact they need to cling to the surface and the lizards had trouble sticking to glass misted with water drops or on glass fully submerged in water.

The key, however, seemed to be whether their feet were wet or not, according to Stark, as they could hang on to a surface with water drops with dry feet. In contrast, they had trouble with dry surfaces when their feet were soaking wet. “This finding suggests that wet toes can significantly impact the gecko adhesive system, to the point where most geckos would not be able to support their body weight.”

So they can walk on damp surfaces, as long as their feet are dry, and the researchers are keen to understand how long it would take the lizards to recover from being soaked by rain. “We know they are in tropical environments that probably have a lot of rain and it’s not like the geckos fall out of the trees when it’s wet,” Stark says.