Locating and Matching Information

Geckos are remarkable little lizards, clinging to almost any dry surface, and Alyssa Stark, from the University of Akron, US, explains that they appear to be equally happy scampering through tropical rainforest canopies as they are in urban settings. A lot of gecko studies look at the very small adhesive structures on their toes to understand how the system works at the most basic level, says Stark. She adds that the animals grip surfaces with microscopic hairs on the soles of their feet, which make close enough contact to be attracted to the surface by the minute forces between atoms.

However, she and her colleagues Timothy Sullivan and Peter Niewiarowski were curious about how the lizards cope on surfaces in their natural habitat. Explaining that previous studies had focused on the reptiles clinging to artificial dry surfaces, Stark says, 'We know they are in tropical environments that probably have a lot of rain and geckos don't suddenly fall out of the trees when it's wet.' Yet, the animals do seem to have trouble getting a grip on smooth, wet, artificial surfaces, sliding down wet vertical glass after several steps. The team decided to find out how geckos with wet feet cope on both wet and dry surfaces.

First, they had to find out how well their geckos clung onto glass with dry feet. Fitting a tiny harness around the lizard's pelvis and gently lowering the animal onto a plate of smooth glass, Stark and Sullivan allowed the animal to become well attached before connecting the harness to a tiny motor and gently pulling the lizard until it came unstuck. The geckos hung on tenaciously, and only came unstuck at forces of around 20N - about 20 times their own body weight. 'In my view, the gecko attachment system is over-designed,' says Stark.

Next, the trio sprayed the glass plate with a mist of water and re-tested the lizards, but this time the animals had problems holding tight. The droplets were interfering with the lizards' attachment mechanism, but it wasn't clear how. When the team immersed the geckos in a bath of room-temperature water with a smooth glass bottom, the animals were completely unable to anchor themselves to the smooth surface. 'The toes are super-hydrophobic,' explains Stark, who could see a silvery bubble of air around their toes. But they were unable to displace the water around their feet to make the tight contact that usually keeps the geckos in place.

Then the team tested the lizard's adhesive forces on the dry surface when their feet had been soaking for 90 minutes, and found that the lizards could barely hold on, detaching when they were pulled with a force roughly equalling their own weight. 'That might be the sliding behaviour that we see when the geckos climb vertically up misted glass,' says Stark. So, geckos climbing on wet surfaces with damp feet are constantly on the verge of slipping and Stark adds that when the soggy lizards were faced with the misted and immersed horizontal surfaces, they slipped as soon as the rig started pulling. Therefore geckos can walk on wet surfaces, as long as their feet are reasonably dry. However, as soon as their feet get wet, they are barely able to hang on, and the team is keen to understand how long it takes geckos to recover from a drenching.