August 16, 2012
How Do Geckos Deal With Wet Feet
redOrbit Staff & Wire Reports - Your Universe Online
Geckos are unique little creatures, having the ability to cling to almost any dry surface. They seem to be just as happy scampering through tropical rainforest canopies as they are in urban settings, explains Alyssa Stark, from the University of Akron, USA. "A lot of work is done on geckos that looks at the very small adhesive structures on their toes to really understand how the system works at the most basic level", says Stark. She adds that the animals grip surfaces using microscopic hairs on the soles of their feet which make close enough contact to be attracted to the surface by the minute van der Waals forces between atoms. Stark and her colleagues, Timothy Sullivan and Peter Niewiarowski, were curious 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 it's not like the geckos fall out of the trees when it's wet". Even though minute patches of the animal's adhesive structures do not slip under humid conditions on moist glass they seem to have trouble getting a grip on smooth wet surfaces, sliding down wet vertical glass after a several steps. The team decided to examine how Tokay geckos cope on wet and dry surfaces with wet feet , and publish their discovery in The Journal of Experimental Biology.
First of all they had to look at how well the geckos clung onto glass with dry feet. Stark and Sullivan put a tiny harness around the lizard's pelvis and gently lowered the animal onto a plate of smooth glass and allowed the animal to become well attached. They attached the harness to a tiny motor and gently pulled the lizard until it came unstuck. The geckos hung on tenaciously, and only came unstuck at forces of around 20N, which is about 20 times their own body weight. "The gecko attachment system is over-designed", says Stark.
In the next step they retested the geckos after misting the glass plate with water. This time the animals had problems holding on. The attachment force varied each time they took a step. The lizards' attachment mechanism was interfered with from the water, 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 hold on to the smooth surface. There was a silvery bubble of air around their toes, but they were unable to displace the water around their feet to make the tight van der Waals contacts that usually keep the geckos in place. "The toes are superhydrophobic [water repellent]", explains Stark.
The team soaked the geckos' feet for 90 minutes and tested the adhesive forces on a dry surface. They could barely hold on and came loose when they were pulled with a force roughly equaling their own weight. "That might be the sliding behaviour that we see when the geckos climb vertically up misted glass", says Stark. So, geckos are constantly on the verge of slipping when climbing on wet surfaces with damp feet. "When the soggy lizards were faced with the misted and immersed horizontal surfaces, they slipped as soon as the rig started pulling," Stark added.
Therefore as their feet are reasonably dry, geckos can walk on wet surfaces. However, they are barely able to hang on as soon as their feet get wet. The team is keen to understand how long it takes geckos to recover from a drenching.