The secrets of how snakes slither across smooth surfaces have finally been pinned down by animal locomotion experts. The finding solves an age-old conundrum and could help engineers build better search-and-rescue robots.
It's clear that snakes push against rocks and other debris when moving across rough ground, but exactly how do they negotiate flat surfaces, such as paved roads?
David Hu, now at the Georgia Institute of Technology, and colleagues at New York University say they have found an answer.
The team administered anaesthetic to 10 young Pueblan milk snakes – relatively small and harmless reptiles about 35 centimetres long. While the snakes were knocked out, Hu measured the force required to slide them in different directions.
He found that it was twice as hard to move the snakes sideways as it was to slide the animals forwards, and 50 per cent harder to push them backwards. The friction is caused by the orientation of the snakes' scales, which are arranged to resist such movements.
This difference is the key to crossing flat surfaces like sand and bare rock, says Hu. The animals propel themselves using their muscles to move their bodies in a wave. As the wave travels backwards through its body, the snake's scales catch the ground, generating a frictional force that propels it forward.
Using video of the snakes in action, Hu's team also found that the snakes lift parts of their body slightly off the ground when moving. This helps reduce unwanted friction and helps apply greater pressure to the parts of the wave that are pushing the snake forwards. When Hu combined these two effects in a computer model of snake movement, it produced simulations that slithered as fast as real snakes do.
Lakshminarayanan Mahadevan, a mathematician at Harvard University who has studied snake locomotion, says that researchers have known about the frictional properties of snake skin since at least the 1940s, but reckons Hu's work is the first to use the lifting and friction together to explain it.
The finding could help engineers working on robots that might one day search wrecked buildings for earthquake survivors: snake-like designs have been explored, but they have trouble crossing smooth surfaces.
Hu suggests that the robots would do better if they were wrapped in a skin that mimicked the action of snake scales.
"It is an interesting thing to explore," says Gavin Miller, a software engineer at Adobe Systems in San Jose, California, who builds snake robots in his spare time.
Miller suggests that a flexible version of the material used to coat the underside of cross-country skis, which has an appropriate scaly structure, might be suitable.