T-Day geek post #1

Why is the Jesus lizard called a Jesus lizard? Because it can walk on water. Well, okay, it doesn't exactly walk on water - it runs on water.

Really. It can. Aside from a few insects and spiders that spread their weight out far enough to not break the surface tension of the water, the Jesus lizard - more usually known as the basilisk lizard - is the only creature that can do it. And now, the BBC for Wednesday reports, scientists may have figured out how it does it.

Harvard University's Dr Tonia Hsieh told the BBC World Service that experiments showed the lizard to be producing massive sideways force to stay upright. ...

The study, which was reported recently in the Proceedings of the National Academy of Sciences, reveals how a large upward force is produced every time the lizard slaps its foot down into the water.

This keeps the animal from sinking straight down into the liquid. But just like we tend to teeter forward when we run on a soft surface such as sand, the lizard would also stumble forward unless it had a mechanism for stabilising itself.

And this is where the sideways force comes in - and it is almost as strong as the initial slap down. ...

Animals that run on land with two legs, such as birds and humans, have little force directed out towards the sides. The basilisk lizard is very different. ...

"Our guess on this is that it appears to help maintain stability ... as they're running across water; they're constantly tripping[," Dr Hsieh said.]

"It's a matter of catching themselves and keeping themselves upright before they actually fall over."

The lizards have long bodies and large feet. On the edges of their toes are fringes that resemble pom-poms with a number of fronds.

That serves to spread the force of the slap over a larger area of water. Think of trying to reach something at the bottom of a sink full of water. Which would be easier: Pushing your hand in fingers first or slapping down with your palm? Well, then why? Combine two things: First, pressure, which is force per unit area (e.g., air pressure at sea level is about 14.7 pounds per square inch - do the metric conversion yourself). And by Newton's Third Law of Motion ("for every action there is an equal and opposite reaction"), the harder the slap on the water, the greater the push of the water on the hand. So compare an Olympic diver with a 300-pound man doing a belly flop. The former has less force (weight is a force) than the latter but still probably generates much greater pressure on the water because that force, even if smaller, is still concentrated on such a small area. Our belly-flopper, then, exerts less presure but hits the water with greater force. Which one will feel the hit more?

In the same way, when the lizard slaps its broad foot on the water, the water pushes up against the foot, supporting the lizard. In fact, any of us could probably do the same by wearing some kind of sufficiently large flipper arrangement on our feet - for about one step. Then we'd fall face forward into the water. What I find so cool about the explanation is that it means that the lizard is actually constantly tripping but because of the distribution of forces it just manages to keep up with itself long enough to keep from actually falling. Neat.

Dr Hsieh said there were "definitely" broader implications coming out of the research that would centre on the study of how animals moved over many different types of surface.

"They have to manoeuvre across all these surfaces on a daily basis - how exactly are they doing this?" she said.

Footnote: There is in mythology a terrible beast called the basilisk that was so deadly its very glance was lethal.