NEW SCIENTIST (London, UK) 28 July 06 Snakes hunt their prey blindfolded (Zeeya Merali)
Pit vipers and boid snakes strike at prey with uncanny accuracy even when blindfolded, a feat that's been hard to explain given the rudimentary nature of their heat-sensing organs. It seems that some rather spectacular image processing may be the key.
To scout for cool shelters, and hunt in complete darkness, pit vipers and boid snakes are known to use infrared sensing organs. But their skill has amazed scientists. "In the lab, blindfolded snakes can strike a running rat behind the ears to avoid its sharp teeth," says physicist Leo van Hemmen of the Technical University of Munich in Germany. "It must be seeing more than just a warm blob."
But how, given that the snakes are saddled with very crude heat-sensing apparatus? On each side of their face, they have a pit organ that is little more than a hole with a heat-sensitive membrane stretched across it. "The eye has a lens to focus a visual image, but these holes can't do that," says van Hemmen.
Instead, the pit organs are supposed to work as "pinhole" cameras, except that the holes are too large at 1 millimetre or more in diameter. "These must produce images that are just fuzzy blurs," says van Hemmen. "So, how can the snakes strike with such precision?"
It could be down to how the snakes process the information reaching their heat sensors. Van Hemmen's team have developed a model that takes into account both the infrared "noise" created by moving prey and any errors generated by the snake's pit membrane itself.
The team recombined the inputs from the membrane's 2000-odd receptors to filter out the noise and create a sharp image. In their model, the heat signal reaching each receptor causes a neuron to fire, but the firing rate depends on the input received by all the other receptors. By tweaking how the receptors interact, the team could create precise images, even when there was a lot of background noise. "The precision is spectacular," says van Hemmen.
Nonetheless, even tiny errors generated by the pit membrane ruin this image. To minimise these errors the pit membrane would have to be no more than 15 micrometres thick - one-fifth the thickness of a sheet of paper. And this is exactly the thickness of the membrane seen in pit vipers and boids, says van Hemmen. Their work will appear in the journal Physical Review Letters.
Bruce Young, who studies snake behaviour at Washburn University in Topeka, Kansas, is impressed. "The team have addressed a fascinating question in sensory biology: what do you do with less than perfect input?" Nonetheless, he would like confirmation that this is what actually happens in real snakes.
Van Hemmen is confident that his team has uncovered the correct mechanism. "We've found a simple way that something seemingly impossible could work in the snake," he says. "If we could work it out, we're sure that nature could too"
Snakes hunt their prey blindfolded

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