Pitvipers quite rightly have a fearsome reputation, injecting their victims with venom from their sharp fangs, causing very painful bites. But there is more to pitvipers than a vicious bite; they are also well known for having heat-detecting organs, called pits, located between their nostrils and mouth,which allow them to sense the temperature of surrounding objects. Conventional wisdom suggests that the pits are highly accurate organs, allowing the snakes to see' a detailed thermal map of their surroundings with pinpoint accuracy. However as George Bakken and Aaron Krochmal show in their latest paper, the vipers' pit organs might not give as clear an image as previously suspected(p. 2801). Pitvipers'pits are irregular, mushroom shaped cavities with a 1-3 mm long opening. A membrane at the back of the pit, covered in sensory receptors, responds to temperature changes. In order to understand what information is reaching the pit membrane, indicating what the snakes can see' thermally, Bakken and Krochmal modelled the pit as an optical system. Thermal radiation travelling through the pit's opening and onto the membrane is much like light travelling through the opening in a simple pinhole camera.

First the model assumed that the pit would work like a perfect optical system, where all light (or heat) entering the pit would form a highly focussed point on the membrane. The second part of the model calculated how the thermal radiation entering the pit would heat up the membrane, stimulating the membrane's receptors. Because pits have no lenses, the thermal radiation entering the pit forms fuzzy, un-focussed points on the membrane. Therefore the third part of the model took into account the fact that each point is fuzzy, and overlaps with the points around it, and calculated the effect this had on the resulting image.

Having developed their model, the team recorded thermograms - essentially heat maps - of natural situations to show the temperatures and contrasts that the snakes are likely to encounter. By putting the temperature information from the thermograms into their model, they could calculate maps of what the snakes could see'.

They found that the size of the pit opening influenced both the resolution- or the fuzziness' - of the image, and signal strength, or brightness. With a bigger opening, images are fuzzier, but brighter. It looks like you lose more by making the image fuzzy than you gain by making it brighter', says Bakken. So on balance, smaller apertures appear to form a better image on the pit membrane. The team also found that prey such as a mouse was hard to detect even if they assumed that the membrane could pick up temperature differences of 0.001°C, because the image smeared' over a large area of membrane. This meant that the mouse didn't show up as a temperature `hotspot',indicating that latching onto the strongest thermal signal is not a reliable way for a pitviper to be sure of getting a meal.

Scenes with larger targets and a strong thermal contrast, such as the cool opening to a rodent burrow, fared much better, supporting the team's idea that pits probably evolved to help the snakes regulate their body temperatures by seeking out warm or cool spots. Given that the model suggests that pitvipers'temperature sensing is not as accurate as previously thought, the team suspect that the snakes' brains might be sharpening up the thermal image formed on the pit membrane. Next they plan to find out whether the snakes do this, and if so, how they do it.

Bakken, G. S. and Krochmal, A. R. (
2007
). The imaging properties and sensitivity of the facial pits of pitvipers as determined by optical and heat-transfer analysis.
J. Exp. Biol.
210
,
2801
-2810.