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First published online August 9, 2007
Journal of Experimental Biology 210, 2801-2810 (2007)
Published by The Company of Biologists 2007
doi: 10.1242/jeb.006965

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The imaging properties and sensitivity of the facial pits of pitvipers as determined by optical and heat-transfer analysis

George S. Bakken^1,* and Aaron R. Krochmal²

¹ Department of Ecology and Organismal Biology, Indiana State University, Terre Haute, IN 47809, USA
² Department of Natural Sciences, University of Houston – Downtown, 1 Main Street, Houston, TX 77002, USA

View larger version (81K): [in this window] [in a new window]   Fig. 1. (A) Close-up view of the head of a western diamondback rattlesnake Crotalus atrox Baird and Girard 1853 showing the location of the pit organ. (B) Frontal section showing the internal structure of the pit organ of a Pacific rattlesnake Crotalus o. oreganus Holbrook 1840. To aid visualization, the anterior air chamber was filled with red acrylic before the entire head was infiltrated and embedded. However, this may have displaced the membrane closer to the back of the posterior chamber. The angular aperture i varies from 23° (included angle 45°) laterally to 10° (included angle 20°) when looking ahead and to the contralateral side.

View larger version (9K): [in this window] [in a new window]   Fig. 2. Spherical coordinate system used in Eqn 2 and Eqn 4 to compute radiance and irradiance. The symbol d=sindd denotes an element of solid angle.

View larger version (45K): [in this window] [in a new window]   Fig. 3. (A) Thermogram of an Ord's kangaroo rat Dipodomys ordii taken in a laboratory enclosure set at 15°C, scaled to represent an 80° field of view with the animal 25 cm distant. (B–D) The results of convoluting that image with circular spread functions with the indicated half-angles i). (E) The same animal imaged at 30°C. Both A and E use the same color representation of temperature and are marked with the full range of temperatures visible in each. (F–H) The results of convoluting that image. The color steps in B–D and F–H indicate temperature contrasts of 0.001°C. While spatial resolution is good at 15°C for the hypothetical i=5°(D), contrast is only 0.003°C and the kangaroo rat is essentially invisible at 30°C (H). Temperature contrast is somewhat greater for i=10° and 20°, 0.006°C in the 15°C cabinet, and 0.002–0.003°C in the 30°C cabinet.

View larger version (53K): [in this window] [in a new window]   Fig. 4. (A) Thermogram of two mice, scaled for an 80° field of view with the animals 45 cm distant. Images were recorded at an air temperature of 15°C in sparse scrub habitat around midnight, following a mostly cloudy afternoon. (B–D) Image A convoluted with circular spread functions chosen to visualize the image along the optic axis of the facial pit. (E–G) Image A convoluted with elliptical spread functions to visualize imaging directly in front of the snake (right column). The i indicated for each row is the aperture angle of a circular spread function (B–D), and the vertical i of an elliptical spread function (E–G). The minor axis i of the ellipse is half that of the vertical axis. The temperature contrast in these images is quite low, so for clarity we have assumed a larger pit and greater membrane sensitivity (color steps of 0.0005°C) than in Fig. 3. Note that, particularly for large i (poor resolution), the warmest part of the image is a large, warm area of ground and not the mice.

View larger version (16K): [in this window] [in a new window]   Fig. 5. Effect of conduction through still air in the pit on the ratio of image to source temperature contrast for facial pits with i=20° and various dimensions and membrane positions. The x-axis is the thickness of the posterior chamber and indicates the position of the membrane from touching the back of the posterior chamber (thickness 0) to the center of the chamber (thickness equal to half the total thickness of the pit), as indicated by the inset drawings based on Fig. 1B. The y-axis is the temperature contrast on the pit membrane for a 1°C source temperature contrast. If conduction through the air surrounding the membrane is neglected as in prior studies, this ratio is 0.058 for pits of all sizes and configurations.

View larger version (57K): [in this window] [in a new window]   Fig. 6. The effect of facial pit size on membrane temperature contrast. The original thermogram (A) used in Fig. 4 has been processed to show membrane temperature contrasts for facial pits both on the optic axis of the pit (B–D) and directly in front of the snake (E–G). The total thickness, including both the outer and inner chambers, is indicated for each row. The membrane is assumed to be 25% of the total diameter from the wall of the posterior chamber. The temperature contrast on the pit membrane is indicated by color steps of 0.0005°C.

View larger version (74K): [in this window] [in a new window]   Fig. 7. Two situations relevant to pitviper biology present particularly strong surface temperature contrasts. (A) A rodent burrow below a desert shrub at high noon, scaled to an 80° view angle with the burrow 28 cm distant. Conditions are warm but not extreme, with air temperature of 33°C and peak ground temperatures near 50°C. (B–D) Pit membrane temperature contrasts visualized using the indicated i. The burrow could be easily identified from a distance by a snake leaving the shade of a bush to seek underground shelter. (E) An American cardinal Cardinalis cardinalis viewed against a clear sky, scaled to an 80° view angle with the cardinal 31 cm distant. Note that the thermal imager does not record radiant sky temperature properly, and so the sky has been rescaled to a radiant temperature of 5°C on the basis of Swinbank's formula (Swinbank, 1963). (F–H) Pit membrane temperature contrasts visualized using the i indicated for that row. The temperature contrast on the pit membrane in B–D and F–H are indicated by color steps of 0.001°C.