Ebert, J. (2008). Infrared sense in snakes: Behavioural and anatomical examinations (crotalus atrox, python regius, corallus hortulanus). Unpublished PhD thesis, Rheinische Friedrich-Wilhelms-Universität Bonn.
Added by: Sarina (2021-01-31 10:33:47) Last edited by: Sarina (2021-02-08 15:44:06)
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BibTeX citation key: Ebert2008
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Publisher: Rheinische Friedrich-Wilhelms-Universität Bonn
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The ability to detect infrared (IR) radiation is a characteristic of boids and pitvipers. These snakes possess highly sensitive IR receptors, often embedded in pit organs, which enable them to locate IR sources independently of visual cues. It was aimed to behaviourally determine IR detection thresholds of two separately evolved IR sensory systems. Electrophysiological studies have been conducted to determine IR detection thresholds in boids and pitvipers. This is the first behavioural study which focuses on the detection thresholds of a pitviper and a boid snake to IR stimuli. Blindfolded Western diamondback rattlesnakes (Crotalus atrox) and Ball pythons (Python regius) were exposed to a moving IR stimulus of constant size and temperature at various distances (C. atrox: 10-160 cm, P. regius: 10-100 cm). The threshold for eliciting distinct behavioural changes during stimulus presentation was used to assess their IR detection thresholds. C. atrox can detect a moving IR stimulus resembling a mouse in temperature and size up to a distance of 100 cm, which corresponds to an irradiance contrast of 3.35 x 10-6 W/cm2. P. regius detected the IR stimulus up to a distance of 30 cm, which corresponds to an irradiance contrast of 3.83 x 10-5 W/cm2. These irradiance contrast detection thresholds reveal a sensitivity 3.2-times higher (C. atrox) or 1.5-times higher (P. regius) than sensitivities found in previous electrophysiological investigations, confirming that behavioural approaches determine sensory sensitivities far more accurately. The differing IR sensitivities of C. atrox and P. regius presumably result from their different habitats, i.e. C. atrox prefers open spaces, whereas P. regius primarily inhabits narrow burrows and rocky areas. Naturally, their IR detection thresholds should be adapted to the distance ranges at which they can be employed usefully. The pit morphology of pitvipers, pythons and boas varies greatly. Pitvipers possess loreal pits, and most pythons have labial pits, whereas most boas lack labial pits or depressions. Pitvipers and pythons are quite well investigated; however, the pit morphology of pit-bearing boas has hardly been examined. The IR sensitive Amazon tree boa (Corallus hortulanus) possesses extraordinarily shaped deep labial scale depressions, which are arranged in a zigzag-formation resulting in backward- and forward-facing scale areas. This unique directional morphology is assumed to be the basis for the IR perception mechanism of C. hortulanus. To build a model for assessing the function of this pit arrangement for IR reception, it was necessary to determine the occurrence, structure and distribution of the IR receptors, i.e. terminal nerve masses (TNMs), within the labial scales. Their structure was investigated by light microscopy and transmission electron-microscopy, whereas the location and size of the TNM areas was determined by succinate dehydrogenase staining. Furthermore, tracing experiments with neurobiotin should have exhibited the central neural projection of the TNMs, however only one trial successfully revealed projections up to the IR processing LTTD. Certain small pores have previously been hypothesised to aid IR reception therefore the surface of the labial scales was inspected with scanning electron microscopy (SEM). These SEM results were combined with a precise 3-D reconstruction of the labial scales, revealing a loose correlation between TNM areas and specific small pores. However, these pores do not seem suitable for reflecting away non-IR wavelengths as hypothesised. Indeed it seems more probable that these pores aid IR reception by homogenising the surface structure, and thereby the surface temperature. To investigate the functional morphology, the SDH staining results were necessary to reconstruct the TNM areas of the supra- and infralabial scales and transfer these onto a plaster head cast of C. hortulanus. This model was used to decode the directional reception characteristics of the 3-D IR field of view, which covers an angular range of up to 300°. It was possible to show that the radiation of an IR source impinges on different proportions of forward- and/or backward-facing TNM areas when coming from different angles, thus giving exact directional information. Additionally, the spatial resolution is enhanced by the sophisticated shape of some labial scales. Moreover, a novel enhancement mechanism was discovered: when a boa turns its head to frontally face an IR source, it receives a sudden drop in IR stimulation at the 10° angle, before receiving doubled IR radiation input when facing the IR source frontally (0° angle). Only then does the IR radiation impinge fully on both sides of the boas head and labial scales. The swift drop at 10° most probably serves as a neural trigger for precise targeting when striking. With C. hortulanus a new IR sensing system has been discovered and the results help to understand how morphology can function to increase the spatial resolution power of sensory systems.