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You can’t see me: background matching in the Western Diamond-backed Rattlesnake

Da Cunha, Oceane

Fournier, Corentin

Horne, L. Miles

Mata-Silva, Vicente

Johnson, Jerry D.

Department of Biological Sciences

University of Texas at El Paso

El Paso, Texas USA

In ectotherms, coloration is under heavy selection as it is linked to two essential functions for survival: thermoregulation and crypsis. As an ectothermic species and an ambush predator, rattlesnakes rely on their coloration for crypsis making them excellent models to study background matching crypsis. The Western Diamond-backed Rattlesnake (Crotalus atrox) possesses the photoreceptors necessary to see colors, UV, in addition to be able to perceive infrared. Presuming this species could perceive colors, it would be advantageous for C. atrox to select backgrounds that match their own coloration to enhance crypsis. To investigate this hypothesis, pictures of 14 radio-tracked C. atrox and their backgrounds were taken at each relocation site when visible with a modified camera with visible and UV filters. For each picture taken, a picture of a random background within the snake home range was also taken for comparison. Pictures were converted to the relative photon catches of a human (Homo sapiens) and a Blue Tit (Cyanistes caeruleus). Achromatic and chromatic just noticeable differences (JND) were then calculated to compare snakes and their backgrounds, and snakes and a randomly chosen background. On average, chromatic JNDs were higher between snakes and a randomly chosen background than their selected backgrounds for both human and avian photon catches. However, these differences were not statistically significant. On the other hand, achromatic JNDs were significantly different between selected backgrounds and random backgrounds showing that rattlesnakes might choose to limit contrast differences between themselves and their backgrounds. Rattlesnakes appear to always be chromatically indiscriminable for human eyes in their natural habitat but appear to be distinguishable for avian eyes under photopic conditions. These preliminary results suggest that C. atrox and other rattlesnakes might select their habitat to enhance their crypsis as shown in other taxa.


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