Effects of Chemical and Visual Stimuli upon Chemosensory Searching by Garter Snakes and Rattlesnakes

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Abstract
Many studies indicate that chemical cues activate garter snake feeding behavior; visual and/or thermal stimuli do so for rattlesnakes. No experiments have factorially combined chemical and visual cues for either of these taxa. The present work explored possible interactive effects of these stimuli on rate of tongue flicking (RTF) in garter snakes (T. r. haydeni) and rattlesnakes (Crotalus enyo enyo, C. viridis viridis and Sistrurus catenatus tergeminus). In experiments 1 and 2 snakes were exposed to 4 conditions representing an orthogonal combination of presence vs. absence of visual and chemical stimuli arising from prey. RTF was recorded for 5 min under each condition. Garter snakes exhibited a significant elevation in RTF in the presence of chemical cues whether or not visual cues were present. There was also an effect of visual cues but no interaction between visual and chemical cues. Rattlesnakes did not respond to chemical cues; these snakes showed only an effect of visual cues. In experiment 3, rattlesnakes were again observed as above, with the exception that test sessions were of 10 min duration. Behavior during the 1st 5 min was identical to that seen in experiment 2, but during min 6-10 the snakes responded more in the condition containing visual plus chemical cues than in any other condition. Visual stimuli alerted rattlesnakes to the presence of potential prey and gave rise to elevated RTF which subsequently allowed these predators to utilize chemical cues that happened to be available. Garter snakes and rattlesnakes use both chemical and visual information but differ in the sequence in which these stimuli are used.