Stimuli Influencing the Release and Aim of Predatory Strikes of the Northern Pacific Rattlesnake (Crotalus viridis oreganus)

Abstract

This study examined the relative importance of visual, chemical and thermal stimuli for releasing predatory strikes of the northern Pacific rattlesnake (Crotalus viridis oreganus). In experiment 1, adult snakes deprived of food for 30 days or longer were randomly presented four stimulus conditions (1 /day) combining visual, visual-chemical, visual-thermal and visual-chemical-thermal stimuli in moving models resembling mice. Experiment 2 replicated experiment 1 except that snakes were deprived of food 9 days and a fifth condition, live mice, was added. The number of strikes elicited by each condition suggested that visual cues alone were ineffective releasers of the strike, the addition of thermal cues dramatically increased the likelihood of attack, and the presence of chemical cues was not essential for striking. However, differences between the two experiments suggested that the contribution of chemoreception to prestrike behavior increased with prolonged food deprivation. The most effective strike-releasing conditions were struck more quickly and after fewer tongue flicks than less effective conditions, but tongue flick rates (recorded only in experiment 2) did not differ between conditions. Increased length of food deprivation appeared to decrease the latency to strike. Most models were struck in anterior regions, which suggested that directional movement contributed to the strike aim. The proportion of models envenomated during the strike differed between two model conditions, implying that rattlesnakes possess control over the mechanics of venom in-