Searching for the Critical Features of Snakes

Research over the last decade has clarified that snakes are efficiently detected by adults, children, and even monkeys which have never seen a real or toy snake. Yet, despite several studies that have attempted to determine what characteristics of snakes attract the attention of humans and primates, no definitive proof has been obtained. The visible characteristics of snakes include their curvilinear shape, limblessness, triangular-shaped head (especially vipers), coloration, specific spatial frequencies, poses and postures, scales, and so on. Any of these could individually or in combination serve as triggers to indicate “snake.” This chapter investigates how each of these characteristic “clues” contributes to snake recognition. Since humans, monkeys, and other animals react strongly even when only a portion of a snake is visible, viewing only a part of a snake body may enable recognition. This is a reasonable conjecture considering that snakes conceal themselves when hunting prey. Our research on humans and monkeys has shown that there is speedier detection of snakes with scales than of other animals not dangerous for participants. Yet, when images were processed to remove the scales and present smooth surfaces, there was no longer immediate detection of a snake. While photographs of snakes with scales elicited stronger event-related brainwave potentials (ERPs) than did photos of birds, when the scales were removed, ERPs ranged in the intermediate area between those for birds and those for snakes with scales. This suggests that a key visual feature for snake recognition is their scales.