Behavioral complexity, prey-handling ability and the evolution of constricting behavior in snakes

A traditional Darwinian view of evolution posits that complex traits evolve gradually from simpler precursors and predicts that as complexity increases, the functionality of traits increases as well. I examined the relationship between the complexity and functionality of different snake prey-handling behaviors. I videotaped predatory encounters between snakes and prey, and then quantified behavioral complexity based on the number of discrete acts composing a behavior and the degree of coordination among acts. I compared complexity and performance among eight snake species including non-constrictors (least complex), species in which constriction has evolved recently (more complex), and long-term constrictors (most complex). Compared to simpler behaviors used by non-constrictors (jaw-hold and body-pinning), complex constriction involving parallel loops of a snake's body significantly reduces escape and struggling by mice and leads to rapid death of prey. However, in a species that has recently evolved constriction, the behavior functions primarily to minimize struggling by prey until they can be grasped and held firmly in the snake's jaws until dead. In this species, struggling by prey is a key stimulus eliciting constriction as these snakes rarely constricted helpless prey (frogs), but regularly attempted to constrict struggling prey (mice). These observations of a recently evolved form of constriction indicate that the ability of some long-term constrictors to "squeeze" prey to death via constricting loops evolved secondarily. I also tested the idea that an evolutionary shift in diet toward helpless prey (bird eggs) led to relaxed selection for complex constriction. Compared to its close relatives, a species that feeds primarily on bird eggs rarely used parallel loops, was more hesitant to attack rodent prey and took longer to subdue them using constriction. In addition to being a key proximate factor eliciting constriction, struggling by prey appears to be an important selective force driving the evolution of this widespread behavior.