Such behavioral contagion probably evolved early for survival, some scientists argue. It is what scatters a flock well before most members see a lunging predator.

• 部分科学家认为，早年这种行为感染力的进化，可能是为了生存。早在多数成员察觉疾冲而来的掠食者之前，行为感染力已让兽群疏散。