The evolution of chemical alarm signals: Attracting predators benefits alarm signal senders

A wide variety of organisms possess damage-released alarm pheromones that evoke antipredator responses in conspecifics. Understanding the evolution of such involuntary alarm signals has been perplexing because it is difficult to see direct benefits to the sender, notwithstanding benefits derived from warning relatives. Recently, it has been proposed that the alarm pheromone, or Schreckstoff, of Ostariophysan fishes may function in a fashion analogous to distress calls of many birds and mammals. The alarm pheromone may attract secondary predators to the proximity of the primary predation event, and, once there, the secondary predators may disrupt the predation event, thus allowing the prey greater opportunity to escape. Previous findings have established that the alarm pheromone of fathead minnows (Pimephales promelas) attracts predators, including northern pike (Esox lucius) to an area. In this study we demonstrate that the probability that fathead minnows will escape after being captured by a northern pike is significantly increased through interference by a second pike. Taken with the previous findings that alarm pheromone attracts predators, these results are the first to provide empirical evidence of benefits to senders of an involuntary alarm signal.