CHEMICAL ALARM SIGNALS - PREDATOR DETERRENTS OR PREDATOR ATTRACTANTS

Although fright responses to alarm signals provide obvious benefits to receivers, benefits to senders of alarm signals are less clear. Ostar iophysan fishes produce a chemical alarm signal (''Schreckstoff'') tha t is released only following mechanical damage to the skin, such as th at which occurs following capture by a predator. Two mutually exclusiv e hypotheses for the evolution of chemical alarm signals in senders ar e predator deterrence and predator attraction. According to the predat or-attraction hypothesis, the alarm pheromone functions to attract add itional predators that interfere with the predation event, which allow s an opportunity for the prey to escape. We used laboratory and field experiments to demonstrate that the alarm substance of fathead minnows (Cyprinidae) is attractive to two different predators, northern pike (Esocidae) and predaceous diving beetles (Dytiscidae). We suggest that damage-released alarm pheromones, such as the alarm substance of osta riophysan fishes, are analogous to the distress calls of some birds an d mammals in that they are produced only after the sender has been cap tured and may function to attract other predators. Alarm signals that are released only following capture by predators may therefore be func tionally distinct from other alarm signals and may have evolved throug h direct benefits to the sender.