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. Ostariophysan fishes produce a chemical alarm signal ("Schreckstoff") that is released only following mechanical damage to the skin, such as that which occurs following capture by a predator. Two mutually exclusive hypotheses for the evolution of chemical alarm signals in senders are predator deterrence and predator attraction. According to the predator-attraction hypothesis, the alarm pheromone functions to attract additional predators that interfere with the predation event, which allows 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 ostariophysan fishes, are analogous to the distress calls of some birds and mammals in that they are produced only after the sender has been captured and may function to attract other predators. Alarm signals that are released only following capture by predators may therefore be functionally distinct from other alarm signals and may have evolved through direct benefits to the sender.