Schreckstoff - Predator Attractant Hypothesis

Predator Attractant Hypothesis

The predator attractant hypothesis proposes that the main purpose of schreckstoff is to attract additional predators to the area (Smith 1992). According to this hypothesis, additional predators will interact with the initial predator, and these interactions will provide the sender with an opportunity to escape. A number of conditions must be met to support this hypothesis. First, schreckstoff must attract predators. Second, subsequent predators must disrupt the predation event, thereby increasing the probability that the prey will escape. Third, the sender must be able to recover from the mechanical damage incurred during the predation event.

A study conducted by Mathis et al. (1995) provides support for the first condition that the release of schreckstoff must attract predators. This experiment revealed that schreckstoff extracted from the skin of fathead minnows attracted both northern pike (Esox lucius) and predaceous diving beetles (Colymbetes sculptilis). Additionally, a natural study showed that predatory fish were seven times more likely to strike a lure baited with a sponge soaked in fathead minnow skin extract than a sponge soaked in either water or skin extract from a non-Ostariophysan convict cichlid (which presumably does not produce schreckstoff) (Wisenden and Thiel 2002).

While the previous two studies provided examples of systems in which schreckstoff acts to attract additional predators, Cashner (2004) found a system for which this was not the case. Spotted bass (Micropterus punctulatus) were exposed to skin (containing schreckstoff) and muscle (control, containing no schreckstoff) extracts from five different co-occurring prey species. The spotted bass were not attracted to any of the schreckstoff treatments. This result indicates that schreckstoff does not always attract relevant predators in the area. Cashner additionally called into question the results of the previous study conducted by Mathis et al. (1995) by pointing out that northern pike are an introduced species in many areas and therefore were not likely to be coevolving with fathead minnows during the evolution of the schreckstoff system. He suggested that his system was more ecologically relevant and that there was little evidence to suggest that schreckstoff evolved as a predator attractant. In conclusion, the debate continues over whether or not the first condition for this hypothesis has been met.

The second condition that needs to be met in support of the predator attraction hypothesis is that additional predators must occasionally disrupt predation events, increasing the probability that prey will escape. In the northern pike/fathead minnow system, it has been proposed that additional northern pike interfere with a predation event in one of two ways (Mathis et al. 1995). First, additional northern pike of the same size interfere with a predation event by coming into contact with the main predator (biting it, etc.). Second, additional pike of larger size attracted to schreckstoff may depredate the initial predator.

Chivers et al. (1996) showed that the probability that fathead minnows escape after being captured by a northern pike significantly increases when a second pike interferes with the predation event. The authors also pointed out that northern pike have an age-structured population that is biased towards younger, smaller individuals. If a younger pike attacks a fathead minnow and attracts an older, larger conspecific, then the younger pike may be at risk of cannibalism and will be inclined to release the prey in order to focus on escape. In regards to the second condition, additional predators do appear to disrupt predation events, increasing the probability that the sender will escape. The final condition, that individuals need to successfully recover from a predation event, appears to be satisfied. Support for this condition comes from the observation that many small fishes in natural populations exhibit scars, presumably from failed predator attempts (Smith and Lemly 1986; Reimchem 1988).

While the evidence that schreckstoff attracts predators is mixed, studies indicate that multiple predators will interfere with each other and that prey can recover from predation events when they manage to escape. The extent to which predators are attracted to a predation event depends upon the speed at which schreckstoff diffuses through its aquatic environment, which in turn depends upon water flow parameters. This hypothesis indicates that schreckstoff evolved as a way of increasing the probability of survival during a predation event and its role as a predator cue for conspecifics evolved subsequently. Supported by more empirical studies than the kin selection hypothesis, the predator attraction hypothesis remained popular for quite some time.