Predator avoidance freeze responses

Essentially all animals experience the threat of predation at some point in their life history (Lima Dill, 1990), and selection seems to favor either avoidance mechanisms that reduce the likelihood of contact or anti-predator mechanisms that reduce predator success after contact has occurred (Sih, 1985 Brodie, Formanowicz Brodie, 1991). Avoidance behavior can vary from elevated activity that is associated with seeking locations of comparative safety ( flight response) to inhibited activity that is associated with natural concealment or death feigning ( freeze response). Species (sea urchins, fishes) may show one or the other of these responses corresponding to their natural history (Parker Shulman, 1986 Smith, 1992, 1997), but some species (salamanders, snails) may shift between response types depending on the ecological context (Sih Kats, 1991 Turner, 1994, 1996). These studies indicate that the avoidance of a stimulus may appear to dissociate from (vary independently of) activity level. Thus an animal s activity level may reveal important information about the fine-tuning of behavioral avoidance in the natural context. 

The lower activity levels at night during the goldfish-fed snake trials were probably normal, low-level activity rather than freeze responses. The latter term implies immobility and certainly a reduction in activity below normal levels, which in the absence of a significant avoidance response was unlikely to have occurred. Some of the salamanders in our study may have momentarily shifted into an anti-predator defense mode upon initial exposure to snake odors, but quickly shifted back into an avoidance mode once contact with the predator appeared less imminent. This ambivalence in some salamanders may also have contributed to the 18 minute delay to peak activity. 

An example for stimulus generalization are responses of rats to stress-inducing odors. Laboratoiy rats of the Wistar strain respond to predator odors, specifically mercapto compounds in fox droppings, with stress reactions, for example avoidance behavior such as freezing and increased plasma corticosterone concentrations (Vemet-Mauiy et ah, 1984). The rats were trained to avoid water scented with a mercapto odorant that contained both a keto- and a sulfhydryl group (4-mercapto-4-methyl-2-pentanone). As the animals licked a waterspout, a mild electric shock was applied to their tongue. When different compounds were tested thereafter, the rats avoided compounds with similar... 

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