Predator diet cues

Because defenses are costly, prey often develop elaborate systems of risk analysis to closely match defense intensity with predation risk.  For example, many species incorporate information about predators’ recent foraging activity and respond more strongly to predators consuming more dangerous diets.  While studies which demonstrate prey use of predator diet cues are numerous, studies which demonstrate these cues are not used are almost as common.  I reviewed the literature on prey use of predator diet cues and described several factors which may explain patterns of diet cue use.  Diet cues are valuable when they provide information about a predator’s future foraging activity.  Therefore, cues which indicate differences in species consumed may be useful when predators specialize, at a species or individual level, on a small subset of prey species.  In contrast, for generalist predators other dietary cues, such as those which indicate general dietary class (i.e. predator versus herbivore, invertivore versus piscivore) or whether a predator has recently eaten, may be more useful.  Diet cues can also be helpful if they extend the benefits of a prey’s plastic defenses.  For example, many prey use diet cues to induce more costly forms of defense or to learn novel risk cues.

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I have investigated several of these ideas through my own work with morphological defenses in the eastern oyster.  Oysters produce heavier, stronger shells in response to crab predators.  I found oysters do not differentiate between crabs fed different species of bivalves, which is expected as crabs are generalist predators.  However, oysters do produce their strongest shells in response to fed crabs which have consumed freshly shucked oyster tissue.  Age of consumed tissue is a previously unstudied dietary difference but may represent important crab preferences for foraging versus scavenging of dead tissue.  Oyster responses to hunger state and prey tissue age support the idea that alternative diet cues (i.e. those not related to species differences) may be more important in systems with generalist predators.  In addition, although responses are weaker to starved crabs, oysters do produce morphological defenses to these predators.  Previous studies of behavioral responses, which are quick and inexpensive to produce, found no response of prey to starved crab predators, supporting the possibility that diet cues are used to induce more costly defenses.