Locating suitable food is one of the pressing problems faced by animals. \_31\_ pets or pigs in factory farms, most animals must spend a large fraction of their waking hours locating food and extracting it from their environment. \_32\_ the case of herbivores, this may seem simple at first thought, but it is seldom an easy matter of wandering about nibbling whatever \_33\_ is encountered. Not all plants are equally nutritious, and some contain distasteful or even \_34\_ substances. Even grazing animals that need only abundant grass do pick and choose which patches are most worth \_35\_ and pay considerable attention to signs that food is available from particular plants. Active predators face more obvious challenges because they must not only locate but capture prey animals that seldom wait passively to be eaten but devote considerable effort to avoiding that fate.