sentences: a wolf is a kind of animal & establishing a territory requires keeping other animals away ->  establishing a territory can require keeping other wolves away  howling is a kind of sound & sound can be used for communication by animals ->  howling is used for communication by animals  howling is used for communication by animals & a wolf is a kind of animal ->  howling can be used for communication by wolves  a wolf is a kind of animal & a warning is used to warn animals ->  a warning can be used to warn wolves  a warning can be used to warn wolves & warning is a kind of communication ->  warning is a kind of communication that can be used to warn wolves  howling can be used for communication by wolves & warning is a kind of communication that can be used to warn wolves ->  howling is used to warn other wolves by wolves  animals usually keep away from danger or problems & warning can be used to indicate danger or problems ->  warning can be used to keep animals away  warning can be used to keep animals away & a wolf is a kind of animal ->  warning can be used to keep wolves away  howling is used to warn other wolves by wolves & warning can be used to keep wolves away ->  howling is used by wolves to keep other wolves away  establishing a territory can require keeping other wolves away & howling is used by wolves to keep other wolves away -> howling is used by wolves to establish a territory