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Play appears to be a developmental characteristic of animals with fairly sophisticated nervous systems, mainly birds and mammals. A. Although play often resembles aggression, flight, or other purposeful activities, researchers do not degree on the reasons for and functions of play B. Although many animals develop physically from play, too many young animals become victims of their natural predators while playing. C. Animals such as rats, dogs, deer, goats and monkeys learn how to be both dominant and submissive during play activities so that they will fit in better with their adult social groups. D. The function of play is still debated in the research literature primarily because each animal species uses so few of the many available types of play behavior. E. Energy expenditure and security risks are some of the costs to animals of play behavior, but the costs are not so great that they outweigh the long-term benefits of play to the species. F. As experiments and observations have shown, animals that play at some stages of their development obtain neurological, muscular, or social benefits from the play behaviors.