A third possible theme is the contrast between civilization and savagery. The story might explore how the savage nature exists in contrast to the norms and values of a more civilized society. For instance, if it's a story about a person who has been isolated from society for a long time and has to revert to a more'savage' way of living to survive. This shows the difference between the two states and how easily one can slip into a more primal, savage nature when the trappings of civilization are removed.
I'm not sure specifically which 'a savage nature true story' you're referring to. It could be about a wild animal's behavior in its natural habitat, or perhaps a story about a person who has a 'savage' or untamed side to their nature.