It's easy to say something like "it's a jungle out there" without really thinking about it, but it's hard to overstate exactly how rough a jungle can really be.
While our imaginations tend to drift to the thought of being eaten by ferocious animals, there's also the intense heat, the crushing humidity, the poisonous plants, and the risk of contracting all sorts of bizarre diseases.
Of course, those things aren't so easy to convey visually, so it's for that reason that we tend to illustrate how rough various types of wilderness are with how intimidating its fauna is.
And the easiest way to get that point across is to show you how they interact when their bellies aren't full and their minds aren't at peace.