I was listening to Ron Magill of Zoo Miami (he's on the Le Batard show weekly) and he made a point that I'll steal for this post:
Humans who are injured, or even just a headache, have no problem showing it (in fact just last week, I chose a bad mix of ingredients for my protein shake and suffered visibly for hours because of it). Animals in the wild, though, cannot afford this luxury. Showing symptoms of injury or weakness in the wild will invite a predator to attack you. They have to appear normal and/or able to defend themselves at all times which keeps the attackers at bay, unwilling to find out what might happen.
This can work for humans in many areas; one that comes to mind is competition. Even when you feel like shit, put on the face of being just fine and keep your opponents guessing.