You would think the bit in Riddick's mouth is there to keep him from biting people. Take a moment and you'll realize it's so he won't talk to anyone because his mouth's just as dangerous as the rest of him. He's dangerously charming, and the children especially seem interested.
Johns shoots morphine into his eyesocket, instead of putting it in his arm like one would expect. Combined with the implication that he stole the morphine from the ship's medical supplies, it becomes clear that he is doing this to avoid leaving visible needle marks on his skin.
At some point in Pitch Black, the creatures turn on each other and become cannibalistic. Well, considering they have to wait 22 years just to ''feed'...
The raptors don't come out during the Eclipse just to breed - they can do that underground just fine. They come out during the Eclipse because they can. It's then that they really have room to get killing and eating each other above ground, which thins out the weak and old, and results in all the skeletons. It's been going on for hundreds of thousands of years. And this 23-year cycle is what prevents any sun-based life from evolving.