In nature, the fittest animals are the ones better able to reproduce their traits to the offspring in a process that nature scientist call "natural selection".
The common term used to define this is "survival of the fittest", which means the way they live in nature.
The process is the natural way to live in the wild. The strongest animal runs faster and kills the weaker animal, in what other studies known as the chain of food. So that is why the fittest animal has the best opportunity to survive.
