The Martial Arts, and this includes Shaolin kung fu, are passed on by oral tradition. They are, at heart, a mythology created to represent a unique universe. They are the science of the times, passed down to us.
Thus, to understand the martial arts, we must ask ourselves what science were the ancients trying to describe. Science is science, however, and I think you will find that the real reasons for the five animals is empirical in nature. Also, you will find that there is a distinct combat reasoning to it all.
The tiger is an aggressive animal, and he is always on the attack. Claws and teeth, he drags down his opponent and tears him apart. Thus, the direction of the Tiger is forward, and all he does is attack.
The Crane is a lighter animal, giving way as he manipulates and uses legs. He does not run, but rather and deflects the attack to set up his kicks. He does not charge, but fights on the edge of a circle, never letting that circle collapse.
The Snake coils and twists and allows the attacker to enter his perimeter. The artful deflections of the crane become full blown enwrappings, trips and locks under the deceptive practices of the snake. Thus, the snake gives way without giving way, allowing the enemy to fall into various tricks and traps he sets up.
There is some argument whether the fifth animal is a monkey or leopard, and I usually choose the monkey. Whichever animal you select as the best representation of Shaolin Kung Fu, the characteristic of the animal is tricky footwork and the ability to acrobat and gyrate out of the fight. Thus, the Monkey (leopard) has the ability to change tactics and reset the whole thing to whatever animal characteristic he is strongest at.
The fifth animal is the dragon, and this is an imaginary animal which represents the best of the previous four animals. The dragon is the embodiment of understanding and employing all animals so that they work together. Run, manipulate, trick or flee, the dragon does them all, smoothly and at the right time.
The five animals of Shaolin Kung Fu represent geometry in space, and a distinct selection of distance of combat. Practice each animal until you have mastered the strength of that animal, then blend them all together. This is the strategy for combat as developed by the Five Animals of Shaolin Kung Fu.