The Martial Arts, and this includes Shaolin kung fu, are passed on through unique patterns of movement. They are, at heart, a mythology created to represent a unique methodology. They are the science of an ancient time, passed down to us by dance.
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 goes forward. Claws and teeth, he drags down the runner and tears him apart. Thus, the direction of the Tiger is forward, and all he does is go forward.
The Crane is a lighter animal, giving way with deflection as he sets up and uses the legs. He does not rely on flight, but rather manipulates the attack to set up his kicks. He does not charge, but fights at a certain distance, never never lets that distance close.
The Snake coils and twists and allows the attacker to get close. The artful deflections of the crane become full blown traps and locks under the trickery of the snake. Thus, the snake gives way without giving way, and the enemy falls into trickiness and deception.
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 all the strengths of the other four Kung Fu animals. The dragon is the embodiment of understanding and employing all animals so that they work as a smooth and embracive strategy. Run, manipulate, trick or flee, the dragon can do anything, smoothly and with perfect timing.
The five animals found in Shaolin Kung Fu represent geometry of motion in space, and a distinct selection of distance of combat. Practice each animal until you have mastered the abilities of that animal, then blend them all together. This is the always winning strategy for combat as developed by the Five Animals of Shaolin Kung Fu.