What allow animals to perform their functions

a nervous system, connecting to their brain which sends signals to move all the  parts around the body, for example, if you want to move your leg forward, your nervous system sends signals to it, same for other animals.

not necessarily. higher fitness requires the organism to be reproductive successful. for example, an organism at age 10 with 15 offspring is much more fit than an organism at age 15 with only 5 offspring.