Of all the living organisms on earth, only multicellular animals show complex behavioral responses to external stimuli. They possess all the parts necessary to do so: a nervous system to receive and process stimuli and send out commands, and a muscular and skeletal system that makes quick and diverse movements possible. Vertebrates have the most complex nervous systems, and, not surprisingly, they also have the most complex behavior of all animals. The ability of animals to learn and to modify their behavior is directly related to how large the associative areas of their brains are and is extremely great in mammals, especially primates. There are limits, however, to what an animal can learn and when it can learn. All behavior also has a genetic component. Both nature and nurture are important in determining how an animal behaves. Much of the behavior of animals centers on communication. The actual form of the communication may vary widely from species to species, but the end results are usually similar and vital. Without communication systems, many animals would be unable or less likely to find mates, reproduce, care for young, locate food, or avoid predators.