Read the passage from The Secret Language of Cats by Susanne Schötz. Not all cats communicate happily or frequently with sounds. Many prefer to be silent. Let us not forget that they are predators, and being a predator is deeply embedded, even in our pet cats. That is why they instinctively try to hide their location and their physical and mental state (especially when they are sick, in pain, or giving birth to kittens) from other animals or humans to avoid conflicts. Still, they sometimes want to communicate with each other with the help of sounds. Cats like to be out and about at night, and sounds are an especially sensible way of communicating over long distances or when visibility is bad. Certainly many are familiar with the nocturnal concerts of cat sounds. Cats have learned that humans react well to cat sounds. We humans do not have the same good noses that cats have, and our eyes are also often elsewhere, so that we do not notice, for example, when our furry friends have sneaked into the kitchen and sit in front of their food bowl. When we are working, are occupied with our computers or smart phones or are sleeping, sounds are especially effective. The four-legged companions have understood that and adapted themselves to us. That is why many cats develop a kind of spoken language together with their humans that is mutually understandable. I have also found that the more I talk to a cat, the more it talks back to me. However, it is important to clarify something here: Do all cats speak the same "language"? Can they understand each other when they communicate with vocal signals? There seem to be signals that are universal and are understood by all cats. But there also seem to be geographic, cultural, and breed-based differences. Maybe cats are even influenced by the language or the accent of the people around them. From Susanne Schötz, The Secret Language of Cats. Copyright 2019 by Susanne Schötz Which two main, or central, ideas are developed in the passage?