Reproductional isolation is a way to prevent closely related species to mate each other. This is due to reproductive barriers which can be either prezygotic (before fertilization) and postzygotic (after fertilization) barrier. Behavioral isolation is an example of postzygotic barrier. It is based on behavior, including mating sounds and rituals. Thanks to behavior isolation, the different species (in this example gentoo penguins and emperor penguins) will have different mating rituals so they could attract only individuals of their own species, respectively.
