Since there are such a wide variety of species considered rats, it is rather difficult to describe their habits. Some, for instance, are highly adaptable omnivores, while others maintain specialized diets and only live in locales that meet very specific requirements. The types of dwellings they inhabit also vary significantly, ranging from burrows and nests to crevices in rocks and caves to structures built by humans. Most rats, however, are nocturnal, social, and prolific breeders. Yet, the brown rat is active in the daytime as well as the night and, though many rats live in groups, there may be a significant amount of fighting between individuals. Some rats are also good swimmers, while other are better adapted for climbing.