Both of the most common types of rats, the black rat and the brown rat, are omnivores, eating fruits, grains and any waste or unprotected human foods they encounter, along with invertebrate prey when necessary or available. Brown rats are more carnivorous than black rats, will eat more types of meat and have even been observed fishing. These animals are ubiquitous and are major pests in destroying livestock feed.
Black and brown rats are both found on every continent on Earth, and travel readily in large ships. They do not occur in the same places often, however, because brown rats will kill off any black rat population they encounter. In general, black rats are more suited to tropical regions, while brown rats dominate in temperate and colder regions. Black rats are also less flexible in their diets than brown rats, and are more reliant on sources of seeds and other plant foods.
Both black and brown rats are significant vectors for many diseases, most commonly transmitted via the fleas and other parasites they carry. Both species commonly form packs, and in black rats, this includes a social hierarchy that determines access to mating opportunities. Large packs of brown rats have been known to attack larger animals, even human infants.