There are over 60 recognized species in the genus Rattus, but only two dominate most of the world: the brown rat and the black rat. Beyond wild species, domestic “fancy” rats come in seven recognized varieties, and laboratory research relies on several distinct genetic strains. Many animals commonly called rats, like kangaroo rats and packrats, aren’t true rats at all.
The Two Wild Species You’ll Actually Encounter
The brown rat and the black rat account for nearly all human-rat interactions worldwide. They look different, behave differently, and prefer different habitats, but both have spread across the globe as invasive species tied to human settlement.
Brown Rat
The brown rat is the larger and more common of the two. Adults measure up to 28 cm (about 11 inches) in body length with a tail slightly shorter than the body, and they weigh between 140 and 500 grams. Despite the name “Norway rat,” the species is originally native to northern China. It now lives on every continent except Antarctica.
Brown rats are ground dwellers. They dig extensive burrow systems and are strong swimmers, comfortable both on the surface and underwater. They’re nocturnal, highly adaptable, and aggressive enough to drive out competing rat species when they move into a new area. The historical shift from wooden buildings with thatched roofs to brick and tile construction actually favored brown rats, since their burrowing lifestyle fit better with solid structures than the tree-climbing habits of black rats.
Black Rat
The black rat is noticeably smaller, roughly half the weight of a brown rat. Its most reliable visual difference is the tail: a black rat’s tail is longer than its head and body combined, while a brown rat’s tail is shorter. Black rats are climbers by nature, preferring rooftops, attics, and upper floors of buildings. They’re sometimes called roof rats or ship rats for this reason. In many urban areas where brown rats have established themselves, black rat populations have declined or disappeared entirely because the larger species outcompetes them for food and space.
Fancy Rat Varieties
Pet rats, all descended from the brown rat, don’t come in “breeds” the way dogs or cats do. They’re all roughly the same size and shape. Instead, the American Fancy Rat and Mouse Association recognizes seven varieties based on coat type, ear placement, or other physical traits:
- Standard: Short, smooth, glossy hair. This is the default look most people picture.
- Rex: Curly hair and curly whiskers, giving them a soft, wavy texture.
- Satin: A thinner, longer coat with a noticeable lustrous sheen.
- Bristle Coat: Stiff, coarse fur that feels rougher to the touch.
- Hairless: Complete absence of hair.
- Dumbo: Larger, round ears set lower on the sides of the head rather than on top.
- Tailless: Born without a tail, similar to Manx cats.
Within each variety, rats are further classified by color and marking patterns. You’ll find solid colors, hooded patterns (a colored “hood” over the head and shoulders with a white body), and dozens of other recognized markings. The variety describes the physical type; the color and markings describe the look.
Domestic vs. Wild Lifespan
Wild brown rats typically live only 6 to 11 months. Predators, disease, competition, and harsh conditions keep their lives short. Domestic pet rats, with proper care and nutrition, average 2 to 3 years. Breeders focused on health and longevity have pushed average lifespans from about 1.5 to 2 years up toward 2.5 or even 3 years over recent decades. That gap between wild and domestic lifespan is one of the starkest in any domesticated animal.
Laboratory Rat Strains
Research labs use carefully controlled rat populations that fall into two broad categories: inbred and outbred strains. Inbred strains are genetically near-identical, which makes experiments more repeatable with fewer animals. Outbred strains have more genetic diversity, making them better models for studying conditions that affect genetically varied human populations.
The Sprague-Dawley is one of the most widely used outbred strains, originally created in the 1920s by crossing Wistar laboratory rats with wild-domestic hybrids. These rats are workhorses in diabetes, obesity, cancer, and cardiovascular research. Even within the same strain name, rats from different suppliers can differ significantly. Sprague-Dawley rats from one major supplier were 21 to 24% heavier than those from another supplier by 24 weeks of age, with nearly 50% more body fat. That kind of variation matters enormously in research and illustrates why “Sprague-Dawley” isn’t as uniform a label as it sounds.
The Wistar rat, recognizable by its wider head and longer ears, was one of the first standardized lab strains. The Lewis rat, an inbred strain, offers more predictable results in surgical models. Each strain has traits that make it better suited to particular kinds of research.
Other Species in the Rattus Genus
Most of the 60-plus Rattus species live in Southeast Asia and the Pacific islands, where the genus originated. These include species like the Polynesian rat, which spread across the Pacific with early human voyagers, and dozens of lesser-known species restricted to single islands or small forest ranges. The majority of these species never interact with people and are studied primarily by ecologists and conservation biologists rather than pest control professionals.
The Giant Pouched Rat
The Gambian pouched rat is one of the largest rodents commonly called a rat, though it belongs to a completely different genus (Cricetomys) and isn’t a true Rattus species. Males weigh up to 2.8 kg (over 6 pounds), with an average around 1.5 kg. Females average about 1 kg. Their tails alone measure 35 to 45 cm, nearly hairless with the last third a creamy white.
Their most distinctive feature is their cheek pouches, used to collect and temporarily store food, similar to a hamster. They reproduce up to five or six times per year with litters of one to five young. These rats have been trained to detect landmines and diagnose tuberculosis by scent, making them one of the few rodent species actively employed in humanitarian work.
Animals Called Rats That Aren’t
Many animals with “rat” in their name aren’t in the Rattus genus or even closely related. Kangaroo rats are desert-dwelling rodents in North America that hop on powerful hind legs and can survive without drinking water. Packrats (woodrats) build elaborate nests called middens from collected debris and belong to the genus Neotoma. Australia alone has rabbit-rats, rock rats, water rats, stick-nest rats, and prehensile-tailed rats, all of which evolved from a separate rodent lineage called Hydromyini and filled ecological niches unique to the continent.
The common thread is size: any medium-sized rodent with a long tail tends to get called a rat in everyday language, regardless of its actual biology. If you’re dealing with a “rat” in North America, Australia, or Africa, it’s worth checking whether the species is a true Rattus member or something else entirely, since their behavior, diet, and the risks they pose can be very different.
Social Behavior Across Types
Brown rats are social animals that live in colonies with clear dominance hierarchies. Males typically form strict linear rankings where every individual knows its place relative to every other. In some colony setups, a single dominant male controls the group in what researchers call a “despotic” hierarchy, where one rat rules and all others are roughly equal subordinates. Females show less rigid ranking systems, especially when males are present.
This social nature is part of what makes domestic rats appealing as pets. They recognize their owners, enjoy interaction, and do best when kept in pairs or small groups. Wild species like the black rat are also social but tend toward smaller, more loosely organized groups, partly because their arboreal lifestyle spreads individuals across more vertical space.
Diseases Carried by Wild Rats
Wild rats, particularly brown and black rats in urban environments, carry a range of diseases transmissible to humans. The CDC lists several bacterial infections spread directly by rodents, including leptospirosis (spread through contaminated water or soil), rat-bite fever, and salmonellosis. Viral threats include hantavirus and lymphocytic choriomeningitis. These risks apply primarily to wild rats. Domestic pet rats bred in clean conditions pose minimal disease risk, which is one of many reasons the wild and domestic versions of the same species lead such different lives.