A rat is generally defined as a medium-sized, long-tailed rodent belonging to the superfamily Muroidea. The common term “rat” is not a precise biological classification, but rather a descriptive label used for hundreds of different species across numerous genera. These animals are typically distinguished from mice primarily by size, with a large muroid rodent earning the name “rat.” The adaptability of these rodents has allowed them to colonize nearly every terrestrial environment on Earth.
The Two Primary Global Pests: Norway and Roof Rats
The two most widely distributed species that interact with human environments are the Norway Rat (Rattus norvegicus) and the Roof Rat (Rattus rattus). Clear physical differences exist between these two species, which often determine their preferred habitat.
The Norway Rat, also called the Brown Rat or Sewer Rat, is the larger and more robust of the two, typically weighing between 12 and 17 ounces. It has a stocky body, a blunt snout, and small ears that do not cover the eyes when pressed forward. In contrast, the Roof Rat (Black Rat or Ship Rat) is smaller, weighing 5 to 9 ounces, and features a pointed snout and noticeably large ears. A key distinguishing feature is the tail length: the Norway Rat’s tail is shorter than its head and body combined, while the Roof Rat’s tail is conspicuously longer.
Behaviorally, the two species occupy different vertical niches, which is a major factor in pest management. Norway Rats are poor climbers but excellent swimmers and burrowers, preferring to live at ground level in basements, sewers, and underground tunnel systems. In contrast, Roof Rats are agile climbers, favoring elevated nesting sites such as attics, walls, trees, and dense vegetation. Their dietary preferences also vary: Norway Rats are true omnivores, while Roof Rats tend toward a more herbivorous diet of fruits, nuts, seeds, and grains.
Rats Bred for Domestic Environments
The domesticated rat, known as the Fancy Rat, is almost exclusively derived from the Norway Rat species (Rattus norvegicus). Selective breeding over centuries transformed the wild brown rat into a docile and intelligent companion animal. These domesticated varieties are all the same species, but they exhibit a vast array of aesthetic variations not seen in the wild.
These varieties are primarily categorized by their coat type, color, and specific physical mutations. Coat types include the standard smooth fur, the wavy and curly Rex, the sparse-coated Double Rex, and the Hairless variety. Colors range from wild-type agouti and black to champagne, blue, and mink. Beyond the coat, some popular varieties possess distinct body characteristics, such as the Dumbo rat, which is bred for large, round ears set low on the sides of the head.
This domestication process also led to the development of the laboratory rat, which shares the same genetic lineage and is important for scientific research due to its manageable size and physiological similarity to humans. The focus in breeding for both pets and research animals is on docility and specific, predictable genetic traits.
Diversity Beyond the Common Rat
The term “rat” extends far beyond the Rattus genus, encompassing a diverse group of rodents with specialized adaptations to specific ecological niches. One of the largest examples is the African Giant Pouched Rat (Cricetomys species), which can reach up to three feet in length from nose to tail tip. These nocturnal animals are notable for their hamster-like cheek pouches, which they use to carry forage back to their burrows.
Their exceptional sense of smell and light body weight (too light to trigger landmines) has led to their training for humanitarian work, such as detecting buried landmines and identifying tuberculosis in human sputum samples. Another distinct group is the North American Wood Rats, or Pack Rats (Neotoma species), which are recognized by their large ears and furry, rather than scaly, tails.
Wood Rats are famous for their hoarding behavior, where they collect materials like twigs, leaves, and shiny objects to build large, elaborate nests called middens. These middens, which can be several feet high, are often built in caves or rocky shelters, and in arid environments, the structures can be fossilized by the animals’ urine. This unique habit has made ancient middens valuable to paleontologists for reconstructing past climates and vegetation.