Brown rat

Rattus norvegicus


Rattus norvegicus (Berkenhout, 1769), Great Britain. OTHER COMMON NAMES

French: Rat d'egout, surmulot; German: Wanderratte; Spanish: Rata noruega.


Body length 8.6-10.2 in (22-26 cm); tail 7-8.6 in (18-22cm); weigh 7-14 oz (200-400 g); can reach 1.1 lb (0.5 kg) on rare occasions. Different than the black rat, in that the brown rat grows larger, has a thicker build, shorter ears and tail, and coarser fur. It is reddish brown to grayish brown on the back and often has darker sides. Fur on its belly is a light slate.


Believed to have originated in the northern regions of Asia, perhaps in China or Mongolia, and spread westward. It reached Western Europe in 1716 via a ship docking in Copenhagen; hit France in 1735, and then the shores of America in 1755, it spread to England by 1750. Ships and ocean vessels have spread the rodents to nearly all of the world's seaports.


Lives just about anywhere, but it does not fare well in hotter climates. It is well adapted for the colder climes, and the species has actually inhabited a whaling station in Antarctica. It lives in close proximity to humans, often found anywhere from burrows and cavities to sewers and cellars. In homes, brown rats, which are poor climbers, are at ground level. Although the species is not aquatic, it can swim well and has been spotted in rivers or ocean waters.


Social animals that live and hunt in groups with close familial relationships, although they do allow non-related animals in on occasion. They can form groups as large as 200 animals usually with a dominant male overseeing the social order. Dominance hierarchies are determined by size and weight rather than age.


Eats a variety of things, although it prefers meat. It can swim, dive, and catch fish. In the 1940s, a pack of 15,000 brown rats decimated the bird population of a sanctuary on the island of Nooderoog, eating eggs and catching seagulls, ducks, passerines, and other species. They have also been known to eat mice, chickens, ducks, and geese, and will gnaw on lambs and piglets. Rat packs have ganged up to kill cats and dogs that have been deployed to keep their populations in check. They have been known to feed on elephants, invalids, and newborn babies. They have also been described as cannibalistic. It will take its catch back to its den for feasting, and can live without water as long as it consumes sufficiently moist food.


They will breed throughout the year, although they peak in the spring and fall. When females go into heat, which lasts for six hours, a collection of males will mate with her. Each female will produce between 2-12 litters each year, with each litter consisting of 6-12—and as many as 22—young. The pups are born blind and pink, they open their eyes after about two weeks, and they leave the nest after three weeks. In a pack, the females give birth to their pups in the same room and assist each other in raising their young. The young are able to breed after 90 days. In optimal conditions, the animals are capable of producing up to 800 offspring per year, but this number is considerably lower in the wild.


Not threatened.


Best known as carriers of the bubonic plague, and for centuries have eaten and contaminated human food. A descendent, the white albino lab rat, has proven invaluable to scientific research and discoveries in human health research. ♦

builds nests, suspended between grass stems, about 3.2 ft (1 m) aboveground. The nests, which can take two days to build, are made from woven leaves, with the inner layer being finely shredded to provide a soft spot for young. The animals also nest in holes or, in some circumstances, on the ground during winter. They may also move into human structures like barns to avoid the cold.


Nocturnal. The animals live in crowded conditions, but are not as social as black or brown rats. Captive males have been known to fight each other, and males and females only come together to build nests and to mate. Females chatter to attract mates and squeal during aggressive outbursts.


Forages in the grass fields primarily before dawn and just after dusk. It feeds on seeds and vegetables, but does on occasion take insects and eggs of small birds. Their unique climbing adaptations, like their semi-prehensile tail and their feet, allow them to climb through grass fields and counterbalance themselves as they move about.


Polygamous. Females reproduce during the warmer months, from May-September, and they are capable of giving birth several times in succession. Gestation time is about 2.5 weeks. Each litter ranges from one to 13 pups, with an average of five. They are blind and naked at birth, but within 10 days can see and have fur. They are weaned at two weeks and start to reproduce at one month of age. They are capable of living to nearly five years in captivity, but rarely live longer than six months in the wild.


Not threatened, but certain populations near farm machinery are in decline due to habitat loss.

SIGNIFICANCE TO HUMANS No major significance. ♦

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