There are two species of rats known as “true rats”—the types recognized worldwide as pest animals: the Black Rat (rattus rattus) and the Brown Rat (rattus norvegicus). Both species are known by a variety of other names. The brown rat is also called the wharf rat, street rat, common rat, hood rat, Norway rat, Norwegian rat, sewer rat and Hanover rat. The black rat species has been referred to as a ship’s rat, roof rat, Alexandrine rat, house rat and Old English rat.
The Black Rat
By the first century of the current era the black rat had spread into all areas of Europe, including the British Isles (thanks once again to Roman invasion). From that time forward, black rats have spread to nearly ever spot on earth, alongside European explorers and settlers.The black rat is believed to have originated in the tropical areas of Asia and began to spread into the Middle East around the time of the Roman military expansion into the area. Black rats are famous for infesting ocean going ships, it is likely that Romans returning from the near East carried the first black rats in invade Europe in the holds of their ships.
The black rat actually varies in color from black to a light brown. The black rat’s underside is usually lighter in color than the rest of its body. In size the black rat is smaller than the larger brown rat. The average adult black rat is between 12.75 and 18.25 inches, which includes the tail which can take up between 6 ½ to 10 inches of its length. Weight varies between 4 to 12 ounces. Black rat fur is often described as scraggly.
Black rats are omnivores, but favor plant material such as grain, fruit and seeds, but has also been known to eat insects, feces and animal derived refuse. They are scavengers who survive by exploiting whatever plant or animal remains they encounter in their environment.
Black rats a good swimmers, and excellent climbers, which helped earn them their alternate name as roof rats. While the brown rat prefers more terrestrial habitats, the black rat’s ability to climb finds them infesting attics and roofs, accessed via power and telephone lines connected to human dwellings. Their favored nesting space is roof rafters where they build their nests from grass and twigs.
Black rats can produce anywhere from three to five litters a year following a breeding season which extends in most areas of the world between March and November. Litters can be as small as one infant or as large as 16 rat pups; the average litter size is around seven babies.
Female rats are able to breed at 12 weeks of age, and can successfully breed and conceive at the same time as they are suckling their current litter. These two factors result in a single female being able to produce an enormous number of young. Black rat lifespans in the wild are just under 18 months, due to predation and pest control.
The black rat species has been historically considered the disease vector of the Bubonic plague—the bacterium Yersinia pestis inhabits the digestive systems of black rat fleas. Black rats with some kind of natural immunity to the disease itself (surviving to spread the infected fleas) are deemed responsible for the dreaded disease’s pandemic in the fourteenth century. However, many experts now believe that this was only one of several paths that Yersinia pestis took during the Black Death. Black rats do transmit several diseases to humans, and also pose significant threats to food stores and property.
The Brown Rat
The designation as “Norwegian” originated with a British naturalist John Berkenhout who hypothesized that the brown rat had invaded England via Norwegian ships in 1728. There were however, no known populations of brown rats in Norway at that time. The name has stuck in spite of the refutation of the animal’s origins.Despite one of their alternate names being the Norway rat, brown rats are believed to have originated in Northern China. They now can be found on every continent except Antarctica.
Brown rat fur varies from brown to a very dark grey and, like the black rat the underbellies tend to be lighter variants of the animal’s predominant color. Brown rat fur is coarse in texture and shot through with black hairs. Brown rats have a body length of around 10 inches, their tails are usually of the same length as their bodies. Weight runs from 550 grams (19 ounces) to as large as 900 grams (32 ounces).
Brown rats are ominvores and scavengers, but prefer cereal grains. A study of brown rats revealed that scrambled eggs were their favorite meal, followed by macaroni (and cheese!), and cooked corn. Brown rats were found to be least fond of vegetables and fruits. Different populations of brown rats tend to exploit different food sources, colonies near fish hatcheries will dine on fingerling fish; riverside populations have been known to dive for mollusks; North sea island dwelling rats have been observed stalking and killing sparrows and ducks.
Brown rats are known as excellent swimmers, but unlike the black rat, are poor climbers. Brown rats are prodigious diggers, they produce sprawling burrow systems. They favor burrowing next to existing structures which they can exploit for food and water sources; they use the burrow systems for food storage, shelter and nesting sites. Inside structures they tend to stick to established routes, running along the bases of walls. When threatened they prefer to withdraw to their burrows. When burrowing is not possible, they prefer damp subterranean nesting sites including cellars and sewers.
Brown rats packs are socially stratified, each individual in a pack has a particular place and is dominant over another rat. Brown rats sleep together in packs and engage in mutual grooming. They huddle together for warmth and social interaction; they have been observed “nosing” that is adult rats nuzzling each another in the neck. They also play, wrestle and chase each other, and squabble over dominance.
Brown rats are not favored by the species of rat fleas that carry Yersinia pestis, and so is not a vector for bubonic plague. They do figure in the transmission of several other diseases than can affect humans, including toxoplasmosis which is often transmitted from the rat to a cat feeding upon it, and from there to a human via exposure to cat feces in litter boxes. Rat bite fever is transmitted by brown rats, as is Weil’s disease, viral hemorrhagic fever, Q fever and Hantavirus pulmonary syndrome.
Brown rats are the source species for Pet or Fancy rats; and laboratory rats. The latter are descended from albino brown rats and have been used in psychological, medical and other biologic experiments. Their rapid rate of reproduction and quick maturation rates make them easy to keep and breed they are considered an important model organism for better understanding human disease and behavior.