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Official Loofah Tester
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Random comparisons between the two most common rats to be found cohabiting with humans.

There are two main types of common rat United States. Neither are native to the US, but have been introduced with exploration and colonization in the last few centuries.

The first is Rattus Rattus, also known as the black rat, roof rat, ship rat, or in unsavory references, 'the plague rat'. Their coat color ranges from a an extremely dark greyish toned agouti brown to black.

The second is Rattus Norvegicus, commonly known as the brown rat, Norway rat, house rat, or wharf rat. Their wild coat color is an agouti brown with a lighter belly.

Agouti is a common 'survival' color of many wild animals...with hair shafts being a lighter grey or yellow base, and then alternating with bands of brown and black before finally tipping out with black. The result is a textured rich light to medium brown with grey black, and yellow undertones mixed throughout. For quick comparison, wild gray wolves and coyotes have an agouti coat pattern.

Physically, the brown Norway rat is larger than the black roof rat, up to two inches longer in the body. The Norway rat is much more muscular and stocky as well and can weigh up to two pounds, while the roof rat often weighs less than a pound. The bite strength of a Norway rat is much more powerful than a roof rat.

The roof rat being smaller, is faster and more agile than the Norway rat. They are also much better climbers and jumpers than the Norway rat. So if looking for a warm cottage in which to bed for the year, roof rats are more apt make like hairy daredevils and climb the tree, cross the telephone wires, and leap improbable distances to the small opening in the attic. The Norway rat will just gnaw through the cinder block and bunk down in the cellar.

Norway rats have a higher cold tolerance than roof rats. It is thought by many that in the middle ages, roof rats spread through Europe during a warming period. The proliferation of roof rats, and the fleas who preferred them, led to the Great Plague. The faster breeding and physically tougher Norway rats came through later. Between a gradual cooling period and resource competition with the Norway rats (among many other factors), the black roof rats were displaced and greatly reduced in number. Norway rats, unlike the roof rat, are not natural carriers of the bacterium Yersinia pestis, which is responsible for the bubonic plague. It is widely theorized that Norway rats are one of the contributing factors to the decline of the plague.

Behavior. Comparing the wild Norway rat to the roof rat, the Norway rat is more xenophobic with non-Norway rats and critters than is the roof rat. Roof rats on the other hand, have not shown a particular bias against 'non roof rat' species. To put it in simple terms, what this means is that if a Norway rat and a dog are raised together as friends, the Norway rat will consider the dog a non rat member of their family. If a roof rat were raised with a dog, the roof rat will consider the dog another big roof rat.

Another difference in behavior is that a roof rat tends to be more aggressive and less fearful of new situations. Since they don't hold a particular species bias, they are more likely to think they can take on a dog and consider it a fair fight...a dog after all is just another rat, right?

They are also more curious and less suspicious/shy/fearful in their behavior. This is a boon for a home owner seeking to rid a wild habitation in their attic, because it's easier to lure roof rats into traps. In this and many other ways in fact, roof rats behave more like mice. Catch phrase of roof rats (and mice): "Oooh Shiny! Can I play with it?" Catch phrase of Norway rats: "Who are you and what did you do to my brother?"

An interesting tactic unique to roof rats is their tendency to sit benignly and quietly, and then in an unexpected explosion of screeching, leaping, and flying teeth, will lunge at a much larger predator. This is theorized to be an arboreal defense/offense tactic of the tree dwelling black rat. If they sit quietly and then suddenly go bug crazy, the approaching predator will likely be startled off of a branch completely and end up sailing speedily towards the ground. This is also the source of stories about crazy screaming and attacking face lunging rats, because frankly Norway rats are much too dignified for such nonsense. Though the black rat really isn't all that tough so the only real damage usually inflicted would be the heart attack from having a screaming rodent try to attach itself to your eyebrows while it pees on your head.

In an encounter, a roof rat will be more likely to attack a Norway rat and will dance circles around him, biting and foot scuffing him with near impunity while the Norway tries to lumber fast enough to catch up. However, thanks to the weakness of the roof rat bite, the damage done by the roof rat will be superficial but all it takes is one good bite from a Norway rat to end matters. In short, a roof rat will start a fight, but a Norway rat'll finish it.

The domesticated rats we see in pet stores and kept in homes are brown Norway rats, over a thousand generations removed from their wild Norway counterparts, and now offered in people loving assortments of sizes, types, and colors. Their temperaments have been altered significantly to make them much more laid back and tolerant of new situations, and to greatly reduce wild shyness and aggressiveness. While they can be bred with a wild Norway rat, pet rats are no longer even the same animal in terms of temperament or often even color. It would be like comparing a grey wolf to a poodle or a coyote to a chihuahua.

As for the black rat, though attempts are periodically made to domesticate black roof rats, they have yet to be domesticated and tend to make really skittish and unpredictable pets.

Assorted sources (and additional reading):
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