As in other pet species, a variety of colors,
coat types, and other features that do not appear in the wild have either been
developed, or have appeared spontaneously. Fancy rats in themselves are a subspecies and as such do not have distinctive breeds. Any individual rat may be defined one or more ways by its color, coat, marking, and non-standard body type. This allows for very specific classifications such as a ruby-eyed cinnamon Berkshire rex Dumbo.
Coloring While some pet rats retain the
agouti coloring of the wild brown rat (three tones on the same hair), others have solid colors (a single color on each hair), a trait derived from rats with black coats. Agouti-based colors include agouti, cinnamon, and fawn. Black-based colors include black, beige, blue, and chocolate. while the interpretations of standards can fluctuate between (and even within) different countries or clubs.
Markings Further dividing the varieties of fancy rats are the many different markings. Fancy rats can appear in any combination of colors and markings. The markings are typically in reference to the patterns and ratios of colored hair versus white hair. Two extremes would be a
self (completely solid, non-white color) and a Himalayan (completely white except blending into colored areas at the nose and feet, called
points, as in a
Himalayan cat's markings). Markings have a strict standard, with detailed terminology, for showing in fancy rat
pet shows. However, many domestic rats are not closely bred to any color standard; many of those found in pet shops will have
mismarkings from a formal breeding perspective, which are defined as variations in markings that are not recognized as conforming to a
breed standard published by a rat fancier organisation. Commonly recognized standards include: • Berkshire – colored top, white belly • Hooded – color runs in a
saddle, a single, unbroken line from the full head down to the spine and possibly partly down the tail Other marking varieties include spotted or Dalmatian (named for the spotted
Dalmatian dog), Essex, masked, Himalayan (typically a
gradient of color along the body, darkest at the base of the tail and nose as in
Siamese cats), and
Down Under or Downunder (an Australian variety that has a solid color stripe on the belly or a color marking there that corresponds to the markings on the top).
Ear and body type Two of the most prominent and standardized physical changes applied to rats through selective breeding are the development of the Manx rat and Dumbo rat. The Dumbo rat, whose origins are in the United States, is characterized by having large, low, round ears on the sides of its head, versus the smaller and less prominent ears seen on a standard eared rat. Dumbo ears are caused by a recessive mutation, and was named for its resemblance to the fictional character
Dumbo the Flying Elephant. The Manx rat is tailless due to a
genetic mutation, and was named for the
Manx cat which shares this feature, Another body type variation that has risen to prominence more recently in the hobby is the Dwarf rat. Dwarfism arose as a mutation among Sprague-Dawley rats kept for research in the 1970s. Adult dwarfs are considerably smaller than their standard counterparts, with males between 100 and 130g as opposed to 300g or more. This strain of dwarfism is caused by reduced production of growth hormone, and this has other effects in addition to changed overall body size. Important for pet-keeping, it reduces the incidence of pituitary and mammary tumours and nephropathy (kidney disease), with the result that Dwarf rats live 20–40% longer in males and 10–20% longer in females than their standard counterparts. However, they may also show some cognitive impairment.
Coat types There is a relatively small variety of coats compared to the number of colors and markings, and not all are internationally standardized. The most common type is the normal or standard, which is allowed variance in coarseness between the sexes; males have a coarse, thick, rough coat, while females' coats are softer and finer. Remaining coat types are not defined by the hair itself, but rather by the lack of it, such as hairless rats.
Hairless rats indicates that it is a hooded rat.|alt=|left Hairless rats are a coat variety characterized by varying levels of hair loss. One type of hairless rat is bred from curly-coated rexes. These range from having areas of very short fur to being completely bald. Since rex is a
dominant trait, there only needs to be one rex parent to produce curly rex-coated offspring. However, when two rex parents are bred, two copies of the trait may be present in the offspring. This causes varying levels of hairlessness, and has earned the colloquial name "double rex". The other type of hairless rat is sometimes referred to as a "true hairless". This is caused by a different gene, and is distinguishable from a hairless double rex by the absence of whiskers. Unlike a double rex, this type of hairless rat is incapable of growing hairs on any part of the body. One additional subset of semi-hairless rats, patchwork rex, constantly lose their hair and regrow it in different "patches" several times throughout their life. On one hand, breeding rats to "conform" to a specific standard or to develop a new one is a large part of
what the fancy was founded on. On the other hand, the process results in many rats who do not "conform", and are then either given away, sold as food, or killed—the latter referred to as
culling. There are concerns as to whether breeding hairless and tailless rats is ethical. The tail is vital for rats' balance and for adjusting
body temperature. Tailless rats have greater risk of
heat exhaustion, poor bowel and bladder control, falling from heights, and can be at risk for life-threatening deformities in the pelvic region, like hind leg
paralysis and
megacolon. Similarly, hairless rats are less protected from scratches and the cold without their coat. Groups such as the
NFRS prohibit the showing of these varieties at their events and forbid advertisement through affiliated services.
Availability Because
R. norvegicus and related species are seen as pests, their intentional import into foreign countries is often regulated. For example, the importation of foreign rodents is prohibited in
Australia, and so various coat types, colors, and varieties have been bred separately from foreign lines, or are just not obtainable within that country (for example, hairless and Dumbo rats do not exist in Australia). In other areas, like the Canadian province of
Alberta, which is considered rat-free, the ownership of domestic fancy rats outside of schools, laboratories, and zoos is illegal. == Health ==