Genetic Anomalies in Rats
The domestication of rats have taught many fanciers the power of genetics and selective breeding, as we have somehow turned an animal with two color phases (agouti and albino) into an animal capable of being a selection of hundreds of colors, coat patterns, and hair, body, tail, and ear types. It's an impressive feat given that the domestication of the modern pet rat only started in the mid 1800s. In this article, I hope to outline the barebones basics of unusual rat characteristics bred solely in captivity, with a dash of history mixed in.
In the Beginning
Reports of people keeping rats, squirrels, mice, and other small animals as pets spans back centuries and possibly millennia but the domestic rat as we know it today originated in England in the mid 1800s. Queen Victoria's royal rat catcher, Jack Black, was in the business of controlling the vermin infestation at the time. He was paid for killing rats but somewhere along the line he learned that catching rats alive was more lucrative. These rats were sold to people who'd throw dogs in a fighting ring filled with rats. Bets would be made on how many rats the dogs could kill. Rat baiting was a popular sport, as was bear baiting, bull baiting, and dog fighting.
The rat fancy has been known for attracting some pretty eccentric people. Jack Black was no exception. He knew the importance of a public image and was said to wear a belt of cast iron rats around his waistcoat. He also appears to have been a keen entrepreneur as he eventually got into the business of breeding his live rats for the pits. We are told through historical accounts that he started to breed the "pretty" rats together to sell as pets to the well to do women of the day. Although we'll probably never know the exact qualifications of "pretty" we can assume with great confidence that these rats at the very least included albinos. There are reports that the first albinos date back to two captured in a cemetery by Mr. jack Black himself. In any event this first step into selective breeding was the cornerstone of rat domestication.
Albinos, Blacks, and Over-Spotting
We know albino rats were being bred by Jack Black because there are many historical accounts that have been written down. Beatrice Potter, the author of Peter Rabbit, was thought to have been a customer of Jack Black's, even casting her white rat as a character in at least one book.
Albino rats looked different from the regular wild rats who were brown (agouti) in coloration hence they were the first to be selectively bred. These albinos were bred first for looks but somewhere down the line they also were bred to increasingly tame stock which resulted in the modern laboratory rat being born, bred, and utilized. Albinos were likely bred back to their agouti counterparts for numerous generations. This cross breeding started to mutate their color even more.
Black rats eventually were born to agouti parents and then those too were added into the collective breeding project (though we may never know by whom.) Domestication really started to set in when white feet and spots became apparent on some of the newer generations. This was due to a masking gene often referred to as the over spotting gene. At first these rats were called piebald and probably only had white stomachs and chests. Selective breeding made their white spread until there was a variety of markings to choose from, including entirely white rats with black eyes and no markings at all which were distinctly different from albinos.
The foundation of all domestic colors was then set. There were albinos, agouti, black, and white. These colors eventually started mutating into other colors. When albinoism and black showed up in the same animal it created the pointed rats, otherwise known as Siamese. The albino gene altered the black genes until their fur appeared to be dark brown on their nose, feet, ears, and tail and lighter brown through their body. Agouti spurned colors like beige and black eventually started showing dilution genes which turned it into several shades of blue. The more complicated rat colors got the more fanciers started to cross them which ended up creating a massive boom in colors. As late as the 1970s rats were only known to come in agouti, albino, black, over spotted, and beige. By the beginning of the millennia there were literally hundreds of colors, but by this time colors were only part of the equation.
Manx rats are those who are born without tails or with shorter tails then their parents. Often manxed rats have stubs or nothing at all. These rats were first recorded in the 1920s when four individuals were born in an American laboratory. These rats were then bred to see if more could be created but the gene proved problematic. Females with no tails couldn't give birth and breeding a tailless male, even to a half-tailed female, didn't guarantee any more manx offspring. Offspring were produced but so sporadically that these researchers lost interest and the manx didn't appear again until it popped up in the pet population at a much later date. There's no evidence to suggest that they are the desendants of the laboratory rats but one cannot completely disregard the possibility. Manx is still a tricky and very little understood gene. It's very possible that manx rats aren't even the cause of one gene, but rather a series of genes. Though they still exist in the pet population, and some breeders do still work with them, they are, and likely always will be, rare. It's far easier to find what I call an accidental manx, that is a rat whose tail was lost do to an overzealous mother or an accident later on in life.
Rex rats are rats born with curly fur. Unlike most genes rex proved to be dominant, meaning that a rex rat could be bred to a standard rat and produce rexed offspring. This reduced the need to inbreed which gave this variety a better chance of being bred for health rather then looks. Rex rats were however bred together for numerous generations. Occasionally a rat would be born that almost looked like it had mange. It would be a sparsely furred rat with curly whiskers who'd grow hair in patches and loose it, only to grow back more hair in different patches. These were called double rexes, as they were known right away to be the cause of over rexing. When the double rexes were bred together for several generations they created babies with even less hair until a few were born who grew no hair at all (save for their curly whiskers.) This is probably the most common type of furless seen in the pet population but not the only ones. Unlike the rexes this new hairless rat proved to be a mostly recessive gene. I say mostly because an over rexed hairless bred to a standard does create rexes, but they don't create more hairless.
Nude hairless rats originated in the laboratory. These animals proved most fragile as they had very little or no immune system. It was because of this these rats were specifically bred by laboratories for experiments. These rats occasionally find their way into the pet population but very rarely do they live past six months, they just don't have the weapons they need to find off even the smallest infection.
It's speculated that there are at least four different strains of genetically different hairless in the pet population. This can cause much confusion when two hairless bred together can create fully furred offspring, as all hairless genes are recessive.
As far as I know satin rats showed up sometime in the 1990s. These rats had smooth, slippery hair, whose shafts were flat rather then round, creating a satin effect. Some breeders bred these rats to rex to create velveteen rats. Satin, like rex, is a dominant gene, and appears to have been spontaneous. I haven't heard anyone claiming to discovered it first, though it seems likely whoever did was a fancier, otherwise this gene probably would have not been noticed.
Harley rats are rats with long hair like a teddy bear hamster. The first Harley rat (named Harley) was a Himalayan found by Odd Fellows Rattery at a pet shop in September of 2002. Harley came home and started a prosperous career as a stud to see if the gene was dominant or recessive. Unfortunately the gene turned out to be recessive so an intense line breeding program had to be started to create more little Harleys. The variety is gaining much popularity in it's few years of existence.
Dumbos Enter the Scene
Dumbo rats entered the scene when a litter was born in California in 1990 that had a male dumbo spontaneously appear. This rat was kept by the breeder and then bred to other regular rats but no more dumbos were created. It's at this point he was bred to either his mother or sisters to create more dumbos, proving it is also a recessive gene. Rat enthusiasts went wild for this new mutation whose ears were rounder, lower on the head, and whose skulls started to resemble Bull Terriers. They spread like wildfire from coast to coast in the US where breeders took to outcrossing and linecrossing to create a stable genetically diverse animal. They grew to be an even bigger phenomenon when dumbos were exported out of the country and set about taking over the world.
It wasn't until 2009 that dumbos were studied in a laboratory setting. Breeders had noted that their dumbos sometimes had smaller lower jaws and that their females didn't wiggle their ears when they were in heat like other rats. Someone made the comparison to a variety of pharyngeal arch development disorders in humans, of which the most recognized is Treacher Collins Syndrome. A study of nine dumbo embryos in a laboratory setting did indeed prove they had a pharyngeal arch development disorder which caused them to develop different than their non-dumbo peers. Although this sounds very scary there's no proof this causes any adverse health effects in the rats besides disallowing some of the muscles in the face from developing normally (making them unable to wiggle their ears and make certain facial expressions.)
Current Laboratory Strains
Laboratory rats have long been bred to be cookie-cutter animals, that is animals whose genetic backgrounds are so similar that they are for all intent and purposes, more or less the same animal. This is important for studies as it greatly reduces the contaminating factors that may give flaws to a study. In order to achieve this goal laboratories first bred albinos sister to brother for at least 300 generations. The result was stock that were 99.9% genetically the same, natural cloning without the use of high technology!
After the cookie-cutter rat process was created researchers started to breed rats that fit their specific study in the same way. One of the most notable is a rat called the Sprague-Dawley, an albino strain that has been bred to be the mother of all rats. By this I mean that Sprague-Dawley were bred from very productive females which were bred to males born to even more productive females until the current day where Sprague-Dawleys routinely give birth to 18-25 pups per litter. This is in comparison to the 6-10 which is average. These rats quickly gained favorability outside the laboratory when feeder breeders got a hold of them. It's now standard practice for large rodent breeders (Who usually breed food for reptiles) to have Sprague-Dawleys or Sprague-Dawley crosses.
Sprague Dewleys are still very popular in labs, providing very quick turn around. It's to these rats that the first spontaneous dwarf was born. These rats had a defect in there genes that caused them to make very little use of their own growth hormones. The result was a stunted rat perfect for yet more studies. Dwarfism in rats is just as complicated as dwarfism in humans. We have no reason to believe that rats have any less capability of producing all the forms of dwarfism we see in humans (a number over 100.) pet breeders have been breeding naturally small rats and dwarves alike since they first made their way out of the laboratory but just like the different strains of furless two individual dwarves may not be compatible enough to create more dwarfed offspring. Current studies suggest dwarf rats my be more susceptible to mammary tumors and various other ailments related to their inefficiency to utilize their own hormones. It remains to be seen whether or not these rats will be healthy enough in the long run to catch on big in the pet market.
On the opposite side of the spectrum is the Zucker rat, a strain of piebald rats bred in the laboratory to be super obese. These rats have very little control of their own weight and can grow to be enormously fat even when fed the same amount of food as a regular thin rat. These rats are used predominantly in diabetes research and are not specimens most enthusiasts would want within the pet trade because of their health. Zucker rats have been seen outside the laboratory and kept by enthusiasts but no concentrated effort has been made to breed them for this market.
The Future of the Domestic Rat
Rats seem to be really catching on around the world and more breeders are concentrating on these little creatures then ever before. New markings and colors appear every day as breeders tinker with triple, quadruple, and sometimes quintuple recessive genes. Breeders are creating the rat types they want with the colors and markings they set forth to cross. This takes a concentrated and immense effort on their part.
Just within the past five years Burmese have appeared on the scene, as have merled, and a few tricolored rats (none which were healthy enough to be successfully bred.) Just before I got out of the hobby a color was created in my lines that I still haven't come up with a name for. It's an exciting time to be a rat breeder and I predict in the next twenty years rats will go through a boom like that of little dogs during the Industrial Age. I predict colors will continue to mutate but also body types may also begin to mutate as well. I would not be surprised if rats become the next dogs as far as genetic and aesthetic diversity go. If breeders continue to do their job responsibly we may even have rats who live longer and suffer from less diseases.
This article is accurate and true to the best of the author’s knowledge. It is not meant to substitute for diagnosis, prognosis, treatment, prescription, or formal and individualized advice from a veterinary medical professional. Animals exhibiting signs and symptoms of distress should be seen by a veterinarian immediately.