Only becoming popular as pets over the last century, rats have actually lived with humans for thousands of years.
The two rat species that have long, intimate histories with humankind are the brown rat (Rattus norvegicus) and the black rat (Rattus rattus). Modern pet rats are descendants of wild brown rats, which today come in a variety of colors in pet stores. While historically rats were viewed as disease-ridden, it was black rats (which can actually be black to dark slate-gray in coloring) that were involved in spreading bubonic plague; modern pet rats normally pose no health risk to their owners.
Both rats originate in Asia, where they have been a nuisance to humans for millennia. Indian scriptures from 3000 BC reference granaries ravaged by rats. It’s even thought that the famous German story of the Pied Piper of Hamelin may have its origins in ancient Persian or Chinese literature. Both rat species once coexisted with each other in human dwellings by filling different niches; black rats are arboreal, preferring dry, thatched roofs and attics, while brown rats are mainly burrowers, seeking cool ground levels and basements.
Black rats were the first to travel from Asia to Europe in significant numbers. They most likely traveled on trade ships from India to Egypt around 3000 BC. The ancient Egyptians’ highly prized cats kept the rodents out of precious grain stores. Egyptian and Roman trade then took black rats to the Mediterranean; evidence shows that by 100 BC they were in Pompeii, Italy. By 300 AD, black rats had followed humans all the way to England. Black rats quickly created a bad reputation for rats in Europe, repeatedly spreading plague throughout the continent (as they had done in Asia previously). The most infamous outbreak was the Black Death, a bubonic plague pandemic that extended throughout Europe in 1347 AD and resulted in the death of 30 percent to 60 percent of Europe’s population.
Many rodents can carry bubonic plague, but only black rats carried the bubonic plague to humans, due to their biology and behavior. Mind, though, it is not the black rats themselves but their infected fleas that are the primary vectors of the bubonic plague: When a flea bites an animal with Yersinia pestis (the bacteria that causes bubonic plague) in its blood, it becomes infected and spreads Y. pestis to the next animal (rat, human, or other) it feeds on.
Over a millennium after black rats, brown rats arrived in Europe. In 1727 AD, an enormous number of brown rats left eastern Asia for Europe. Although small groups of brown rats probably already lived in Europe, there had never been a mass exodus like this: Witnesses saw thousands of brown rats swim across the Volga, the largest European river, as they headed westward across Russia. Some speculate that an earthquake caused this migration, but it remains a mystery. Reaching England in the late 1720s, their arrival from the East was misattributed to passage on Norwegian ships and so they were labeled Rattus norvegicus.
Brown rats quickly spread throughout Europe, displacing the black rat, and entrenching themselves inseparably with the European lifestyle by 1800. Favorable factors helped brown rats displace black rats. The Great Fire of London in 1666 caused previously wooden buildings and thatched roofs, perfect for black rats, to be replaced by lead, tile, and brick. Burrowing brown rats could still dig into and around these new buildings, but black rats lost their upper-story lodgings. Similar housing updates were occurring throughout Europe. This, combined with brown rats’ physiological advantages over black rats, helped clinch the victory. While black rats weigh around half a pound, brown rats weigh nearly one pound, with records of 3.5 pounds. Brown rats can also survive harsh weather and eat nearly anything. While black rats are still dominant in the tropics today, brown rats are now found nearly worldwide thanks to their travels with humans.
But how did brown rats become pets? The victorious brown rats became a major problem in much of Europe, Victorian England in particular. Seasoned ratcatchers faced a new, larger species of rat, and with one rat pair potentially producing 15,000 descendants in a year if left unchecked, they could barely stem the tide. In response, a new kind of entertainment sprouted up in England: Ratcatchers not only made money catching rats, but found they could make additional money by then selling them for blood sport, dubbed rat baiting. Dozens of rats would be put in a pit with a vicious, trained terrier and bets placed on how many rats the dog could kill in a period of time. At the height of its popularity in the 1840s, London had over 40 rat-baiting pits.
One of these business-oriented Victorian ratcatchers started the domestication of the brown rat into the pet rats we know today. Jack Black, so renowned a ratcatcher that he became Queen Victoria’s royal ratcatcher, was quite a showman. He trained rats to live in his shirt and go up and down his arms. Though he used these rats to demonstrate how effective his brand of rat poison was, he undoubtedly became attached to some, and their theatrics and shock value certainly gained them a public spotlight.
While Black poisoned rats and sold them for use in rat-baiting entertainments, he began keeping unusually colored rats. Taming these “prettier” rats, he decorated them with ribbons and sold them to women as pets. Rats slowly became more accepted pets in Europe, and in 1869, Charles Baudelaire, a French poet, in Paris Spleen called the rat “the poor child’s toy.” By 1901, pet rats had become so popular that a British woman, Mary Douglas, successfully led efforts to show pet rats at the National Mouse Club in England. After her death, pet rats went out of fashion, but interest was revived in the 1970s when the English National Fancy Rat Society (NFRS) was created.
Pet rats today come in a great array of colors, color patterns, and even fur patterns, with strict standards given by NFRS. Colors can range from white to cinnamon, fawn, beige, black, chocolate, and many more. Color patterns can be completely solid; have small markings (the most common being the “hooded” pattern, with coloration on the head and a stripe down the back); have gradient “points” of color on the nose and feet (called a Himalayan); or other designs. While most rat coats are straight and short, they can be curly, long-haired, or hairless. However, the NFRS prohibits the showing of hairless rats and tailless rats because these genetic traits can harm the animal’s health.
These thoroughly modern, “fancy” domesticated brown rats make great pets. Rats are genetically very similar to humans, even more closely related to us than cats or dogs are, which may be a large part of why we’re attracted to them as pets. They’re curious, intelligent, trainable, omnivorous, social, and eat their food sitting on their haunches, grasping it with their tiny front feet. They’re also calmer and less likely to startle than other domesticated rodents. (It is because of their genetic similarities to humans, as well as their low space requirements and amazing reproductive abilities, that they’re often used in research laboratories.) All in all, pet rats make better companions than pests, and likely have a long history ahead of co-existing with humans.
For more on pet rats, see the Web site for the National Fancy Rat Society (NFRS), Wikipedia’s article on the “Fancy Rat,” Charles Golding’s Rats: The New Plague, or Robert Hendrickson’s More Cunning Than Man: A Social History of Rats and Men.
Biology Bytes author Teisha Rowland is a science writer, blogger at All Things Stem Cell, and a graduate student in molecular, cellular, and developmental biology at UCSB, where she studies stem cells. Send any ideas for future columns to her at email@example.com.