The genus Rattus includes 300 species although, for research purposes, the most common are the brown Norway rat (Rattus novegicus), black rat (Rattus rattus) and albino strains of these two species. Today, the vast majority of rats used in biomedical research, testing and education are domesticated strains purchased from animal breeders or bred in-house. Popular strains include Sprague-Dawley, Holtzman, Long-Evans or hooded rat, and Wistar, which takes its name from the Wistar Institute in Philadelphia, the world's largest supplier of rats for scientific research.
Coming to America
Because they are able to adapt to many habitats and climates, both the Norway and black rat species are present on every continent in the world. The black rat first reached North America in the 16th century as an uninvited traveler on trading ships. The Norway rat, which has no connection to Norway, originated in central Asia and was carried on ships from Europe to North America in the 18th century, reaching the eastern shore by 1775. The Norway rat has spread to most of the world over the past three centuries and has gradually displaced the black rat, a smaller, less aggressive species, in many places. The black rat is now most prevalent in tropical and subtropical America.
Sight and Sound in the Rat's World
All rats share a keen sense of hearing, but have poor vision—about 20/600 for most rats and 20/1200 for albino rats. Until recently, rats were thought to be completely colorblind, but studies of rat vision show that they can perceive ultraviolet light and are able to distinguish between different colors of blue and green. Rat vision is very blurry but the position of their eyes on each side of the head allows them a wide field of vision, which is helpful in scanning their environment for possible predators.
Rats emit ultrasonic squeaks, clicks and whines from 22 to 8 kilohertz (inaudible to the human ear) when in fear, during aggression, and during mating behaviors. They can detect frequencies up to 80 kilohertz.
The Nose Knows
Like most rodent species, the rat's behavior is based primarily on olfactory signals. In fact, about one out of every 100 genes in the rat is involved in the sense of smell. Besides their nasal passages, rats have an additional olfactory apparatus called the vomeronasal organ (VNO). The VNO is situated in a pouch off the nasal cavity and is used primarily to detect pheromones—chemical signals transmitted from other rats. Pheromones are critical to communication regarding mating, courtship, aggression, and parental care, and are found in urine, feces, and secretions of rats' skin glands. From pheromones of other individuals of the same species, rats can detect subordinate or dominant status, family members or strangers, stress level, reproductive status, and sex. Newborn rats will learn what to eat based on odor cues from their mother's milk. They will choose to eat foods that the mother consumed while feeding them.
Tails and Whiskers
The rat's tail is a long, nearly hairless appendage that accounts for about 80% of the animal's total body length. The tail is important for balance and thermoregulation—blood circulation to the tail can be increased to dissipate body heat and decreased to conserve body heat. Rats are agile climbers because they are able to use their tails to adjust their center of gravity and maintain balance similar to the way a tight rope walker uses a pole.
Whiskers are also very important to the rat, being essential to a rat's interaction with the environment and other creatures. Rats have 15 whiskers located on each side of the nose, eyebrows, and cheeks that are useful—particularly when underground or in other low light conditions—in navigation, orientation, and balance, as well as finding food and mates and avoiding predators. Rats also use their whiskers to determine that their noses are above water, allowing them to swim underwater without drowning.
Compared to their mouse cousins, rats have smaller ears, a longer, pointier snout and a generally larger body size, about 12 ounces (300 grams) for females and one pound (500 grams) for males, with a body length between 9 and 11 inches, not counting the tail. Although rats are capable of high-speed bolts in the face of potential threats, rats are generally much calmer than mice and will casually lope along when simply exploring. The rat and mouse have similar body temperatures, around 100 degrees Fahrenheit. Rats typically live about a year longer than mice, with an average life span of 3-4 years.
Rat Reproduction
Rats are among the most prolific breeders of all mammals. With adequate food and shelter, rats may breed year-round, and a single female may produce up to 12 litters a year. Gestation lasts for 21 to 22 days and the typical litter ranges from 6 to 14 pups, who are born blind, deaf and hairless, weighing 5-6 grams.
Young rats have teeth and can begin eating solid foods as soon as their eyes open, but they need their mother's milk for three to four weeks after birth. Female rats are nurturing mothers and create nests out of soft debris such as paper and rags to shelter and hide their pups while they are away. Both male and female rats will protect their young if a danger is present.
Habitat and Diet
In the wild, rats dig burrows beneath protective cover and construct elaborate tunnel systems with separate chambers for food storage and nesting. The rat is very territorial and will quickly defend his nest and its surrounding area. Rats live in colonies that consist of several families made up of one male and a harem of females. Dominance in a rat colony is established through size and fighting, with defeated individuals often being chased off and banished to a solitary life outside the colony.
The rat's nutritional requirements are rather similar to those of humans. Rats are omnivores who can adapt their diet to available food and remain properly nourished. The rat's diet typically includes seeds, nuts, grains, vegetables, fruits, meats and invertebrates. They consume about one-third of their weight in food every 24 hours. Because of their inability to vomit, rats are very hesitant to try new foods that may be poisonous. They will take a small nibble and wait to see if they feel sick and, if so, will avoid that food in the future.
Rats are nocturnal animals who have three major activity periods equally spaced at night, during which they may consume up to five meals. Rats have extremely powerful teeth and healthy rats must grind their teeth to keep them the proper length and sharpness.
Rats in the Scheme of Things
Like other rodents, rats are an important food source for predators, including snakes, hawks, eagles, and many larger mammals. Rats are also one of nature's "sanitation workers" because of their ability to "clean up" excess or waste material. But problems occur when rats make themselves at home near or in human dwellings to take advantage of easy access to food and shelter. Perhaps the rat's relationship with humans has been permanently jaded by their role in disease transmission to people. But the rat's role in these disasters has been greatly exaggerated, and today we are aware that the rat may more accurately be considered a symptom of dysfunctional city life than a cause of disease.
The Bubonic plague or Black Death killed millions of people in Europe in the 14th century, when cities were polluted by human waste and garbage thrown into the streets. Those conditions invited many unwelcome guests like the black rat, but rats were not entirely responsible for the spread of plague. Plague resulted from bites of oriental rat fleas carrying the bacterium Yersinia perstis. These fleas infested black rats but were likely carried by many other animals, including wild rabbits, chipmunks and prairie dogs.
During the plague outbreaks a fearful population blamed all sorts of animals including cats, dogs and, of course, the rat, for the outbreaks. However, sanitary disposal of trash and human waste was the most essential factor in eliminating the plague. By the 17th century, European cities and towns began to improve sanitation and construct sewer systems. In the 18th century, the spread of Norway rats in Europe even helped to control plague, because the oriental rat flea does not live on the Norway rat.
Rats and Humans
People began trapping large numbers of wild rats and keeping them in captivity during the early 19th century, a time when the "sport" of rat-baiting was popular. A terrier dog was placed in a pit with up to 100 rats and bets were taken on how long it would take the dog to catch and kill all the rats. Individuals involved in this activity began to look for additional uses for the rat and soon the Norway rat became the first mammalian species to be domesticated primarily for scientific purposes.
Also during the 1800s "fancy" rats—those selectively bred to exhibit various colors, traits and temperaments—became popular as show novelties and the occasional pet. As people became better acquainted with this intelligent animal, myths and misconceptions about rats began to diminish.
Today, rats are kept as companion animals by many people who enjoy their playfulness and inquisitive nature. Any rat owner will proclaim that each one has its own unique personality and that rats will bond to human caretakers much the same way as dogs. Rats can be taught to come when their name is called and can learn other tasks. Rats are social animals and who usually enjoy the company of other rats and an interesting environment. They enjoy chasing, grooming each other, sleeping piled into a heap, wrestling, and playing with food.
In Japan, the rat is honored every 12 years as a symbol of wealth. Egyptians honored the rat as a symbol of destruction, but also wise judgment because rats would only eat the best food. And followers of Hinduism believe the rat is the living representative of the goddess Karniji of wisdom and prosperity. There is even a temple in Deshnok, India, dedicated to rats.
Keeping Rats out of Homes
Although pet rats can be charming, wild rats taking up residence in a home is another story. The biggest problem rats cause for humans is evident from their label as rodents, a name which is formed from the Latin word rotere. meaning "to gnaw." Rats can cause serious damage to property by chewing belongings and spoiling food. These gnawings are the most obvious sign of rat presence, as are fecal pellets (much larger than those of a mouse) and spoiled food.
The best way to deal with rats, or any rodent, in human dwellings is to avoid attracting them and to prevent their entrance in the first place. Avoid leaving human and pet food out, and eliminate shelter for rodents around homes, other buildings, and outside facilities such as campgrounds and picnic areas to which rats may have access. Do not allow brush, rock or junk piles to accumulate near dwellings where rats could take shelter. Rats are able to squeeze through any opening they can fit their head through, so homeowners should make sure their house is not accessible to rats by sealing off cracks and crevices.
If rats are already inside a house, the most humane options are live trapping followed by release in the wild or euthanasia at a local animal shelter, whichever is more appropriate to the situation. Poison rat bait is very dangerous to other animals and humans, especially children, and also causes the animals that consume it to suffer a slow and painful death. Traps also permit rats to be removed from the home, thereby avoiding problems from animal bodies decaying inside walls and floors.
Life in the Laboratory
Rats used in research are most often kept in small, shoebox-like cages made of opaque material. The cage is lined with absorbent shavings or pellets, and has a water spigot and food pellets in a hopper in the grid ceiling of the cage. Some rats are kept on grid flooring, which can cause sores and calluses on their feet.
While laboratory conditions generally provide the basics of food, water, and shelter, they are ill-suited to the rat's propensity to play, explore and forage for food. In addition to the barrenness of their cages, lab life for rats often involves social isolation, environmental disturbances and lack of a stimulating environment. Isolation at any age can have permanent effects on rat physiology and behavior. In fact, rats isolated for as little as two hours show anxiety and aggressive behavior. And routine lab procedures, such as moving cages and holding rats, have been shown to produce significant stress and fear responses.
For the intelligent and inquisitive rat, enrichments in the cage environment can be more important than an increase in cage size. Providing proper bedding material gives rats the opportunity to carry out some of their favorite activities such as tunneling, nest building and foraging for food. Toys such as ladders, balls, and tubes also facilitate play and exploration of their environment.
Recent studies have shown that lab rats, despite generations of domestication, will quickly adapt to life in the wild when given the opportunity. A zoologist from Oxford University released 50 lab born-and-bred rats into an outdoor enclosure and followed their activity for six months. He found that within several days the rats were able to locate food, water and shelter and began to form hierarchical social groups—just as rats born in the wild do.
Experiments and Testing on Rats
While mice are the most frequently used animals in medical research, rats are a close second. Researchers point to rats' short gestation period, rapid growth rate, relative low maintenance, and mild temperament as reasons for choosing this species. Rats are considered a better "model" than mice for studying certain human diseases and conditions including memory loss, muscular dystrophy, cystic fibrosis, cancer, AIDS, substance abuse, genetic disease, spinal cord injuries, and heart disease. Rats are used in research on diseases such as Alzheimer's, diabetes, and Parkinson's.
Some of these diseases either do not occur in rats naturally or may affect rats much differently than humans. Consequently, diseases are often artificially induced in the rat by inserting genes into the rat's genome or creating transgenic rats that have a specific gene turned off or made useless. Mutant strains of rats and those born with deformities are also used in research. The rat was the third animal to have its genome sequenced, which is likely to lead to even more research on this species.
Toxicity testing is performed on rats exposing them to extremely painful experiments such as the LD50 test, which is used to determine what amount of a chemical will kill 50 percent of a group of animals.
Space Travel and Education
Rats were used as test subjects in early space programs. A rat named Hector became a national hero in France in 1961 after becoming the first animal to travel into the lower atmosphere in a rocket. Rats were also the first animal to undergo anesthesia and surgery in space during research to determine whether the absence of gravity affected muscle development in young rodents.
In many schools rats are used as educational dissection models. However, non-animal alternatives such as diagrams, computer simulations and interactive videos are beginning to replace dead animals as anatomy and physiology learning tools. For example, in 1999 a hand-crafted rubber rat anatomy model was introduced in the Netherlands as an alternative to the 3,000 or so rats killed for educational purposes each year in that country. Models can even be used to learn introductory surgical techniques, such as stitching and nerve fusion. Unlike actual rats, the models may be used repeatedly, reducing lab costs.
Protection for Rats in Laboratories
Research institutions receiving government funding for animal research are required to follow the Public Health Service Policy on Humane Care and Use of Laboratory Animals, which calls for adherence to the Institute for Laboratory Animal Research's Guide for the Care and Use of Laboratory Animals in testing, research, and training. While the policy strictly applies only to animals used in government-funded projects, most research institutions choose to apply it to all their research projects.
However, rats are not protected under the Animal Welfare Act, the primary federal law governing the treatment of laboratory animals. Consequently, millions of rats used each year in research fail to receive the minimum protections afforded to rabbits, guinea pigs, hamsters, and most other animals used in research.
Suggested reading: The Story of Rats by S. Anthony Barnett. Allen & Unwin, publishers.
