Rats

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Rats

Physical characteristics

Rats are members of the order Rodentia, which also includes beavers, mice, hamsters, and porcupines. Two major subfamilies of rats and mice are recognized within the family Muridae: the Sigmodontinae; the New World rats and mice, comprising about 370 species in 71 genera, and the Murinae, the Old World rats and mice, comprising more than 520 species in 122 genera. The major taxonomic difference between the two subfamilies is the presence of a functional row of tubercles on the inner side of the upper molars in the Murinae.

Physical characteristics

Rats are generally small animals. A typical rat, Rattus norvegicus or the Norway rat, is about 9 in (23 cm) from the nose to the base of the tail when fully grown and weighs about 2 lb (1.8 kg). One of the largest species, the southern giant slender-tailed cloud rat Phloeomys cumingi, has a head-body length of 19 in (48 cm) and a tail that ranges between 8-13 in (20-33 cm) long.

Rats have brown, gray, or black fur covering their body, except for their ears, tail, and feet (the familiar white lab rat is an albino form of R. norvegicus). Their hearing is excellent, and their eyes are suited for a nocturnal lifestyle. Rats typically have 16 teeth, the most prominent of which are their ever-growing incisors. The outer surface of the incisors is harder than the inner side, much like a chisel. The incisors grow throughout life from the base and are nerveless except for at the base. Rats must gnaw continually to keep the incisors down to a manageable length; if rats fail to gnaw, the teeth can grow rapidly and curl back into the

roof of the mouth, or (with the lower incisors) up in front of the nose, making biting and eating difficult.

The teeth, combined with the rat’s powerful jaw muscles, allow them to chew through almost anything; even concrete block and lead pipe have been found bearing toothmarks. The jaw muscles exert an extraordinary 24,000 lb (12 tons) per square inch (for comparison, a great white shark bites with a force of 20 tons per square inch). One of the masseter muscles responsible for this tremendous biting power in the rat passes through the orbit, or eye socket, a feature unique among the mammals.

A rat will bite a perceived enemy, particularly if cornered or if its nest is threatened. It also often bites out of curiosity, when exploring the edibility of unfamiliar things. Unfortunately, a sleeping child or unconscious derelict may be the subject of this investigation, with potentially serious consequences. Rats do carry a variety of zoonoses (animal-borne diseases) in their saliva, on their fur, and in their external parasites (such as fleas), that can and do infect humans. Best known are rat-bite fever and bubonic plague, transmitted to humans by rat saliva and rat fleas, respectively. When a rat walks though garbage in which salmonella bacteria are present, the microbes can latch onto the rat’s fur. When the rat later investigates a pile of human food, the salmonella moves from the fur to the food, and whoever eats it may develop food poisoning.

Behavior

Rats are social creatures, living in colonies that are housed in a complex network of underground burrows similar to the warrens dug by wild rabbits. To protect the colony from predators, the entrances to the burrows are well-hidden among rocks, the roots of shrubs, or under other thick vegetation. In temperate regions, most of the burrow is below the frost line, ranging from a few inches to several feet below the surface. Inside, the rats build nests of shredded vegetation, feathers, paper, and various other materials and huddle together for warmth.

One colony may consist of hundreds of rats of both sexes and all ages. According to observations made by zoologist S.A. Barnett, the colony is a relatively peaceful place. Due to an established social hierarchy among the males, there is little infighting for the right to mate with the females. Among rats, familiarity breeds content: seldom do rats that have grown up together in the colony fight with each other, although they may play in a rough-and-tumble fashion.

Conflict usually occurs when a new rat, especially an adult male, appears and tries to join the colony. The newcomer’s status, and sometimes its fate, is determined by the first few encounters it has with the colony residents. Fights that occur are seldom intense or bloody. Dominance is quickly established, and once the newcomer adapts to its new place in the colony the issue is settled. Male newcomers that lose the fight seldom remain for long; soon after the fight they either leave the colony or die, although they are uninjured. Some zoologists hypothesize that they die of social stress.

Reproduction

The colony’s size depends on two factors: the density of the population and the food supply. When the colony’s population is low, such as at winter’s end, the females will bear more young and thus the population increases steadily throughout the summer. As the population and density increase, the pregnancy rate declines accordingly.

Similarly, the greater the food supply, the larger the rat population. Female rats living near an abundant supply of food bear more young than females living further away from or without such a supply. If there is little food available, both sexes will become infertile, postponing reproduction in favor of individual survival.

The female’s estrus lasts about six hours, during which she mates with several males, copulating frequently during the heat. After a gestation period of 22-24 days, the female gives birth to 6-12 blind, naked, pinkish, helpless young. By the time they are two weeks old, the young are fully furred and their eyes are open. After 22 days, they leave the nest. Males are sexually mature at three months, females slightly later.

Diet

The rat’s nutritional requirements are similar to those of humans, which makes it a useful subject for scientific experimentation. They have been known to carry off beef bones left by picnickers, eating not only the remaining meat but also the bone as well, for the calcium and phosphorus it provides.

Rats will eat just about anything, including things that humans would consider far past being edible. However, they prefer grain and consume or spoil (by their hair and droppings) millions of tons of stored food each year worldwide.

Rats possess remarkable physical abilities. Rats can: swim for half a mile, and tread water for three days; survive falling five stories and run off unharmed; fit through a hole the size of a quarter; and scale a brick wall. Years after the nuclear testing ceased on Engebi Island in the western Pacific Ocean, scientists found rats, “Not maimed or genetically deformed creatures, but robust rodents so in tune with their environment that their life spans were longer than average,” one researcher observed.

Species

The most widespread species of rats are Rattus norvegicus, the Norway or brown rat; Rattus rattus, the black, ship, roof, or alexandrian rat; R. exulans, the Polynesian rat; and Bandicota bengalensis, the lesser bandicoot rat. Both R. norvegicus and R. rattus are found around the world, and these are the two commensal species found in North American cities. They are longtime residents, firmly established on this continent by 1775. The Norway rat is found in temperate areas worldwide, although it originated in Japan and Eastern Asia, where it lived in burrows along river banks and later in rice fields.

Rattus rattus, like R. norvegicus, originated in Asia. It is thought to have been brought to Europe during the Crusades, although some records indicate it was present in Ireland as early as the ninth century. Rattus rattus arrived in North America with the early settlers, and its presence is recorded as of 1650. Early explorers brought R. rattus with them to South America as early as 1540. The two species spread worldwide, traveling in sailing ships to new ports.

Less global but no less commensal is the Polynesian rat, found from Bangladesh to Vietnam, throughout the East Indies, and in Hawaii and on other Pacific islands. The lesser bandicoot rat has been found in its natural habitat of evergreen jungle and oak scrub in Sumatra, Java, Sri Lanka, Pakistan, Burma, and Penang Island off the Malay Peninsula, but in this century it has also become common in urban areas in India. (It reproduces more quickly than any other rodent; a female lesser bandicoot rat can have a litter of seven every month.)

Rats and humans

These four commensal species of rat together destroy about one-fifth of the world’s food harvest each year. In the United States alone, the Norway and black rat damage or destroy a billion dollars worth of property each year, not counting the accidental fires that start when they chew through electrical insulation.

These commensal rats succeed because they are generalists and opportunists. The Norway rat, for instance, adapted its natural ground-dwelling habit to take advantage of many environments: cellars, sewers, even among the bushes in front of nicely landscaped homes and apartment buildings. In some buildings, the basement is home to Norway rats while black rats inhabit the upper stories.

Rats are present in almost every major city in the world. A study of Baltimore during World War II (done in reaction to fear that the Axis would attempt rat-borne germ warfare) discovered that many blocks in “good residential areas” harbored 300 or more rats. In poorer, run-down neighborhoods, the number was doubtless much higher. Some cities in North America have an estimated population of two rats for every person.

Sanitation is the major contributing factor to the number of rats that will be found in a city, but new construction in an urban area will also force rats into areas where they have not traditionally been found, as digging unearths their traditional burrows.

Most rat control efforts involve poison bait. The most common type is an anticoagulant, usually rote-none, which causes fatal internal bleeding after the rat eats it.

However, there are formidable obstacles to effective rat control. First is the rats’ innate fear of anything new. Even if something as innocuous as a brick is placed near a rat colony, they will go out of their way to avoid it. So merely placing the poison does not guarantee results. In 1960, rats that were apparently unaffected by anticoagulant poisons were found on a farm in Scotland. They had evolved a genetically based resistance to the anticoagulants. These so-called super rats are now found in several places in Great Britain.

Rat-control experts in New York City’s Central Park noticed something curious about the rats they had been poisoning: the rats abandoned their normal shy, nocturnal habits and began appearing in the park in broad daylight. Rather than killing the rats, the poisons apparently acted like a stimulant to them.

Poisons obviously have their limits. The most effective method of rat control has proved to be a general clean-up to reduce the habitat quality for the pest rodents. Members of the Inspectional Services department must supplement their poisoning efforts with the education of local residents, telling people how to store their trash in rat-proof containers and how to rat-proof buildings by plugging all entry holes with steel wool.