An editorial published in a prominent medical journal more than 100 years ago called for exterminating all species of rats to stop the spread of diseases.

It was noted that rats breed three or four times a year, with females breeding when about 4 or 5 months old. (The gestation period for a pregnant female rat is 21 days.) Male and female rats may have sex 20 times a day, and a dominant male rat may mate with up to 20 female rats in just six hours.

The average litter size for rats is 10 or more, and thus a single pair, breeding three litters a year, would in three years have a progeny numbering up to 20 million.

For a number of reasons, including their sheer numbers, rats were, and continue to be, the most despised and feared creatures that plague mankind. They carry bacteria, viruses, protozoa, fungi, mites, fleas and ticks. According to one reference, rats have been responsible for the deaths of more than 10 million people in just the past century. Moreover, rats often bite youngsters and infants on the face because of the smell of food residues on the children. Approximately 50,000 people are bitten by rats every year.

Despite their frightening and abhorrent characteristics, rats have a redeeming quality not described in the editorial written a century ago. For example, rats were the first mammals domesticated for research purposes, and rats in the laboratory may well have saved as many human lives through the years as they have taken.  

Today, there are 51 species within the genus Rattus. Norway rats originated on the plains of Asia (northern China), while black rats originated further south in the Indo-Malayan region. Both traveled to Europe with humans, although Norway rats came somewhat later. The first reliable accounts of the presence of the more aggressive and larger Norway rats in Europe date back to the 18th century, and it was during this time that they began to displace black rats all over Europe. Today, Norway rats have almost completely replaced black rats in Europe and America, where black rats are now rare or absent. In contrast, in tropical zones black rats are more common.

 A rat is an amazing creature; it can collapse its skeleton, which allows it to wriggle through a hole as narrow as three-quarters of an inch. An adult’s jaws are hundreds of times more powerful than a human’s. Rats can gnaw through bone, wood, iron or concrete. They also can alert family members by making high pitched sounds to alert them of danger or presence of food.

Rats are nighttime animals and detect motion 30 feet away, even in nearly total darkness. For locations in the dark, rats also rely on the long whiskers on their faces and hair on their coats to feel their way. As acrobats, rats can scale a brick wall straight up and survive a fall from a five-story building. They are skilled climbers and find it easy to board ships by ascending mooring ropes. Rats are indefatigable, and have been known to run 5 to 10 miles a night in wheel cages.

Fortunately it is possible to kill rats by poisoning them because they do not have the ability to vomit or burp. Rats, however, have an acute sense of smell and taste and have learned to scrupulously avoid food that makes them sick.

Rats are vectors and carry at least 18 diseases that affect people. The most serious are rat bite fever (streptobacillary and spirillary), Lassa fever, typhus, poliomyelitis, meningitis, trichinosis, Salmonella poisoning, hanta virus, leptospirosis (infectious jaundice) and, of course, plague. It is for the latter that rats continue to carry the heinous reputation as they were at the scene of a number of humanity’s greatest calamities. Rats and other rodents are the natural reservoir of plague, a disease that during the middle ages killed as much as one-third of the population of most towns and villages.

Rats can learn to crave the same drugs as humans do — including alcohol, cocaine, nicotine and amphetamine — and have been known to overindulge. Rats are the preferred model for the study of genetics of many complex diseases, including hypertension, non-insulin dependent diabetes, renal disease, and autoimmune and behavioral disorders. Rats and humans absorb and eliminate a number of drugs at a similar rate. Their findings suggest that testing new drugs on rats and perhaps one other species of animal may satisfy FDA requirements for preclinical studies.

The basic internal structures of rats and humans are similar despite the disparities in length and diameter. Similarities extend far beyond gross anatomy. Rats even have personalities; they can be sullen or cheerful depending on their upbringing and circumstances. Surprisingly, rats are generally sociable, curious and love to be touched.

It is unlikely that we will ever be witness to eliminating all species of rats despite what the medical journal advised a century ago. In fact, a recent book about the effects of the earth's climate on extinction predicts that the year 1 million will be witness to islands full of novel tree rats, ground rats, slow rats, vegetarian and predatory rats, even diving rats — a scenario too grim to imagine.

Max Sherman is a medical writer and pharmacist retired from the medical device industry. He has taught college courses on regulatory and compliance issues at Ivy Tech, Grace College and Butler University. Sherman has an unquenchable thirst for knowledge on all levels. Eclectic Science, the title of his column, will touch on famed doctors and scientists, human senses, aging, various diseases, and little-known facts about many species, including their contributions to scientific research. He can be reached by email at