From Alzheimer’s to Zebrafish: Eclectic Science and Regulatory Stories 60
In an editorial published in the Journal of the American Medical Association more than 100
years ago, the writer described the prolific breeding habits, crop destruction, disease and
deaths caused by rats.1 The editorial called for their extermination by strict obedience to
laws of sanitation. It was noted that rats breed three or four times a year, with females
first breeding at the age of about four or five months. (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 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 respon-
sible for the deaths of more than 10 million people in just the past century.2 Moreover,
rats often bite youngsters and infants on the face because of the smell of food residues.
Approximately 50,000 people are bitten by rats every year.3 Despite their frightening and
abhorrent characteristics, rats have redeeming qualities 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.4
This article examines the early history and characteristics of rats, the diseases they
cause and the part they play in animal research. The latter information is of interest to
those of us involved in the product approval process.
History and Characteristics
True rodents are widely considered to have originated in Asia and first appear in the fossil
record about 54 million years ago. Murids, the family that gave rise to present-day rats,
house mice, hamsters, voles and gerbils, first appeared approximately 34 million years
ago. The genus Rattus first emerged about five million years ago and appears to be native
to the Mediterranean countries, the Middle East, India, China, Japan and Southeast Asia.
The ancestors of the most common rats, Rattus norvegicus (Norway rats) and Rattus rattis
(black rats), diverged from one another about two million years ago. Today, there are
51 species within the genus Rattus. Norway rats originated on the plains of Asia (north-
ern 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, 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.5 Today, Norway rats have almost completely replaced black rats in
Europe and America, where black rats are now rare or absent. In contrast, black rats are
more common in tropical zones.6 Pet rats are variants of the Norway rat, bred to have a
docile temperament.7
A rat is an amazing creature it can collapse its skeleton, allowing 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 con-
crete. They also can signal family members by making high-pitched sounds to alert them
to danger or the presence of food. Rats are nocturnal animals and can detect motion 30
feet away, even in near total darkness.8 In darkness, 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 five to 10 miles a night in wheel cages and to tread water for three days.
When it comes to diet, rats eat any sort of vegetable matter, from the obvious—grain,
seeds, nuts, leaves and fruits—to such unlikely choices as paper, soap and beeswax. They
In an editorial published in the Journal of the American Medical Association more than 100
years ago, the writer described the prolific breeding habits, crop destruction, disease and
deaths caused by rats.1 The editorial called for their extermination by strict obedience to
laws of sanitation. It was noted that rats breed three or four times a year, with females
first breeding at the age of about four or five months. (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 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 respon-
sible for the deaths of more than 10 million people in just the past century.2 Moreover,
rats often bite youngsters and infants on the face because of the smell of food residues.
Approximately 50,000 people are bitten by rats every year.3 Despite their frightening and
abhorrent characteristics, rats have redeeming qualities 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.4
This article examines the early history and characteristics of rats, the diseases they
cause and the part they play in animal research. The latter information is of interest to
those of us involved in the product approval process.
History and Characteristics
True rodents are widely considered to have originated in Asia and first appear in the fossil
record about 54 million years ago. Murids, the family that gave rise to present-day rats,
house mice, hamsters, voles and gerbils, first appeared approximately 34 million years
ago. The genus Rattus first emerged about five million years ago and appears to be native
to the Mediterranean countries, the Middle East, India, China, Japan and Southeast Asia.
The ancestors of the most common rats, Rattus norvegicus (Norway rats) and Rattus rattis
(black rats), diverged from one another about two million years ago. Today, there are
51 species within the genus Rattus. Norway rats originated on the plains of Asia (north-
ern 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, 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.5 Today, Norway rats have almost completely replaced black rats in
Europe and America, where black rats are now rare or absent. In contrast, black rats are
more common in tropical zones.6 Pet rats are variants of the Norway rat, bred to have a
docile temperament.7
A rat is an amazing creature it can collapse its skeleton, allowing 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 con-
crete. They also can signal family members by making high-pitched sounds to alert them
to danger or the presence of food. Rats are nocturnal animals and can detect motion 30
feet away, even in near total darkness.8 In darkness, 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 five to 10 miles a night in wheel cages and to tread water for three days.
When it comes to diet, rats eat any sort of vegetable matter, from the obvious—grain,
seeds, nuts, leaves and fruits—to such unlikely choices as paper, soap and beeswax. They