They are the world's fattest rats, Indian scientists say, and they may hold the key to a gene which causes obesity.
The "sumo" rats become ill and die earlier than their leaner counterparts
Scientists at India's National Institute of Nutrition are maintaining a colony of some 500 obese rats - most of the animals weigh anything between 900g to 1kg.
The fattest rat that has been bred at the institute was 1.4kg, four times the standard weight of a rat.
These rodents are fed a diet set down by international animal research standards - a mix of proteins, fats and carbohydrates.
That means a diet of wheat, roasted gram, casein and skimmed milk.
Scientists say the rats have a gene making them prone to obesity which is why they eat four times the amount that a normal rat consumes.
Rats have always been a favoured animal with scientists for research on nutrition.
Scientists at the institute have found that as the rats become obese, they develop degenerative diseases.
Some 60% of them suffered from mammary tumours, 20% developed cataracts, and most of them had kidney abnormalities. All of them are infertile.
Against the average life of a rat up to three years, these "sumo" rats, as some scientists describe them, do not live beyond 18 months.
"In other words, obesity can lead to these diseases," says Nappan Veettil Giridharan, a rat geneticist at the institute.
"The gene that we are looking for could solve all these problems."
Indian and US scientists are expected to work soon on a $500,000 project on identifying and cloning a gene responsible for the fat rats.
The world's first gene found to be related to obesity was discovered by American scientists in 1994.
Researcher Jeffrey Friedman, of New York's Rockefeller University, and his colleagues cloned the gene and identified its product, the protein leptin.
Leptin is a hormone released by fat cells and is an appetite regulator, and now forms the basis of an anti-obesity drug.
Scientists say hormones play an important role in how and when we eat.
Dr Friedman had showed that mice which could not make leptin responded to leptin injections by losing up to 30% of their weight in two weeks.
But India's fat rats show no signs of losing weight in spite of having high leptin levels.
"So there must be another gene which we are looking for which could solve the obesity problem," says Dr Giridharan.
'Obesity is a disease'
"The challenge is to isolate these genes. The obesity gene can lead to a protein which can lead to a drug," he says.
Dr Friedman echoes the sentiment.
"The possibility of identifying another obesity gene is very exciting," he told the BBC News website.
Obesity is a widespread problem
So do genes decide whether a person is fat or thin?
"That is true to a large extent, though not entirely," says Dr Friedman.
Obesity is a huge problem in countries like US where, according to some studies, over 50% of the population suffers from it.
"Obesity is a metabolic disease, not simply a problem that beauticians can fix," says Dr Giridharan.
There could be other, unusual spin-offs for India from the fat rats.
Scientists say the National Institute of Nutrition could patent and export them as a laboratory animal around the world, bolstering its $140,000 a year revenue from selling animals for biomedical research.