Remember the town of Hamelin, plagued by rats, and the story of the Pied Piper? Films and cartoons show rats as frightening, hateful creatures. People fear rats and avoid them with a certain contempt. The fight between humans and rats is many years old, probably going back to the Neolithic age. The two species have always wanted to destroy each other and wipe each other off the face of the earth. Yet, ironically, they also live closest together, almost cheek by jowl.

Rats have tried to destroy humans by spreading various diseases. At least twice in history, plague almost wiped out mankind. Besides plague, rats carry many other diseases, bringing 18 kinds of deadly bacteria from dirty drains. At night they climb up water pipes and spread those germs from house to house, a kind of revenge on humans. But though rats carry diseases, not all of them do, and not all of them are a nuisance. Some rats help human research: rats are among the live animals most often used in laboratory studies. So at times rats mean a great deal to humanity.

Since ancient times, white fur on animals has been taken to mean purity and virtue. The white rat is a domesticated animal of white and grey colour, with bright blood-red eyes. White rats are now considered among the most popular pets. In the eighteenth century the Japanese were very curious about white house rats; rats were kept and even cared for in homes in Japan. These white rats were later taken to Europe and America, where they came to be called albino rats, known simply as white rats. Why are they white? Researchers say white, or albino, rats lack the ability to produce melanin.

Pet white rats and wild rats differ in colour characteristics and habits. In suitable conditions, a pair of adult wild rats can multiply to 2,000 to 3,000 rats in a year. A female rat can bear young once she is three months old and can become pregnant again within two days of giving birth. Gestation lasts 18 to 22 days depending on the species. They can breed all year round, but usually give birth five to seven times a year, with six to 10 young each time. The mother cares for them. Their eyes open at 14 to 18 days; they suckle for up to four weeks and start eating solid food after three weeks. After five weeks they leave their mother. A rat usually lives two to three years.

Although white rats look remarkable, they do not give off an odour, as many other pets do. Male white rats weigh 400 to 650 grams on average and females 300 to 450 grams. White rats cannot bear sunlight, so they are more active at night and hide in quiet places by day. Pet white rats, though, sleep at night and stay awake only in the day, so their lives are very short. They can give their owners pleasure for a year to a year and a half, though this depends on proper care. It is very rare for white rats, or albinos, to live as long as three years. White rats are therefore scarce and valuable animals, highly prized by researchers, and there is strong demand for them in the market.

Cloning is the scientific process of creating an exact copy of an animal from its genes. Recently, scientists have been able to produce bright white rats by applying genetic engineering technology in genetic experiments. The white rats were produced by inserting a new type of DNA into rat sperm. The scientists produced them to put the new genetic engineering method into practice and improve it. The method is technically very simple and promising, and its success rate is much higher than that of other current genetic methods. In the new method the genes are pushed directly into the animal's body with the help of a virus. Professor Anthony Perry of the University of Hawaii and his colleagues were the first to carry out this cloning of white rats, in Honolulu.

The method has been applied to several other rodents. In the new experiment, rat sperm is first frozen below freezing point. DNA from another species is then inserted into it. Finally, the altered sperm is introduced into white rat eggs, and the eggs are then placed in a female rat using a standard embryo transfer method. The gene inserted into the rat sperm in this case was taken from the jellyfish, a sea creature; it is this gene that gives the jellyfish its shiny, bright green colour.

Researchers say the purpose of using the jellyfish gene was to see whether it would integrate successfully and work in the rat's body. More than 80 per cent of the embryos prepared in the laboratory took up the gene. But only one in five of the embryos survived pregnancy, and the effect of the jellyfish gene was seen in them. Among the cloning methods now in use, this new one has a much higher success rate, about 20 per cent. The genes are placed in such a way that they remain in the bodies of future generations, so that the descendants of white rats are also white.

Researchers say that because rat genes are quite similar to human genes, white albino rats are in great demand for the early stages of research. Swiss albino rats are used for testing in the early stages of any research in the biological and biomedical sciences. In Bangladesh, however, white rats are bred only at icddr,b and Jahangirnagar University, far fewer than are needed. If people interested in rearing white rats were trained by the government to set up farms, rats could make an even more important contribution to research, and Bangladesh's research sector could be enriched.

The writer is a science writer, researcher and MPhil scholar at the University of Dhaka.

Source: Jagonews24. First published in Bengali on The Agro News.