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The laboratory ratSecond only to the mouse, the rat is probably the most commonly used laboratory animal. The wild rat is a dangerous experimental animal and may attack a handler, but the domesticated brown rat takes readily to laboratory conditions. The albino, which has been developed by selection over the last seventy years from Rattus norvegicus, differs in many important aspects of behaviour from its wild progenitor. It has been selected for tameness and differs from the wild rat in its behaviour towards man. There is a reduced tendency to attack smaller animals, to flee from man, or to struggle or bite on being handled. However, if it is badly handled or suffers from certain nutritional deficiencies, it can become savage. Fighting between adult males is less ferocious, and fear of new objects has been greatly modified. Its choice of food may also differ from that of a wild rat. Physically, the laboratory rat attains a lower body weight, and it has smaller adrenals and a smaller brain and spinal cord. It is also less resistant to cold as it seems to lack the ability to grow a thicker coat in cold conditions. The rat is used for an enormously wide variety of purposes, including experimental pathology, biological assay, toxicity studies, nutritional research, cancer research and teaching. The rat is a traditional animal for behavioural research. Its ability to find its way to and through the branching passages of the burrow, enable it to find its way readily through a maze. Similarly, its ability to handle food and to build nests is basic to the quite complicated manipulations required in other behavioural studies. There is a growing use of gnotobiotic rats in various research fields - these are germ-free animals which have been deliberately exposed to a known microbial flora. Germ-free or axenic rats have no microbial flora and are obtained by a special aseptic Caesarian operation on the full-term dam. Gnotobiotic guinea-pigs, pigs, dogs, cats and chickens are also used, but not as frequently as rats and mice. They are expensive to produce and maintain but for some experiments they are essential. There are many colonies of rats in existence, but the best known of the albino rats is that bred at the Wistar Institute of Anatomy and Biology at Philadelphia, U.S.A. Apart from the familiar albino, with pink eyes and poor sight, there are various other strains of laboratory rats, some hooded, piebald, grey or black, many of which have pigmented eyes and, therefore, better vision. Under the controlled conditions of the laboratory there is no particular breeding season. Breeding begins at about eleven weeks of age and, if advantage is taken of the post-partum oestrous, a litter could be produced every 21-25 days. The female wild brown rat can breed at 3 months' old - about the same as the albino. In the laboratory, the expected life span is about three years, the female usually living longer than the male. By comparison, a wild rat may survive for 12-18 months, or longer in a safe environment, with the female outliving the male.
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23-04-09
