Rodents

Environmental control has another very important function. By producing controlled environments that differ in some single variable (e.g., temperature) it is possible to isolate that variable and determine its effects upon the health, behavior, or longevity of the animals being studied. Variables that can be tested in this way range from simple aspects of the environment such as day length, temperature, and humidity to more complex variables such as feeding regimen and nutrition, drug dose or route of administration, or social interaction with cage mates.

The power of environmental control in these rodent models (and in some lower species such as fruit flies as well) is greatly increased by the genetic control that can be applied at the same time. By a process of inbreeding (mating to close relatives) mice and rats can be made to share all or most of their genes. Inbred strains are the most commonly used examples of such control. Inbred strains are the result of at least twenty generations of brother-sister mating. After twenty generations new mutations are adding genetic diversity, in the form of new genes at the same rate that gene differences are being eliminated by the inbreeding process. However, at this point the differences between the animals within the inbred strain are very slight. The animals are as genetically identical as are identical twins. Thus, it is possible to produce large numbers of genetically identical mice or rats, all the same age and at the same time.