Small animals suitable for experimental use must reproduce fast and their genes should be as similar to humans as possible. Some small animals reproduce fast but have few genes similar to humans. 
some fast-breeding small animals are not suitable for experiments.
Generally speaking, computers that do not undergo system maintenance every week must be repaired in specialty stores after one year of use. Some computers do not need to be repaired in specialty stores after being used for a long time. Therefore, some computers undergo system maintenance every week.