INTRODUCTION
There are well over 100 species categorized in the genus Rattus. However, rats used in research laboratories today evolved from the Brown Norway (BN) rat, Rattus norve- gicus. This species originated in central Asia, and disseminated worldwide relatively recently within the last 200 years.
Early in the 19th century, “rat baiting” was a popular spectator sport in Western Europe. Albino rats were occasionally trapped during the acquisition of live rats for this “sport” and selectively bred as pets or show animals. They are considered to be a primary source of the laboratory rat stocks for use in research laboratories today. The Brown Norway rat appears to be the first mammalian species to be domesticated primarily for scientific purposes. Fancy rats coevolved during this time. Laboratory rats are represented by fewer numbers of outbred stocks and inbred strains than laboratory mice. Small numbers of transgenic rats now exist, and it is likely that transgenic rats will become more prevalent, but never to the degree of mice. The genetic background of rats is as important a consideration as in mice for expression of disease. Rats are subject to infection with fewer viruses than mice but seem to make up for this with more clinically significant bacterial infections.Wild rats tend to live in association with human environments. They live in social hierarchies consisting of a dominant male with a number of females and younger or submissive males. Intraspecies communication is complex, utilizing olfaction, pheromones, ultrasonic vocalizations, and tactile cues. Wild rats inhabit complex burrow systems, usually near water, with chambers for nesting and food storage. Domesticated rats are remarkably more docile than their wild counterparts, but aggressiveness varies among stocks and strains of laboratory rats. Brown Norway and F344 rats tend to be more aggressive than outbred stocks.