Most scientific studies of rat behavior have been on domestic varieties of Rattus norvegicus. All laboratory rats are of this species. They can easily be crossed with wild ones, but their behavior differs from that of wild rats in many important respects, not all of them obvious.
They have been selected for tameness that is for a reduced tendency to flee from man or to struggle or bite when handled. In addition, strange objects which would in duce avoidance (fear) by wild rats provoke mainly approach (curiosity) by tame ones; and tame males do not attack other males with anything like the same intensity as wild males.
Their selection of food, when they are faced with a choice, may also differ from those wild rats. A similarity of behavior, which they are rarely allowed to display, is that they burrow readily.
Differences in behavior have been paralleled by changes in growth, described by Donaldson. There are differences in the relative weights of organs, such as the adrenal glands. A population of wild rats is genetically heterogeneous, and generalizations about the behavior of wild rats often have to be qualified by reverence to individual variation.
In an inbred strain genetically variation is greatly reduced. This has advantage, for certain kinds of experiment, that differences between experimental and control group are more likely to be the effect of the experimental procedure, and less likely to be due genotype variation with which the experimenter is usually not concern.
