Rats are not generally considered either popular or useful animals. But to scientists, who use them for biological and psychological experiments, they are both. In many laboratories, in which dozens of rats are kept in one cage, it is essential that they be marked in some way so that any one rat can quickly be singled out from all the other rats. Labels attached to the rats or marks painted on them are not entirely satisfactory. A more popular method is notching or perforating the ears according to a code. Another is cutting off various combinations of toes, or different...
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