Domestic animals significantly differ from their wild forebears. It is difficult to reveal all differences, because there are no comparison objects for a domestic cow, dog, or horse - their predecessors are extinct. Changes that happened to domesticated animals thousands years ago can be restored in experiments on domestication, which have been conducted during more than 40 years in the Institute of Cytology and Genetics in Novosibirsk. Taming begins with the disappearance of animal fear of men. Novosibirsk scientists sorted rats by aggressiveness to men and obtained two selection lines: aggressive and tame animals. A change in behaviour always causes numerous alterations in functioning of nervous, endocrine, and immune systems that are to be revealed. By the present time, it has been found out that the immune response in tame rats is much weaker than that in aggressive ones.
The researchers worked with adult males of the 56th-58th generations of grey rats. Tame and aggressive animals were treated (injection in peritoneum) with a male sheep's erythrocytes, which contain proteins-antigens. Cells of the rat spleen produced antibodies. It was revealed that the number of cells producing antibodies is 20 times greater in aggressive animals than in tame ones. Moreover, each cell of a tame rat produces fewer antibodies. As a result, the immune response of tame rats is very weak. Following the first attack of a foreign protein-antigen, the body acquires immunological memory, which provides for a more intensive formation of antibodies in case of another attack of the same antigen. This phenomenon is called the secondary immune response. The number of cells having the immunological memory in tame rats is lowered too. Hence, their secondary immune response is several times weaker than that in aggressive animals.
Thus, several decades of domestications have caused changes not only in behaviour, but also in the immune status. The researches have concluded that domestication of wild rats through selecting non-aggressive individuals causes degradation of functional activity of the whole immune system, which primarily depends on functions of hyphophysis and adrenal glands. Several thousand years ago, wild predecessors of practically all domestic animals came through a similar selection. Apparently, their immune system is weakened too, and they are more susceptible to infections in comparison with wild forebears. Life conditions in captivity (dense flock, unnatural habitat, artificial food, continuous stress, etc.) only favour epidemic outbreaks of diseases that are usually lethal for poor weakened animals.
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