foxes,and connection of this variability with that of re-~
productive characters (date of sexual activity and ferti-
lity).

Analysis of crosses of the parents with different re-
actions as well as heritability and repeatability coeffi-
cients,point to a significant influence of heredity upon
the character of defensive behaviour in silver foxes. It
has been found that the time of sexual activity in females
as well as their fertility level,depends upon the type of
their behaviour.The most calm females exhibit the best
fertility.Besides,they have earlier dates of mating in
the reproductive season as compared with aggressive fe-
males,

For many years a special experiment is being carried
out,in which foxes are selected for the type of their de-
fensive behaviour;the aim of this experiment is to repro-
duce in these animals the main features of domestication
process and hereditary reorganization of behaviour, and
to try to reveal the influence of this reorganization on
the character of reproduction in these animals.We managed
to select animals which were close in behaviour to the
dog.It has been established an additive effect of selec-
tion for this character.It was also shown that selection
for hereditary ability to domestication,because of corre-
lation of this trait with the best reproductive qualities,
leads to significant increase of fertility and to earlier
reproductive dates in the mating season.

As it is known,sexual and endocrine systems in sea-
sonally reproducing animals undergo certain cyclic chan-
ges.We investigated morphologically the genital and endo-
crine systems in females from seiLected for behaviour po-
pulation,and in comtrol females of non-selected popula-
tion at one and the same moment of the breeding cycle
(end of November - beginning of December,that is 2-2,5

1678