[The reproductive function of young male silver foxes Vulpes vulpes after long-term selection for domesticated behavior]

Sperm morphology of spermaries, testosterone plasma level and sexual activity of young silver fox males selected for domestic behavior have been studied. It is established that a number of spermatozoa in semen of males from selected population is lower as compared with the control one, whereas abnormal spermatogenesis level is significantly increased. The testosterone plasma level in males after a female introduction increased to the same extent in both groups. Sexual activity in males from selected population was lower than in the control one during the first mating season. The data obtained suggest a depressing effect of selection for domestic behavior on spermatogenesis and sexual activity in the young silver fox males and point to the hereditary changes of some chains of reproductive functions of males under the selection for domestic behavior.

[Adrenal function in the mink Mustela vison selected for the type of behavior]

Glucocorticoid function of adrenals and its reactivity to stress caused by immobilization and to ACTH stimulation have been studied in the male minks selected both for domestic and aggressive behavior towards man. In autumn period the regression of cortisol content in adrenals was observed in domesticated males and particularly in aggressive ones compared with control. At the same period the increased level of cortisol in blood as the result of immobilization was noticed in aggressive minks compared with the domesticated and control ones. At ACTH stimulation in spring time the increase of cortisol level in blood was shown for animals of all the groups but it was more marked for the aggressive ones.

Gonadal function in mink under artificial photoperiods

Exposure of juvenile minks to artificial light conditions imitating early autumn (additional illumination from 20 June to 20 July followed by 8 h daylight from 21 July to 10 October--photoperiod I; 8 h daylight from 21 July to 10 October--photoperiod II) induced the prolonged modification of sex steroid concentrations. The increase in plasma testosterone concentrations during the prepubertal period was followed by a decrease before and during the mating period in experimental males. The concentration of oestradiol in plasma increased just before the start of the mating period (January and February) in females under photoperiod I, and these hormonal changes were accompanied by an increase in fertility. The conditions of photoperiod II (involving fewer hours of daylight) had less effect on the sex steroid concentration in the blood of females, and fertility was not changed. The response of gonads to hCG injection at the beginning of puberty (November) was higher in males exposed to photoperiod I conditions and in females exposed to both photoperiods than in control animals.

Testosterone production in fetal testes of the silver fox

Testosterone content was studied in fetal serum and testes of silver foxes during different periods of prenatal life. The first sign of testosterone synthesis in the testes could be detected starting from the 31st day of gestation by radioimmunoassay. Subsequently its content sharply increased and showed maximum value on the 50th day. Detectable serum testosterone level in male silver fox fetus was noted from the 40th day, its serum concentration increased in prenatal life, but only insignificantly. Testicular production of testosterone was stimulated by hCG at the stages studied from Day 40 on. It is supposed that silver fox testes were able to respond to gonadotrophin not earlier than by Day 40 of prenatal life, but the establishment of pituitary-testicular interrelationship was clearly demonstrated at the end of embryogenesis (Day 50).

[The testosterone level in the testes of silver foxes during prenatal development]

The level of testosterone in serum and testes of the silver fox fetuses on days 31, 35, 40, 45, and 50 of gestation was determined using radioimmunoassay. In testes, testosterone was first detected at day 31; then its level gradually increased. Serum testosterone was detected only at day 40. Subsequent increase in its concentration was insignificant. Human chorionic gonadotropin stimulated testicular testosterone production in vitro beginning from day 40. We suggest that in the silver fox, testes are capable of responding to gonadotropins already by the day 40 of prenatal development but the formation of pituitary-gonadal axis proceeds until the end of embryogenesis.

[Testosterone production by the fetal testes of silver foxes selected for their domesticated behavior]

Testosterone content was determined by radioimmune assay in the testes of silver foxes between the 31st and 50th days of gestation. Small quantities of testosterone were found in the testes already at the 31st day of prenatal life, gradually increasing up to a maximum value at the 50th day. No significant difference in testosterone content was found between domesticated and undomesticated silver foxes during prenatal life. It is suggested that selection for domesticated behaviour affects rather central control of endocrine functions than steroid biosynthesis in the testes.

[Sex steroid hormones and brain serotonin in the estrous cycle of silver foxes]

A significant increase in hypothalamic serotonin and estradiol levels was found in silver foxes during proestrus. During estrus, the plasma estradiol concentration was reduced and a significant increase in progesterone level was correlated with a decrease in the brain serotonin content. The data obtained suggest that the brain serotonin plays a mediating role in the control of ovarian hormonal activity and ovulation in silver foxes.

[Hormones, reproductive behavior and fertility in male silver foxes]

The relationships between gonadal and glucocorticoid hormones and reproductive behaviour were estimated in two groups of silver fox males classified as either low or high sex activity. Levels of testosterone and cortisol in peripheral plasma measured by radioimmunoassay did not differ between the groups. However, when receptive females were introduced to experimental males, levels of testosterone in plasma increased significantly only in the males with high sex activity within 1 hr after a test of sexual behaviour. Low sex activity of silver fox males correlated with an increased level of aggressive behaviour and decreased fertility.

[Gonadal endocrine function in female silver foxes selected for domesticated behavior]

Selection for domesticated behavior in female silver fox is accompanied by a number of changes in endocrine function of ovaries. Significant changes in peripheral blood estradiol and progesterone levels and their production by gonads during anestrous may be an endocrine basis for out-of-season activation of reproductive system and earlier beginning of seasonal reproduction in domesticated females. The increase in the hormone level during preimplantation period of pregnancy and before parturition in tame foxes may influence their productivity.

[Effect of adrenocorticotropin on the progesterone level in the peripheral blood and on adrenal progesterone production in vitro in female silver foxes]

The plasma progesterone increased following a single injection of ACTH (3 ME/kg). The progesterone production was also increased by ACTH after incubation of adrenals in vitro. The data suggest that the adrenal glands of female silver foxes are capable not only of secreting progesterone, but also of responding to ACTH with a further increase in the progesterone secretion.

[Photoperiodic control of gonadal endocrine function in silver-gray foxes and its alteration during domestication]

Studies have been made on the level of estradiol and progesterone in domesticated and nondomesticated female silver foxes under different photoperiodic regime. The duration of the "light--darkness" cycle was equal to 24 h. Total duration of the light period was equal to 9 1/2 h; illumination was natural for 7 h, being artificial for the rest 2 1/2 h in a dark part of a day. It was demonstrated that additional illumination affects the hormonal level and sexual activity, the effect being dependent on the particular time of illumination. Nocturnal illumination significantly increased the level of sex hormones in the blood, domesticated animals being more sensitive as compared to nondomesticated ones.

[Relationship between the hypophyseal-adrenal and the genital systems in silver-black foxes]

Interaction between the hypophysial-adrenal system (HAS) and the hypophysis-gonad system was studied. For this purpose the foxes were subjected to total gonadectomy 8 months before they were sacrificed. Functional activity of the HAS was judged by the hypophysis ACTH content, corticosteroid production by the adrenal cortex in vitro, and by the glucocorticoid concentration in the peripheral blood. Castration of male foxes foxes failed to influence the activity of various HAS links. Gonadectomy of females was accompanied by changes in the activity of individual HAS links in different direction--some reduction of ACTH in the hypophysis, a sharp and significant fall of the peripheral blood glucocorticoid level and a marked significant elevation of hydrococortisone production in the adrenal cortex in vitro.