[The effect of the photoperiod on steroid hormone secretion, on seasonal reproductive rhythms and on fertility in female silver foxes].

The silver fox is a strictly seasonal breeder and photoperiod is a main factor that regulates reproduction in this species. The aim of the present work was to study the effects of different skeleton photoperiods on the induction of early estrus, hormonal activity of gonads and fertility in anestrous silver foxes. Adult females (n = 48) were exposed to short days (9.5L:14.5D) from September until March. Females (n = 12) kept under the natural daylight conditions served as the control. The experimental groups received illumination in two fractions: 7- and 2.5-hour long. The 7-hour period comprised natural daylight from 10:00 to 17:00 and an additional light pulse at night (00-02:30, group 1), in the morning (06:00-08:30, group 2) and in the evening (17:30-20:00, group 3). There are three types of response in group 1. A half of animals showed signs of estrus in November-December, 2.5-3.0 months before the natural reproductive season. The second part displayed no changes until the end of the experiment indicating photorefractoriness. The third part had estrus during the natural reproductive season. Photoperiod 2 and 3 did not affect the temporal pattern of reproductive rhythm. All artificial photoperiods increased the estradiol level and decreased the progesterone, cortisol and testosterone levels during the anestrous phase and suppressed the ovarian hormonal activity during the follicular and luteal phases but photoperiod 1 had the most pronounced effects. Fertility was decreased in all experimental groups. Our results suggest that the photoperiod with a light pulse at night can induce early estrus and ovarian hormonal activity in anestrous females. The data obtained suggest that decreased fertility under these conditions could be due to altered ovarian hormonal secretion.