3. Results
Table 1 shows that the ewes that were kept on natural photoperiod but had been previously exposed to a long photoperiod initiated their first ovulatory season at a
Ž .
Ž .
significantly earlier date April 3 than those in the control group July 31 . However, as shown on Table 2, they required a significantly longer period of exposure to decreasing
Ž .
photoperiod before their first ovulation 83 7.5 days than the ewes in the control Ž
. group 40 5.8 . The first ovulatory season of the ewes that were transferred from long
to natural photoperiod ended only 3 weeks after that of control ewes, despite having
Fig. 3. Mean melatonin concentrations during 24-h periods on four different dates in Pelibuey ewes maintained Ž
X
. under natural or inverse photoperiod 19
813 N . Shadows represent periods of darkness. The numbers on the horizontal axis represent the hours at which the lights went off or on.
Table 3 Duration of night-time melatonin elevations on the dates of the natural equinoxes and solstices in Pelibuey
ewes kept under natural or inverse photoperiod Values are mean datestandard error.
Ž .
Ž .
Different literals superscripts a, b, c indicate significant differences P - 0.05 . Ž .
Date Duration of melatonin elevation h
Natural photoperiod Inverse photoperiod
b b
September 21–22, 1997 11.30.4
11.70.2
c a
December 21–22, 1997 12.30.3
9.50.4
b b
March 21–22, 1998 11.00.1
11.10.1
a c
June 21–22, 1998 9.70.2
12.30.1
Ž .
started with a difference of 4 months Table 1 . The duration of the anovulatory season Ž
. was similar for both groups Table 2 , and their second ovulatory season began almost
Ž .
Ž .
simultaneously Table 1 , about 1 month after the summer solstice Table 2 . There was little individual variation in the beginning or the end of ovarian activity within each of
Ž .
the two groups kept under natural photoperiod Figs. 1A and 2A .
Fig. 4. Average monthly prolactin concentrations in Pelibuey ewes kept under normal or inverse photoperiod Ž
X
. 19
813 N . Data are normalized to the longest day of the year for each treatment. Month 0 is June for the natural photoperiod group and December for the inverse photoperiod group. Values are meanstandard error.
Ž .
Values that do not share at least one literal are significantly different P - 0.05 .
Exposure to inverse photoperiod caused a gradual shift on the annual reproductive Ž
cycle, so that in each inverse group with or without previous exposure to artificial .
photoperiod , the first ovulatory season ended about 3 months before that in its corresponding group kept on natural photoperiod, while the second ovulatory season was
Ž .
advanced about 5 months in the inverse groups Table 1 . There was little individual variation in the starting dates of the first period of ovarian activity within each of the
two groups kept under inverse photoperiod. However, both the end of the first ovulatory season and the start of the second one showed ample variation within each of these
Ž .
groups Figs. 1B and 2B . Two ewes kept on inverse photoperiod continued ovulating throughout the experimental period, at the end of which they had been continuously
Ž .
cyclic for more than 18 months Fig. 1B . Another animal was continuously cyclic for Ž
. 19 months before it died from causes not related to the study Fig. 2B .
Fig. 3 shows the profiles of melatonin on four different dates in the ewes kept on natural or inverse photoperiod. Table 3 shows that the duration of the night-time
Ž .
elevations were significantly different P - 0.05 between groups during the solstices Ž
. Ž
. December and June , but not during the equinoxes September and March . Mean
melatonin concentration during the night-time elevations was 101.3 18.2 pgrml, with no differences between groups or dates.
Fig. 4 shows the mean prolactin concentrations during the second year of the study, normalized to the longest day, month 0 being June for the natural photoperiod group and
December for the inverse group. The prolactin profile of ewes kept on inverse photoperiod was 6 months out-of-phase in relation to that of the control group. In both
Ž .
groups, the concentrations of the hormone were highly correlated P - 0.01 with the duration of the day, but this correlation was higher for the ewes kept on natural
Ž .
Ž .
photoperiod r s 0.61
than for those on inverse photoperiod r s 0.48 . Prolactin
concentrations always tended to be higher in the ewes on natural photoperiod than in Ž
. those on inverse photoperiod. However, the differences were only significant P - 0.05
during the month with the longest day, and 1 and 5 months after the longest day.
4. Discussion
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