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
The present study is the first one to demonstrate that the relatively small variation in Ž
.
X
day length 2 h and 12 min that occurs during the year at 19 813 N, can be translated by
Pelibuey sheep into significant changes in the duration of melatonin secretion, strongly affecting the breeding season and prolactin profiles of ewes kept on inverse photoperiod.
In ewes without a previous history of exposure to artificial photoperiod, the mean dates for both the onset and the end of ovarian activity were significantly different in the
animals exposed to inverse photoperiod than in those kept on natural photoperiod. Although the difference in the date of onset of ovarian activity between these groups
Ž .
was much larger during the second year than during the first one Table 1 , this was due to the fact that in 1997, the photoperiod in the inverse group started to decrease on
Ž .
March 22, whereas in 1998 the reduction started on December 22 Fig. 1 . Reproductive seasonality in sheep from temperate regions is regulated by the change
Ž in the direction of photoperiod, rather than by the absolute duration of the day Robinson
. Ž
and Karsch, 1987; Malpaux et al., 1989 , or by the rate of daily change Chemineau et .
al., 1992, 1995 . This appears to be the case also in Pelibuey ewes, since in all the groups exposed to artificial photoperiod, either before or during the experiment,
ovulatory activity started on average after 2 or 3 months of exposure to a decreasing photoperiod, independently of the length of the longest day from which this reduction
Ž .
started Table 2 or of the daylength that was present on the date of the first ovulation. Ž
. Furthermore, Porras 1999 found a similar period of latency when Pelibuey ewes were
Ž .
abruptly transferred to a short 8L:16D photoperiod after 3 months of exposure to a Ž
. long one 16L:8D .
The 50 to 90 days that elapsed from the start of a gradual photoperiod reduction to the onset of ovarian activity in the ewes kept on inverse photoperiod, as well as during
the first year in those animals transferred from an artificially long photoperiod to natural photoperiod, is similar to the interval from the summer solstice to the onset of
Ž reproductive activity that occurs in many breeds of sheep in temperate regions Hafez,
. 1952; Robinson et al., 1992 . However, this was not the case in the Pelibuey ewes that
Ž .
were always kept on natural photoperiod control group , whose ovarian activity on both Ž
. years started quite rapidly average of 30 to 40 days after the longest day, with some
individuals ovulating few days after the summer solstice, especially during the second Ž
. year Fig. 1A . This ability to start their natural ovulatory season around the time of the
summer solstice, or even slightly before it, appears to be usual in Pelibuey ewes Ž
. Martınez, 1998; Porras, 1999 , and since it does not conform to the norm for other
´
types of sheep, it has been used to suggest that the reproductive seasonality of Pelibuey sheep is not driven by photoperiod, but rather by the increased forage availability
derived from the rainy season, which in Mexico starts on late May or early June Ž
. Gonzalez et al., 1991; Cruz et al., 1994 . However, since the direct effect of photope-
´
riod on reproductive activity of Pelibuey ewes has been clearly demonstrated in the Ž
. present study, as well as by Porras 1999 , it would seem likely that under natural
photoperiod the rapid onset of ovarian activity after the summer solstice in Pelibuey ewes is due to refractoriness to long days, rather than by exposure to short days
Ž .
Robinson et al., 1985 . If this is the case, refractoriness would naturally occur in Pelibuey sheep after 5 to 6 months of exposure to increasing photoperiod. This would
Ž .
explain why the animals exposed to only 3 months of long 16L:8D photoperiod needed Ž
. Ž
. more time 60 to 90 days to start ovulating after an abrupt Porras, 1999 or gradual
Ž .
animals with previous exposure to artificial photoperiod in this study shift to short photoperiod than the time required by the ewes kept on natural photoperiod.
Refractoriness to long days appears to have acted in all ewes kept on natural Ž
. photoperiod for several months before the onset of ovarian activity Figs. 1A and 2A .
In contrast, the ovulatory season did not start shortly after the longest day in most of the ewes exposed to inverse photoperiod, and some of them only ovulated after several
Ž .
months of decreasing photoperiod Figs. 1B and 2B , suggesting that there is an Ž
. endogenous circannual rhythm Malpaux et al., 1989 that needs more than 1 year to
adjust to an inverse photoperiod, or that there are other factors that may modulate the effects of photoperiod.
The presence of modulating factors could explain why three animals kept on inverse photoperiod maintained continuous ovarian activity for at least 18 months, and four
Ž .
more had very short anovulatory periods around 35 days , situations that never occurred in the animals kept on natural photoperiod. Furthermore, the variation on the dates at
which the anovulatory periods started and ended was much wider among the ewes on inverse photoperiod than among those on natural photoperiod, which suggests a conflict
between the information conveyed by the inverse photoperiod and that derived from other sources. This conflict does not appear to be important when Pelibuey ewes are
Ž .
Ž .
Ž .
exposed to extreme long 16L:8D or short 8L:16D photoperiods, since Porras 1999 found uniform responses after each abrupt change in photoperiodic regime, regardless of
the time of the year at which the change was provided. This would suggest that Pelibuey ewes can respond primarily to photoperiodic information when the variations are large
enough, but when the annual variation is small, as in the present study, the response to photoperiod could be modulated by other factors. Thus, the relative importance of
photoperiod would decrease, and that of other factors would increase, as the animals are closer to the equator.
The importance of food availability as a modulating factor does not appear to be of great significance, since it was kept constant both in the present study and in that of
Ž .
Porras 1999 . On the other hand, temperature and humidity were not controlled in the present experiment, and they could have provided information about the time of the year
Ž that conflicted with that provided by inverse photoperiod Pevet, 1987; Bronson and
. Heideman, 1994 .
It is clear than any difference in the reproductive response to photoperiod between the control and the inverse groups was not accounted for by a different pineal responsive-
ness to photoperiod, since both groups responded similarly, with an almost immediate increase after the lights were turned off and an equally rapid decrease when the lights
Ž .
were on Fig. 3 . Despite a 6-month difference in calendar time, and thus in other variables that depend on it, Table 3 shows that the duration of melatonin elevation was
Ž .
identical on the longest day of both groups 12.3 h , and almost identical on the shortest Ž
. day 9.7 vs. 9.5 h for the natural and inverse groups . The changes in melatonin
secretion in response to the light:dark cycle in Pelibuey sheep conform to the classic Ž
. pattern for other types of sheep Lincoln, 1992; Malpaux et al., 1993 , thus confirming
that African hair sheep can perceive photoperiodic information and translate it into a melatonin signal in a similar way as sheep from temperate regions do.
The prolactin profile under natural photoperiod and its response to inverse photope- Ž
. Ž
riod Fig. 4 conform to those found on other types of sheep Pelletier, 1973; Poulton et .
al., 1987; Curlewis, 1992 , and may indicate that the transduction of photoperiodic information through melatonin to other physiological processes in Pelibuey sheep is
Ž similar to that in breeds from temperate regions Daveau et al., 1994; Lincoln and
. Clarke, 1994 . However, the response of prolactin to a given daylength was slightly
affected in the ewes exposed to inverse photoperiod, since the correlation between daylength and prolactin concentrations was lower in these ewes than in the control
animals. Also, concentrations of the hormone were always lower in the inverse group than at the corresponding point, in terms of photoperiodic cycle, of the control group.
This effect of the time of the year on the prolactin response to photoperiod was not
Ž .
Ž .
observed when Pelibuey ewes were exposed to long 16L:8D
or short 8L:16D
Ž .
photoperiods Porras, 1999 , where a given photoperiod caused the same response when
applied at different times of the year. This difference between our results and those of Ž
. Porras 1999 again suggests that the relative importance of photoperiodic information
with respect to other modulating factors may be reduced as Pelibuey ewes are kept closer to the equator.
5. Conclusion
